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Accepted papers to appear in an upcoming issue

Optica Publishing Group posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

Optimization of a fiber Fabry-Perot resonator for low-threshold modulation instability Kerr frequency combs

Germain Bourcier, Safia Mohand Ousaid, Stéphane Balac, Julien Lumeau, Antonin Moreau, Thomas Bunel, Arnaud Mussot, Matteo Conforti, Olivier LLOPIS, and arnaud fernandez

DOI: 10.1364/OL.523291 Received 07 Mar 2024; Accepted 17 May 2024; Posted 17 May 2024  View: PDF

Abstract: We report a theoretical and experimental investigation of fiber Fabry-Perot cavities aimed at enhancing Kerr frequency comb generation. The modulation instability (MI) power threshold is derived from the linear stability analysis of a generalized Lugiato-Lefever equation. By combining this analysis with the concepts of power enhancement factor (PEF) and optimal coupling, we predict the ideal manufacturing parameters of fiber Fabry-Perot (FFP) cavities for MI Kerr frequency comb generation. Our findings reveal a distinction between the optimal coupling for modulation instability and that of the cold cavity. Consequently, mirror reflectivity must be adjusted to suit the specific application. We verified the predictions of our theory by measuring the MI power threshold as a function of detuning for three different cavities.

Surface plasmon contribution to the binding force and energy potential

Mauro Cuevas and Hernán Ferrari

DOI: 10.1364/OL.524963 Received 27 Mar 2024; Accepted 16 May 2024; Posted 16 May 2024  View: PDF

Abstract: In this work we have proposed a graphene planar structure as an optical binding device of dielectric nano--particles. The surface plasmons on graphene sheet, generated thanks to the near field scattering of the incident plane wave by the nano--particles placed close to the graphene sheet, act as a powerful intermediary for enhancing the optical force between nano--particles to organize the particle structure at length scales comparable with the plasmon wavelength, this is, at the light sub--wavelength scale. In particular, we have paid attention to the formation of one dimensional arrays of nano--particles. Our results show that both the equilibrium separation between particles and the energy potential binding depend on the number of particles forming the array, and that the former tends to the plasmon wavelength (the array constant) for a number of particles large enough. We have obtained simple analytical expressions that explain the main results obtained by using the rigorous theory. Our contribution can be valuable for the knowledge in the low frequency optical binding framework, from terahertz to far infrared spectrum.

Waveguide-integrated van der Waals heterostructure photodetector on a lithium niobate on insulator platform

Ke Xia, Huayou Liu, yang qiu, Shaonan Zheng, Yaping Dan, Qize Zhong, Yuan Dong, Xingyan Zhao, and Ting Hu

DOI: 10.1364/OL.522343 Received 27 Feb 2024; Accepted 16 May 2024; Posted 17 May 2024  View: PDF

Abstract: Lithium niobate (LN) photonics have gained significant interest for their distinct material properties. However, achieving monolithically integrated photodetectors on lithium niobate on insulator (LNOI) platform for communication wavelengths remains a challenge due to the large bandgap and extremely low electrical conductivity of LN material. Two-dimensional (2D) material photodetector is an ideal solution for LNOI photonics with a strong light-matter interaction and simple integration technique. In this work, a van der Waals heterostructure photodiode composed of a p-type black phosphorus layer and an n-type MoS₂ layer is successfully demonstrated for photodetection at communication wavelengths on LNOI platform. The LNOI waveguide-integrated BP-MoS₂ photodetector exhibits a dark current as low as 0.21 nA and an on/off ratio exceeding 200 under zero voltage bias. A responsivity as high as 1.46 A/W is achieved at -1V bias with a reasonable dark current around 2.33 μA. With the advantages of high responsivity, low dark current and simple fabrication process, it is promising for the monolithically integrated photodetector application for LNOI photonic platforms at communication wavelengths.

Multicolor tunable persistent luminescence mechanism in the well-designed inorganic composites

Yongmin Duan, Shugang Li, Keyi Gu, Zhaojing Kuang, Shiqing Xu, and Junjie Zhang

DOI: 10.1364/OL.522446 Received 27 Feb 2024; Accepted 16 May 2024; Posted 17 May 2024  View: PDF

Abstract: Herein, by ball milling CsPb(Br/I)3 quantum dots glasses powder with Sr2MgSi2O7:Eu2+, Dy3+ phosphor, wide-spectrum tunable (From 466 nm to 630 nm) long persistent luminescence (LPL) in inorganic composites with more than 700 min attenuation time can be obtained via radiation photon reabsorption process. Attractively, the wide color gamut of LPL spectra overlaps the National Television System Committee space 74%. Notably, the luminescence intensity remains stable when the inorganic composites are composed with UV light for 100 h. Finally, practical anti-counterfeiting application is successfully realized based on the prepared LPL inorganic composites. This work provides a new perspective to achieve polychromatic adjustment of LPL.

Multi-site microendoscopic imaging probe for simultaneous three-dimensional imaging at multiple locations in tissues

Guigen Liu, Sebastian Ahn, Jeon Woong Kang, Sharath Bhagavatula, Destiny Matthew, Samantha Martin, Courtney Marlin, Peter So, Guillermo Tearney, and Oliver Jonas

DOI: 10.1364/OL.525945 Received 11 Apr 2024; Accepted 16 May 2024; Posted 16 May 2024  View: PDF

Abstract: Systems that can image in three dimensions at cellular resolution and across different locations within an organism may enable insights into complex biological processes, such as immune responses, for which a single location measurement may be insufficient. In this Letter, we describe an in vivo multi-site imaging probe (MIP) that can simultaneously image multiple spatially separated anatomic sites. The MIP consists of two bendable graded index (GRIN) lenses and is demonstrated by a two-photon two-color fluorescence imaging system. A blind linear unmixing algorithm is applied to suppress bleedthrough between channels. We use this system to successfully demonstrate multi-site two-photon two-color imaging of two biomedically relevant samples, i.e., (1) mixture of two autofluorescent anti-cancer drugs and (2) a live hybrid tumor consisting of two spectrally distinct fluorescent cell lines.

On the origin for the enhanced light extraction efficiency of DUV LED by using inclined sidewalls

Liu Wang, tong jia, Zhaoqiang Liu, Chunshuang Chu, Kangkai Tian, Yonghui Zhang, and Zi Hui Zhang

DOI: 10.1364/OL.526100 Received 15 Apr 2024; Accepted 15 May 2024; Posted 16 May 2024  View: PDF

Abstract: It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) can be enhanced by using inclined sidewall of mesa. However, the reported optimal inclined angles are different. In this work, to explore the origin for enhancing the LEE of DUV LED by using inclined sidewalls, we investigate the effect of inclined sidewall angle on the LEE for AlGaN-based DUV LEDs with different mesa diameters by using ray tracing. It is found that when compared to large-size DUV LEDs with inclined sidewall, the LEE of small-size DUV LEDs with inclined sidewall is enhanced from both the bottom and side surfaces due to the reduced scattering length and material absorption. Additionally, the optimal inclined sidewall angle is recommended within the range of 25°-65°, and the optimal angle for DUV LEDs decreases as the chip size increases. It can be attributed to the fact that there are two scattering mechanisms for the inclined sidewall. For smaller chip sizes, the most of the light is directly scattered into escape cones by the inclined sidewall, resulting in a larger optimal angle. For larger chip sizes, the light firstly experiences total internal reflections by the out-light plane and then is scattered into escape cones by the inclined sidewalls, leading to a smaller optimal angle.

2.6 W average output power, dual-wavelength synchronously self-mode-locked Ho:LLF laser operating at 2068.5 nm and 2069.2 nm

Panqiang Kang, Xinlu Zhang, Xiaofan Jing, Longyi Zhang, Changchang Shen, Binxu Gu, Jinjer Huang, Yulei Wang, Li Li, and Zhiwei Lv

DOI: 10.1364/OL.527220 Received 11 Apr 2024; Accepted 15 May 2024; Posted 16 May 2024  View: PDF

Abstract: A dual-wavelength synchronously self-mode-locked Ho:LLF laser operating at 2068.5 nm and 2069.2 nm was demonstrated. The maximum average output power was as high as 2.6 W with a pulse width of 74.6 ps and a pulse repetition frequency of 3.03 GHz. Meanwhile, the output power ratio of the dual wavelength lasers can be effectively controlled by varying the incident pump power. To the best of our knowledge, this is the first dual-wavelength synchronously self-mode-locked Ho-doped fluoride solid state laser, moreover, our current experimental results represent the highest average output power from a GHz SML oscillator in the 2 µm waveband.

An Error-Compensation Network for Ringing Artifacts Reduction in Holographic Displays

Ganzhangqin Yuan, Mi Zhou, Yifan Peng, Mu Ku Chen, and Zihan Geng

DOI: 10.1364/OL.519519 Received 19 Jan 2024; Accepted 15 May 2024; Posted 16 May 2024  View: PDF

Abstract: Recent advances in learning-based computer-generated holography (CGH) have unlocked novel possibilities for crafting phase-only holograms. However, existing approaches primarily focus on the learning ability of network modules, often neglecting the impact of diffraction propagation models. The resulting ringing artifacts, emanating from the Gibbs phenomenon in the propagation model, can degrade the quality of reconstructed holographic images. To this end, we explore a diffraction propagation error-compensation network that can be easily integrated into existing CGH methods. This network is designed to correct propagation errors by predicting residual values, thereby aligning the diffraction process closely with an ideal state and easing the learning burden of the network. Simulations and optical experiments demonstrate that our method, when applied to state-of-the-art HoloNet and CCNN, achieves PSNRs of up to 32.47 dB and 29.53 dB, respectively, surpassing baseline methods by 3.89 dB and 0.62 dB. Additionally, real-world experiments have confirmed a significant reduction in ringing artifacts. We envision this approach being applied to a variety of CGH algorithms, paving the way for improved holographic displays.

Attosecond Ionic Photoionization Spectroscopy: Ion RABBITT

Yidan Xu, Lulu Han, Wenyu Jiang, Zitan Zuo, Shengzhe Pan, Avner Fleischer, Kiyoshi Ueda, and Jian Wu

DOI: 10.1364/OL.523947 Received 18 Mar 2024; Accepted 15 May 2024; Posted 15 May 2024  View: PDF

Abstract: Photoionization is one of the most fundamental processes in light-matter interaction. Advanced attosecond photoelectron spectroscopy provides the possibility to characterize the ultrafast photoemission process in the extremely short attosecond time scale. Following the scattering symmetry rules, the residual ions encode the ultrafast photoionization prints at the instant of electron removal forming an alternative electron emission chronoscope. Here, we experimentally illustrate the attosecond ion spectroscopy through the development of high-resolution ion momentum detection in atomic photoionization processes. Our ion RABBITT measurement presents identical energy- and time-dependent scattering phase shift, as we observed in photoelectron spectroscopy and thus demonstrates that ion RABBITT can be an alternative attosecond approach to resolve the photoionization process, without the electron homogeneity limitation.

Accelerated Phase Retrieval Using Adaptive Support and Statistical Fringe Processing of Phase Estimates

Mario Juvenal III Onglao and Percival Almoro

DOI: 10.1364/OL.522321 Received 26 Feb 2024; Accepted 14 May 2024; Posted 14 May 2024  View: PDF

Abstract: A technique for accelerated multiple-plane phase retrieval is demonstrated by creating adaptive support through statistical analysis of phase estimates. Its technical advantage arises from, what we believe to be, the first time use of both phase estimates and a statistical metric, enabling the fast generation of noise-robust support masks. This results in a fourfold improvement in convergence speed when compared to the conventional multiple-plane method. Evaluating data fitting performance with fewer intensity recordings showed that using four or more recordings resulted in accurate fitting, three recordings caused overfitting, and two recordings led to underfitting for the test object waves used. The adaptive support strategy based on phase estimates is compatible with other iterative phase retrieval methods.

Ultra-high precision laser nanoprinting based on defect compensated digital holography for fast fabricating optical metalenses

KAIWEN JIE, ZHUOFAN YAO, YIYIN ZHENG, MINGHUI WANG, DIEFENG YUAN, ZEDA LIN, SHANTONG CHEN, Fei Qin, HUASE OU, Xiangping Li, and Yao-Yu Cao

DOI: 10.1364/OL.522575 Received 18 Mar 2024; Accepted 14 May 2024; Posted 15 May 2024  View: PDF

Abstract: 3D structured light field manipulated by digital-micromirror-device(DMD)-based digital hologram has demonstrated its superiority in fast fabricating stereo nanostructures. However, this technique intrinsically suffers from defects of light intensity in generating modulated focal spots, which prevents from achieving high-precision micro/nanodevices. In this paper, we have demonstrated a compensation approach based on adapting spatial voxel density for fabricating optical metalenses with ultra-high precision. The modulated focal spot experiences intensity fluctuations of up to 3% with changing the spatial position, leading to a 20% variation of the structural dimension in fabrication. By altering the voxel density to improve the uniformity of laser cumulative exposure dosage over the fabrication region, we achieved an increased dimensional uniformity from 94.4% to 97.64% in fabricated pillars. This approach enables fast fabrication of metalenses capable of sub-diffraction focusing of 0.44λ/NA with the increased mainlobe-sidelobe ratio from 1:0.34 to 1:0.14. A 6x5 supercritical lens array is fabricated within 2 minutes, paving a way for the fast fabrication of large-scale photonic devices.

Polarization enhancement mechanism from tissue staining in multispectral Mueller matrix microscopy

Yuzhu Shi, Chunyan Chen, Liangyu Deng, Nan Zeng, Hongtao Li, Zhiyan Liu, Honghui He, Chao He, and Hui Ma

DOI: 10.1364/OL.523570 Received 11 Mar 2024; Accepted 14 May 2024; Posted 15 May 2024  View: PDF

Abstract: Mueller matrix microscopy can provide comprehensive polarization-related optical and structural information of biomedical samples label-freely. Thus, it is regarded as an emerging powerful tool for pathological diagnosis. However, the staining dyes have different optical properties and staining mechanisms, which can put influence on Mueller matrix microscopic measurement. In this Letter, we quantitatively analyze the polarization enhancement mechanism from hematoxylin and eosin (H&E) staining in multispectral Mueller matrix microscopy. We examine the influence of hematoxylin and eosin dyes on Mueller matrix derived polarization characteristics of fibrous tissue structures. Combined with Monte Carlo simulations, we explain how the dyes enhance diattenuation and linear retardance as the illumination wavelength changed. In addition, it is demonstrated that by choosing an appropriate incident wavelength, more visual Mueller matrix polarimetric information can be observed of the H&E stained tissue sample. The findings can lay the foundation for the future Mueller matrix assisted digital pathology.

Multi-spectrum compatible metasurface with low infrared emissivity, independent microwave complex-amplitude control and high visible transparency

juanna jiang, Ruichao Zhu, Huiting Sun, yuxiang jia, Yina Cui, Xu Cuilian, Shaobo Qu, and Jiafu Wang

DOI: 10.1364/OL.524625 Received 01 Apr 2024; Accepted 14 May 2024; Posted 17 May 2024  View: PDF

Abstract: With the rapid development of communication technology and detection technology, it is difficult for devices operating in a single spectrum to meet the application requirements of device integration and miniaturization, resulting in the exploration of multi-spectrum compatible devices. However, the functional design of different spectrum is often contradictory and difficult to be compatible. In this work, a transparent slit circular metasurface with high filling ratio is proposed to achieve the compatibility of microwave, infrared and visible light. In microwave, based on Pancharatnam-Berry phase theory, the continue amplitude and binary phase can be customized only by rotating the slit angle to achieve Airy beam function at 8-12 GHz. In infrared, the mean infrared emissivity is reduced to 0.3 at 3-14 μm by maintaining high conductive filling ratio. And in visible light, based on the transparency of materials, the mean transmittance can achieve 60% at 400-800 nm. All the results can verify the multispectral compatibility performance, which can also verify the validity of our design method. Importantly, the multi-spectrum compatible metasurface contributes an option for multifunctional integration, which can be further applied in communication, camouflage and other fields.

Broadband chaos of an interband cascade laser with 6-GHz bandwidth

Yi-Bo Peng, zhecheng dai, Kaili Lin, penglei wang, Zhijian Shen, Baile Chen, Frederic Grillot, and Cheng Wang

DOI: 10.1364/OL.525636 Received 03 Apr 2024; Accepted 14 May 2024; Posted 15 May 2024  View: PDF

Abstract: Near-infrared semiconductor lasers subject to optical feedback usually produce chaos with a broad bandwidth of a few GHz. However, reported mid-infrared interband cascade lasers (ICL) only show chaos with a limited bandwidth below 1 GHz. Here we show that an ICL with optical feedback is able to generate broadband chaos as well. The mid-infrared chaos exhibits a remarkable bandwidth of about 6 GHz, which is comparable to that of the near-infrared counterpart. In addition, the spectral coverage in the electrical domain reaches as high as 17.7 GHz. It is found that the chaos bandwidth generally broadens with increasing feedback ratio and/or increasing pump current of the laser, while it is insensitive to the feedback length.

Enhancing Field-of-View of Spectral-Scanning FMCW LiDAR by Multipass Configuration with an Echelle Grating

Yaqi Han, Zihan Zang, Lican Wu, Yi Hao, Qingyang Zhu, Connie Chang-Hasnain, and Hongyan Fu

DOI: 10.1364/OL.525191 Received 02 Apr 2024; Accepted 13 May 2024; Posted 17 May 2024  View: PDF

Abstract: We present a spectral-scanning frequency-modulated continuous wave (FMCW) 3D imaging system capable of producing high-resolution depth maps with an extended field of view (FOV). By employing a multipass configuration with an echelle grating, the system achieves an FOV of 5.5° along the grating axis. The resulting depth maps have a resolution of 70 ×40 pixels, with a depth resolution of 5.1 mm. The system employs an echelle grating for beam steering and leverages the multipass configuration for angular FOV magnification. Quantitative depth measurements and 3D imaging results of a static 3D-printed depth variation target is demonstrated. The proposed approach offers a promising solution for enhancing the FOV of spectral-scanning FMCW LiDAR systems without the need for a widely tunable source, thereby reducing system complexity and cost, paving the way for improved 3D imaging applications.

A three-dimensional ranging system based on Fresnel incoherent correlation holography

Zhang Pengwei, Fengying Ma, Zekai Li, Nan Zhao, Fuyou Gao, Jianpo Su, Yongsheng Hu, and Jiuru He

DOI: 10.1364/OL.519014 Received 16 Jan 2024; Accepted 13 May 2024; Posted 14 May 2024  View: PDF

Abstract: We proposed a three-dimensional (3D) ranging system based on Fresnel incoherent correlation holography (FINCH). Distinct from displacement measurement based on coherent digital holography (DH), our system simultaneously achieves 3D range measurement using incoherent illumination. The observation range is obtained by the holographic reconstruction, while the in-plane range is determined using the two-dimensional digital imaging correlation (2D-DIC) technique. Experimental results on the resolution target demonstrate precise 3D ranging determination and improved measurement accuracy.

Sensitivity, precision, and accuracy of fs-LIBS for heavy metal detection in flowing aqueous solutions

Yutong Chen, Yuanfei Jiang, Jianhui Han, Anmin Chen, and Mingxing Jin

DOI: 10.1364/OL.526093 Received 11 Apr 2024; Accepted 13 May 2024; Posted 14 May 2024  View: PDF

Abstract: This investigation employs femtosecond laser-induced breakdown spectroscopy (fs-LIBS) to measure the concentrations of Chromium (Cr), Lead (Pb), and Copper (Cu) in flowing aqueous solutions. The fs pulsed laser excites the water, generating plasma in a dynamic setting that prevents liquid splashing—a notable advantage over static methods. The flowing water column maintains a stable liquid level, circumventing the laser focus irregularities due to liquid level fluctuations. Calibration curves, based on a linear function, reveal Limits of Detection (LOD) as low as 0.0179 μg/mL for Cr, 0.1301 μg/mL for Pb, and 0.0120 μg/mL for Cu. The reliability of the experiment is confirmed by R² values exceeding 0.99. These findings offer valuable insights for the analysis of trace heavy metals in flowing aqueous solutions using fs-LIBS, demonstrating the technique’s potential for environmental monitoring.

All-Dielectric Super-Lattice Metasurfaces with Fivefold Spatial Resolution Enhancement for Structured Illumination Microscopy

Jiayu Ding and Siying Peng

DOI: 10.1364/OL.524514 Received 22 Mar 2024; Accepted 13 May 2024; Posted 15 May 2024  View: PDF

Abstract: Structured illumination microscopy (SIM) is a promising imaging technique for high-resolution imaging with a wide field of view. Although periodic nanostructure is a versatile platform for engineering the spatial frequency of structured illumination patterns in SIM, challenges remain, including artifacts from Fourier space gaps. We designed an all-dielectric super-lattice metasurface (ADSLM) to generate structured illumination patterns with enhanced spatial frequency and broadened spatial frequency coverage with no intermediate frequency gaps. Our numerical simulations reveal that ADSLM-based image reconstruction is capable of producing high-contrast, artifact-free images, resulting in enhanced spatial resolution up to 5.7-fold for coherent SIM at 450 nm. Our results show that the ADSLM-SIM technique may facilitate high-resolution imaging using CMOS-compatible substrates, offering potential for compact miniturized imaging applications.

Efficient (∼10%) Generation of Vacuum Ultraviolet Femtosecond Pulses via Four-Wave Mixing in Hollow-Core Fibers

Ruaridh Forbes, Paul Hockett, Quentin Leterrier, and Rune Lausten

DOI: 10.1364/OL.524719 Received 25 Mar 2024; Accepted 13 May 2024; Posted 15 May 2024  View: PDF

Abstract: We report the generation of the 5th harmonic ofTi:sapphire, at 160 nm, with more than 4 µJ of pulseenergy, and a pulse length of 37 fs with a 1 kHz repetition rate. The vacuum ultraviolet pulses are producedusing four-wave difference frequency mixing in a Hefilled stretched hollow-core fiber, driven by a pump at267 nm and seeded at 800 nm. Guided by simulationsusing Luna.jl, we are able to optimize the process carefully. The result is a conversion efficiency of ∼10% fromthe 267 nm pump beam

Graphene-enhanced lateral photovoltaic effect observed in the Ag nanoparticles covered graphene/n-type silicon

Shuai Liu, Feiyu Ren, Xinyuan Dong, Diyuan Zheng, yiru niu, Lu Jiao, Weiwei Tu, Zhikai Gan, and Hui Wang

DOI: 10.1364/OL.524723 Received 25 Mar 2024; Accepted 13 May 2024; Posted 15 May 2024  View: PDF

Abstract: Graphene is a kind of two-dimensional material with single-layer carbon structure and has been investigated in many high-performance photodetectors. Lateral photovoltaic effect (LPE) is widely used in the position-sensitivity detectors (PSDs) owing to its linear reponse of photovoltage to the light position. In this letter, a type of graphene-enhanced LPE is observed in the Ag nanoparticles covered graphene/n-type Si. The LPE sensitivity can reach 97.3mV/mm, much higher than the sensitivity of 1.3mV/mm in the control sample of Ag/Si and 5.2mV/mm of graphene/Si. Based on photocarriers’ diffusion mechanism, tailoring photocarriers transfer at the interface of heterojunction plays a key role for the enhancement. These findings exhibit great application potential of graphene in the field of PSDs and offer an effective method for the optimization of LPE devices.

On the Maximization of Entropy in the Process of Thermalization of Highly Multimode Nonlinear Beams

Fabio Mangini, Mario Ferraro, Wasyhun Gemechu, Yifan SUN, Mikhail Gervaziev, Denis Kharenko, Sergey Babin, Vincent COUDERC, and Stefan Wabnitz

DOI: 10.1364/OL.521563 Received 16 Feb 2024; Accepted 13 May 2024; Posted 13 May 2024  View: PDF

Abstract: We present a direct experimental confirmation of the maximization of entropy which accompanies the thermalization of a highly multimode light beam, upon its nonlinear propagation in standard graded-index optical fibers.

Event-based depth estimation with dense occlusion

Kangrui Zhou, Taihang Lei, Guan Banglei, and Qifeng Yu

DOI: 10.1364/OL.521988 Received 22 Feb 2024; Accepted 13 May 2024; Posted 13 May 2024  View: PDF

Abstract: Occlusions pose a significant challenge to depth estimation in various fields, including automatic driving, remote sensing observation, and video surveillance. In this Letter, we propose a novel depth estimation method for dense occlusion to estimate the depth behind occlusions. We design a comprehensive procedure using an event camera that consists of two steps: rough estimation and precise estimation. In the rough estimation, we reconstruct two segments of the event stream to remove occlusions and subsequently employ binocular intersection measurement to estimate the rough depth. In the precise estimation, we propose a criterion that the maximum total length of edges of reconstructed images corresponds to the actual depth and search for the precise depth around the rough depth. The experimental results demonstrate that our method is implemented with relative errors of depth estimation below 1.05%.

Single wavelength MDM-PDM 800-Gb/s net data rate trsnsmission over standard multimode fiber employing Kramers-Kronig receiver

Xinkuo Yu, Li Jianping, Jianqing He, Yuwen Qin, Jianbo Zhang, Gai Zhou, Meng Xiang, Songnian Fu, Kang Ping Zhong, and Chenglin Bai

DOI: 10.1364/OL.523331 Received 07 Mar 2024; Accepted 13 May 2024; Posted 13 May 2024  View: PDF

Abstract: We propose a high-speed multimode fiber short-reach optical interconnect system based on Kramers–Kronig (KK) field reconstruction with mode division multiplexing (MDM) and polarization division multiplexing (PDM) technology. In this work, the LP01, LP21a, LP21b and LP02 modes are selected as independent channels to carry information. The demonstration achieved the 800 Gb/s net data rate per wavelength with a bitrate-distance-product (BDP) of 8 Tb/s*km. To the best of our knowledge, this is the highest experimental record of single wavelength BDP over the SMMF with KK detection. In addition, we discuss the system performance after all multiple-input multiple-output (MIMO) and partial MIMO processing, and give guidance on the tradeoff between system performance and computational resource.

Secure Optical Communication System Based on Polarization Regulation of Data Fragmentation Multipath Transmission Technology

Kun Peng Zhai, XINYAN ZHANG, Sha Zhu, Yu Liu, Huashun Wen, and Ninghua Zhu

DOI: 10.1364/OL.524408 Received 20 Mar 2024; Accepted 13 May 2024; Posted 13 May 2024  View: PDF

Abstract: We propose and demonstrate a data fragment multipath transmission scheme to achieve secure optical communication based on polarization regulation. A dual-polarization Mach–Zehnder modulator (DPMZM) is driven by digital signals which are scattered by field-programmable gate array (FPGA) and transmitted in multiple paths. By utilizing two orthogonal polarization states, we have achieved signal transmission across different optical parameters, and the transmission rate of the two paths can reach over 10 Gbps through 20 km fiber with 2.5 Gbps hopping rate. In addition, we establish a theoretical model to analyze the security of the system, and simulated brute force cracking, the probability of cracking the minimum information unit is 1.53×10-53. This proves that it was difficult to obtain user data even using the fastest computers. Our scheme has provided a new approach for physical layer security.

Realizing highly efficient vertical coupling with dielectric deflective metasurfaces

Minjun Xie, Bo Xiong, and Tao Chu

DOI: 10.1364/OL.524721 Received 26 Mar 2024; Accepted 13 May 2024; Posted 13 May 2024  View: PDF

Abstract: Using dielectric deflective metasurfaces, we propose a novel out-of-plane modulation scheme to realize vertical coupling on 220 nm silicon-on-insulator platform. The metasurface is used to deflect vertical incident light to an oblique angle with high efficiency in the cladding layer. This deflection introduces a lateral wave vector component, thus preventing bi-directional transmission of traditional vertical coupling due to the second-order Bragg diffraction of the grating. Additionally, an apodized design is employed for the subwavelength grating to improve mode matching with deflection angle incident. The integration of the metasurface and subwavelength grating enables a new vertical coupling scheme with high efficiency. After global optimization, we achieved a simulation coupling efficiency of -2.19 dB. The measured coupling efficiency is -3.36 dB with a center wavelength of 1545.6 nm and a 1-dB bandwidth of 32 nm. The results confirm the feasibility of the proposed new architecture.

Tunable intrinsic strong light-matter coupling in transition-metal-dichalcogenide nanoresonators

Qi Ding, Ruiyang Zhang, Wenrui Bao, Peng Xie, Ling Yue, Shiyu Shen, hong zhang, and wei wang

DOI: 10.1364/OL.524391 Received 20 Mar 2024; Accepted 12 May 2024; Posted 14 May 2024  View: PDF

Abstract: Self-hybridizing structures based on metal dichalcogenides(TMDCs) are becoming promising candidatesfor the study of intrinsic strong light-matter couplingbecause of the efficient mode overlap with much simplifiedgeometries. However, realizing flexible tuningof intrinsic strong coupling in such TMDC-based structuresis still challenging. Here, we propose a strategyfor flexible tuning of intrinsic strong light-matter couplingbased on bulk TMDC material. We report thefirst demonstration of strong coupling of intrinsic excitonsto whispering gallery modes (WGMs) supportedby an all-TMDC nanocavity. Importantly, by simplycontrolling angles of incidence, a selective excitation ofWGMs and anapole can be realized, which enables adirect modulation of self-hybridized interactions frombright WGM-exciton coupling to dark anapole-excitoncoupling. Our work is expected to provide unique opportunitiesfor engineering strong light-matter couplingand to open exciting avenues for highly integrated novelnanophotonic devices.

Dyakonov surface waves in thin interfacial waveguide formed by negatively anisotropic dielectrics

Dmitry Chermoshentsev, Evgeny Anikin, Nikolay Gippius, and Sergei Dyakov

DOI: 10.1364/OL.524964 Received 28 Mar 2024; Accepted 11 May 2024; Posted 13 May 2024  View: PDF

Abstract: Dyakonov surface waves are electromagnetic surface waves that exist at the interface of two dissimilar materials, with at least one material being anisotropic. Although there are various types of these waves, they all exist in anisotropic materials with positive anisotropy. The requirement for positive anisotropy limits the choice of materials that can support these waves. In this study, we present a type of Dyakonov surface waves that occur at the interface of negatively anisotropic materials. Specifically, we demonstrate their existence in a system consisting of two negatively anisotropic slabs confined between two perfect electric conductor walls. By assuming a small distance between the walls, we derive analytical expressions for the propagation constant, penetration depth, and field distribution of these surface waves. Our analytical model exhibits excellent agreement with full-wave electromagnetic simulations conducted in Comsol. Furthermore, we numerically demonstrate that these surface waves can also exist in structures beyond the approximations used to develop the theoretical framework. The existence of Dyakonov surface waves in negative crystals broadens the range of materials suitable for their practical implementation, with potential applications in biosensing, long-range optical communications and imaging.

Enhancing secure transmission and key distribution for ultrahigh-order QAM via delta-sigma modulation and discrete memristive-enhanced chaos

Jianye Zhao, Bo Liu, Jianxin Ren, Yaya Mao, yiming ma, Qing Zhong, xiangyu wu, shuaidong chen, Yongfeng Wu, xiumin song, and Lilong Zhao

DOI: 10.1364/OL.527328 Received 15 Apr 2024; Accepted 10 May 2024; Posted 10 May 2024  View: PDF

Abstract: In this letter, we propose a method for ultrahigh-order QAM secure transmission and key distribution based on delta-sigma modulation (DSM) and discrete memristive-enhanced chaos (DMEC). The disturbance vectors generated by DMEC scramble the DSM signals in both frequency and time domains, resulting in highly secure DSM signals. Through the key modulation and power adjustment, and then superimposing them on the encrypted signals, the method achieves simultaneous transmission of keys and signals without the need for additional spectral resources. This approach allows for secure communication with continuous key iteration and updates, offering an effective solution for implementing "one-time pad" encryption. In the experimental demonstration, we achieved a secure transmission and key distribution of a 16384QAM signal at a rate of 17.09 Gb/s over 25 km in an intensity-modulated direct detection (IMDD) system, based on DSM.

High sensitivity distributed dynamic pressure sensor based on dual-linear frequency modulated optical frequency domain reflectometry

Yuyao Wang, Hua Zheng, Huan Wu, Dongmei HUANG, Changyuan Yu, and Chao Lu

DOI: 10.1364/OL.525224 Received 29 Mar 2024; Accepted 10 May 2024; Posted 16 May 2024  View: PDF

Abstract: In this work, we propose and experimentally demonstrate a highly sensitive distributed dynamic pressure sensor based on a dual-linear frequency modulated optical frequency domain reflectometry and a coating thickness-enhanced single-mode fiber (SMF). A dual-sideband linear frequency modulation (LFM) signal is used to interrogate the sensing fiber, which allows us to obtain a dual-sideband Rayleigh backscattering signal. Due to the opposite slopes of the two LFM sidebands, the Rayleigh backscattering spectra of the two sidebands drift in opposite directions when the fiber is disturbed. By subtracting the frequency shifts of the two spectra, we can double the system's sensitivity. We further enhance the sensitivity by using an SMF with a coating thickness of 200 μm. This results in a pressure sensitivity of 3979 MHz/MPa, a measurement accuracy of 0.97 KPa, and a spatial resolution of 35 cm over a 500 m optical fiber. Our system successfully detected a dynamic pressure change at a sampling rate of 1.25 KHz, demonstrating the sensor's excellent dynamic measuring capabilities.

Loss tailoring single-mode high-power supersymmetric lasers

Lichang Wang, Yufei Wang, Fengxin Dong, Ting Fu, Mengna Li, Kang Zhang, Kai Gong, Xuyan Zhou, and JIANXIN ZHANG

DOI: 10.1364/OL.523144 Received 06 Mar 2024; Accepted 09 May 2024; Posted 13 May 2024  View: PDF

Abstract: Diode lasers with high beam quality and high power have many promising applications. However, high beam quality is always in conflict with high power. In this letter, we theoretically and experimentally confirm the mode instability property of supersymmetric structures at higher operating currents. Meanwhile, we propose a loss-tailoring diode laser based on supersymmetric structure, which enables the higher-order lateral modes to obtain higher losses, raises the excitation threshold of the higher-order lateral modes, and achieves a stable fundamental-lateral-mode output at higher current operation. The device obtained a quasi-single-lobe lateral far-field distribution with the full width at half maximum (FWHM) of 7.58° at 350 mA under room temperature, which is a 65% reduction compared to the traditional Fabry-Perot diode lasers. Moreover, the M² of 2.181@350mA has an improvement of about 37% over traditional FP and supersymmetric structure lasers.

Secure transmission in W-band RoF system based on delta-sigma modulation

Yaoqiang Xiao, Chunxi Zhou, Linrong Jiang, Zhaoyu An, and Yuqing Li

DOI: 10.1364/OL.525571 Received 03 Apr 2024; Accepted 09 May 2024; Posted 13 May 2024  View: PDF

Abstract: In this letter, a delta-sigma modulation (DSM) encryption technique in W-band RoF system is proposed. By performing DSM with different over-sample ratio (OSR) on the OFDM signal based on the controlled keys generated by the chaotic system at the transmitter, and performing constellation masking to disturb the transmitting signal for encryption, a high order QAM-OFDM-DSM encrypted signal is achieved. In order to further improve the security of system, bit bidirectional diffusion scrambling is used to resist chosen-plaintext attacks. After experimental tests, under the same transmission power, the encrypted DSM signal can achieve better security than single OSR of DSM signals through 50-km standard single-mode fiber (SSMF) and 3-m wireless channel, with the gain of sensitivity increased by ~1dBm. And from the reliability of system, the encrypted signal of proposed scheme can be recovered, which meets hard decision–forward error correction (HD-FEC) threshold of 3.8 × 10-3.

On-chip Kerr parametric oscillation with integrated heating for enhanced frequency tuning and control

Jordan Stone, Daron Westly, Gregory Moille, and Kartik Srinivasan

DOI: 10.1364/OL.523704 Received 15 Mar 2024; Accepted 09 May 2024; Posted 09 May 2024  View: PDF

Abstract: Nonlinear microresonators can convert light from chip-integrated sources into new wavelengths within the visible and near-infrared spectrum. For most applications, such as the interrogation of quantum systems with specific transition wavelengths, tuning the frequency of converted light is critical. Nonetheless, demonstrations of wavelength conversion have mostly overlooked this metric. Here, we apply efficient integrated heaters to tune the idler frequency produced by Kerr optical parametric oscillation in a silicon-nitride microring across a continuous 1.5 terahertz range. Finally, we suppress idler frequency noise between DC and 5 kHz by several orders of magnitude using feedback to the heater drive.

Quantum State Tomography in a Third-Order Integrated Optical Parametric Oscillator

Roger Kögler, Gabriel Rickli, Renato DOMENEGUETTI, Xingchen Ji, Alexander Gaeta, Michal Lipson, Marcelo Martinelli, and Paolo Nussenzveig

DOI: 10.1364/OL.521339 Received 09 Feb 2024; Accepted 09 May 2024; Posted 09 May 2024  View: PDF

Abstract: We measured the covariance matrix of the fields generated in an integrated third-order optical parametric oscillator operating above threshold. We observed up to (2.30 ± 0.3) dB of squeezing in amplitude difference, inferred (4.9 ± 0.7) dB of on-chip squeezing, while an excess of noise for the sum of conjugated quadratures hinders the entanglement. The degradation of amplitude correlations and state purity for the increasing of the pump power is consistent with the observed growth of the phase noise of the fields, showing the necessary strategies for noise control aiming at entanglement generation in these systems.

Fiber-interfaced hollow-core light cage: a novel platform for on-fiber integrated waveguides

Wenqin Huang, Diana Pereira, Jun sun, Matthias Zeisberger, and Markus Schmidt

DOI: 10.1364/OL.525328 Received 01 Apr 2024; Accepted 09 May 2024; Posted 09 May 2024  View: PDF

Abstract: Here, we demonstrate the realization of hollow-core light cages on commercial step-index fibers using 3D nanoprinting, resulting in fully fiber-integrated devices. Two different light cage geometries with record-high aspect ratio strands and unique sidewise access to the core have been implemented, exhibiting excellent optical and mechanical properties. These achievements are based on the use of 3D nanoprinting to fabricate light cages and stabilize them with customized support elements. Overall, this approach results in novel fiber-interfaced hollow-core devices that combine several advantages in a lab-on-a-fiber platform that is particularly useful for diffusion-related applications in environmental sciences, nanosciences and quantum technologies.

Single-exposure Multi-wavelength Optical Diffraction Tomography based on Space-Angle Dual Multiplexing Holography

Hong-Yi Huang, Qing-Yang Yue, Yang Yang, Ruo-Xi Wang, and Cheng-Shan Guo

DOI: 10.1364/OL.519248 Received 19 Jan 2024; Accepted 09 May 2024; Posted 10 May 2024  View: PDF

Abstract: We present a space-angle dual multiplexing holographic recording system for realizing single-exposure multi-wavelength optical diffraction tomographic (ODT) imaging. This system is achieved by combining the principle of single-exposure multi-wavelength holographic imaging technique based on angle division multiplexing with the principle of single-exposure ODT imaging technique based on microlens array multi-angle illuminations and space division multiplexing. Compared with the existing multi-wavelength ODT imaging methods, it enables the holographic recording of all the diffraction tomography information of measured specimen at multiple illumination wavelengths without any scan mechanism in a single camera exposure. Using our proposed data processing method, the multi-wavelength 3D refractive index tomograms of specimen can be eventually reconstructed from the single recorded multiplexing hologram. Experimental results of polystyrene bead demonstrate the feasibility of the system.

Depth-of-field extended Fourier ptychographic microscopy without defocus distance priori

Yanqi Chen, Jinghao Xu, and An Pan

DOI: 10.1364/OL.524267 Received 19 Mar 2024; Accepted 09 May 2024; Posted 10 May 2024  View: PDF

Abstract: Fourier ptychographic microscopy (FPM) provides a solution of high-throughput phase imaging. Thanks to its coherent imaging model, FPM has the capacity of depth-of-field (DOF) extension by simultaneously recover the sample’s transmittance function and pupil aberration, which contains defocus term. However, existing phase retrieval algorithms (PRs) often struggle in the presence of significant defocus. In this letter, different PRs with embedded pupil recovery are compared, and the one based on the alternating direction multiplier method (ADMM-FPM) demonstrates promising potential for reconstructing highly defocused FPM images. Besides, we present a plug-and-play framework that integrates ADMM-FPM and total variation or Hessian denoiser for pupil function enhancement. Both simulations and experiments demonstrate that this framework enables robust reconstruction of defocused FPM images without any prior knowledge of defocus distance or sample characteristics. In experiments involving USAF 1951 targets and pathologic slides, ADMM-FPM combined with the Hessian denoiser successfully corrected defocus up to approximately 200 μm, i.e., extending the DOF to 400 μm.

Vertical directional coupling based grating emission engineering for optical phased array

Jiazhu Duan, Weiming Yao, Xiangjie Zhao, cangli Liu, Wanchang Gao, yibo chen, Qiqi Hu, dapeng li, Dayong Zhang, and Xiaochuan Xu

DOI: 10.1364/OL.524198 Received 18 Mar 2024; Accepted 08 May 2024; Posted 08 May 2024  View: PDF

Abstract: In this paper, a novel vertical directional coupling waveguide grating (VDCWG) architecture is proposed to increase the length of waveguide grating antennas for large aperture on-chip optical phased arrays (OPAs). In this new architecture, the grating emission strength is engineered by the vertical directional coupler, which provides additional degrees of design freedom. Theoretical analysis and numerical simulation show that the VDCWG can adjust the grating strength in the range of more than two orders of magnitude, corresponding to an effective grating length more than a centimeter. For proof-of-concept, a VDCWG antenna with a length of 1.5 mm is experimentally demonstrated. The grating strength is measured to be 0.17/mm, and the far-field divergence angle is 0.061°. A 16-channel OPA is also developed based on the proposed VDCWG, which proves the potential of the new architecture for large aperture OPAs.

Analytical target-agnostic piston sensing for segmented telescopes using sparse redundant baseline pairs.

li ming, weilong wei, xiafei ma, Kaiyuan Yang, ren ge, Haotong Ma, and Zongliang Xie

DOI: 10.1364/OL.523585 Received 12 Mar 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: Piston correction is the key to achieving high resolution of segmented telescopes. Phasing with extended objects is still challenging. In this letter, we propose an analytical target-agnostic phasing approach. By theoretical derivation, it is demonstrated that a piston phase matrix of non-full rank can be analytically extracted from captured images generated by sequential piston modulations of redundant subaperture. For practice use, after imposing mathematic constraints by paired sparse redundant pupil masks, the unique piston phases of all the sub-apertures can be analytically obtained. We validate this theory through simulations and experiments. It does not require iteration or additional optical paths, and is less affected by noise, thus providing a simple, fast and low-system-complexity solution for piston monitoring of the segmented telescope over the imaging period of complex scenes.

Compensation of the multipolar polarizability shift in optical lattice clocks

Artem Golovizin

DOI: 10.1364/OL.524104 Received 18 Mar 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: In neutral atom optical clocks, the higher-order atomic polarizability terms lead to the clock transition frequency shift which is motion-state dependent and nonlinear with the optical lattice depth. We propose to use an auxiliary optical lattice to compensate the influence of the E2-M1 differential polarizability or tune the associated coefficient to a favorable value. We show that by applying this method to Sr and Hg optical lattice clocks, the low or even sub-10-19 clock transition frequency uncertainty from the optical lattice becomes feasible. Finally, the proposed scheme is simple for experimental realization, compatible with the use of an enhancement cavity and can be implemented and tested in the existing setups.

Multi Gigabit X-Band transmitter for satellite communication using optical phase locked loop

Bhooma G, Sameer Ahmad Mir, SREERAJ S J, Yogesh Prasad K R, and Deepa Venkitesh

DOI: 10.1364/OL.524150 Received 21 Mar 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: Optical generation of microwave signals using photonictechniques offers benefits of frequency agility, ease offrequency scaling, and reduced hardware complexity.We demonstrate the generation and detection of QPSKmodulated with symbol rates up to 5 GBaud at carrierfrequencies of 8-12 GHz through optical heterodyningof two-phase locked lasers. The received data is demodulated through appropriate post-processing to correctfor the phase noise and IQ Imbalance. The approach isscalable to mmWave and THz communication.

Multi-order orbital angular momentum mode generators based on integrated long period fiber gratings

yuehui ma, Chen Jiang, Siyu Chen, Chengbo Mou, Kaiming Zhou, and Yunqi Liu

DOI: 10.1364/OL.520507 Received 31 Jan 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: We propose an integrated long-period fiber gratings (LPFGs) fabricated by a CO2 laser to realize a multichannel and multi-order OAM mode generator. The integrated LPFG is inscribed on multiple surfaces of the few-mode fiber by rotating the fiber in the opposite direction at an angle θ. By controlling the rotation angle, the number of integrated LPFGs can be set. The selected rotation angle is 43°, which can integrate up to 9 LPFGs, that is, realize the number of channels forfirst order OAM mode conversion is 9. The integrated LPFGs fabricated in this method allow flexible design of channel spacing. In addition, the flexible selection of the integrated grating period achieves the simultaneous generation of multi-channel second-order and third-order OAM mode conversion. The multi-channel and multi-order OAM mode generators have important application in optical communication multiplexing systems and OAM sensing.

SiPM Joint Likelihood Reception for UAV LED-Optical Communication under Pointing Errors

Dapeng Wang, Chao Wang, Yan-Yu Zhang, Junbin Fang, and Zhu Yi-Jun

DOI: 10.1364/OL.521566 Received 16 Feb 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: Unmanned aerial vehicle (UAV) light-emitting diode (LED)-optical communication is a novel of spectrum communication with wide field of view, light weight, and long-distance free-space capabilities. Due to atmospheric turbulence attenuation and pointing errors caused by long-distance communication, this letter proposes a multi-pixel channels joint maximum likelihood (JML) reception method using a highly sensitive silicon photomultiplier (SiPM). To evaluate the performance of SiPM under the UAV jittering communication, we analyze the effect of optical transmitting power, pointing errors, and signal-to-noise ratio (SNR) gain on UAV optical communication by comparing JML with signal channel using maximum likelihood (ML) algorithm. Both simulation analysis and experimental results demonstrate that the proposed JML algorithm to process signals received from SiPM multi-pixel channels can effectively mitigate the impact of pointing errors on the BER of UAV optical communications by two orders of magnitude at large jitter radians and SNR.

Strong enhancement of third harmonic generation from Tamm plasmon multilayer structure with WS2

Jianxu Zhao, Hua Lu, Jiadeng Zheng, Dikun Li, yiqiao zhang, Xuetao Gan, and Jianlin Zhao

DOI: 10.1364/OL.524772 Received 25 Mar 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: Improving the conversion efficiency is particularly important for the generation and applications of harmonic waves in optical microstructures. Herein, we propose to enhance the efficiency of third harmonic generation by integrating a monolayer WS2 with the metal/dielectric/photonic crystal multilayer structure. The numerical simulations show that the multilayer structure enables to generate Tamm plasmon mode between the metal film and photonic crystal around the telecommunication wavelength, which is consistent with the experiment result. By measuring with a self-built nonlinear optical micro-spectroscopy system, we find that the third harmonic signal can be reinforced by 16-fold with inserting the monolayer WS2 in the dielectric space. This work will provide a new way for improving nonlinear optical response, especially THG in multilayer photonic microstructures.

Excitation power-dependent multicolor upconversion in NaLnF4:Er3+ under 1532 nm irradiation for anti- counterfeiting application

wen You, Chennan Zhang, Ruoxi Yu, xiaomin Zhang, Jiacai Li, Mingxing Li, Zhili Xu, Pingping Fan, Gencai Pan, and Yanli Mao

DOI: 10.1364/OL.525417 Received 02 Apr 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: Upconversion (UC) materials are renowned for their ability to convert low-energy photons into high-energy ones. The manipulation of parameters allows for the observation of multicolored UC luminescence (UCL) within a single material system. While modulation of multicolored UCL commonly relies on excitation at approximately 980 nm, investigation into multicolored UC materials activated by a 1532 nm excitation source remains comparatively scarce. In this work, we introduce NaLnF4:Er3+ as a novel class of smart luminescent materials. When the power density of a 1532 nm laser increased from 0.5 to 20.0 W/cm2, the emission peak positions remained unchanged, but the red-to-green (R/G) ratio decreased significantly from 18.82 to 1.48, inducing a color shift from red to yellow and ultimately to green. In contrast, no color variation was observed when NaLnF4:Er3+ was excited with a 980 nm laser at different power densities. This power-dependent multicolored UCL of NaLnF4:Er3+ excited at 1532 nm can be attributed to the competitive processes of upward pumping and downward relaxation of electrons on the 4I9/2 level of Er3+. By utilizing the unique UC characteristics of NaLnF4:Er3+, its potential utility in anti-counterfeiting applications is demonstrated. Our research highlights the distinctive optical properties of NaLnF4:Er3+ and provides novel insights into the use of luminescent materials in optical anti-counterfeiting technologies.

Highly efficient silicon modulator via slow-wave Michelson structure

Jianing Wang, Xi Wang, Jian Li, Yanfu Yang, Jiangbing Du, Qinghai Song, and Ke Xu

DOI: 10.1364/OL.527292 Received 12 Apr 2024; Accepted 07 May 2024; Posted 08 May 2024  View: PDF

Abstract: The weak free carrier dispersion effect significantly hinders the adoption of silicon modulators in low-power applications. While various structures have been demonstrated to reduce the half-wave voltage, it is always challenging to balance the trade-off between modulation efficiency and the bandwidth. Here, we demonstrated a slow-wave Michelson structure with 1 mm-long active length. The modulator was designed at the emerging 2-m waveband which has stronger free carrier effect. A record high modulation efficiency of 0.29 V·cm was achieved under carrier depletion mode. The T-rail traveling wave electrodes were designed to improve the modulation bandwidth to 13.3 GHz. Upto 20 Gb/s intensity modulation was achieved at wavelength of 1976 nm.

Optimisation of Cr/Sc-based multilayer mirrors for soft x-ray water window

Evgueni Meltchakov, Blandine CAPITANIO, DE ROSSI Sébastien, Eirini Papagiannouli, Pascal Mercère, and Franck Delmotte

DOI: 10.1364/OL.523431 Received 08 Mar 2024; Accepted 07 May 2024; Posted 13 May 2024  View: PDF

Abstract: Development of efficient multilayer mirrors for the water window (a spectral region between absorption edges of carbon and oxygen, from 282 to 533 eV) remains a challenge. A best candidate, the Cr/Sc multilayer provides maximum theoretical reflectivity about 60% at near-normal incidence around the Sc L2,3 absorption edge. However, the maximum measured peak reflectance published so far just slightly exceeds 20%. We report on a new approach to design more efficient Cr/Sc-based multilayer coatings using a process of nitridation of chromium during deposition and adding boron carbide as a third material in the multilayer structure. We discuss our strategy of optimisation of the CrN/B4C/Sc multilayer system based on experimental studies. Peak reflectance as high as 32% at 396 eV was measured with this type of coating which is of main interest for various water window applications such as x-ray microscopy.

Single-frequency orbital angular momentum switchable modes from a microchip laser

Chen Li, Ziyu Hua, Qing Wang, lang li, zhichao zhang, Lan Hai, Yidong Tan, Chunqing Gao, and Shiyao Fu

DOI: 10.1364/OL.526168 Received 11 Apr 2024; Accepted 07 May 2024; Posted 13 May 2024  View: PDF

Abstract: We demonstrate the direct generation of single-frequency switchable orbital angular momentum (OAM) modes in 1 μm wavelength range using a Nd: YVO4 microchip laser. The 808 nm laser diode pump beam is shaped into annular through an axicon associated with a lens. By adjusting the diameter and power of the annular pump beam, various OAM modes with different mode volumes can oscillate inside the Nd: YVO4 microchip. Moreover, single-frequency output is also available due to the short cavity of microchip. In the proof-of-principle experiment, single frequency two-fold multiplexed OAM modes |±1> and |±2> are generated, with experimentally measured fidelity 97% and 95%, respectively. This work presents a compact and versatile single frequency OAM source and will inspire multiple advanced scenarios ranging from classical to quantum photonics.

Optical properties of fs laser-irradiated spheres inside CYTOP fiber by Mueller polarimetry

Ruyue QUE, Enrique Garcia-Caurel, Kyriacos Kalli, Robert Pansu, Jean-Frédérique Audibert, Matthieu Lancry, and Bertrand Poumellec

DOI: 10.1364/OL.519955 Received 19 Feb 2024; Accepted 07 May 2024; Posted 14 May 2024  View: PDF

Abstract: Optical elements embedded in an optical fiber can be used to shape and modulate the light transmitted within. We consistently observe via Mueller polarimetry that the optical properties of a femtosecond (fs) laser-created spherical cavity within the core of a perfluorinated fiber, exhibit predictable patterns. Specifically, linear birefringence is always induced at the periphery of the cavity, with its value showing a bell-shape distribution and LB (linear birefringence) component of the Mueller matrix in cylindrical coordinate peaking at approximately +0.065 radians. The peak value of LB showed an increase correlating with the laser fluence and power, but the full width at half maximum (FWHM) of the Gaussian distribution remains unchanged. Furthermore, it is important to highlight that when the cavity is disrupted, forming a channel to the fiber’s surface, a significant, yet negative, LB is observed at the cavity’s periphery, with value reaching up to -0.4 radians. These optical phenomena may pique the interest of engineering and technical fields, potentially inspiring innovative approaches in optical fiber technology and its associated applications.

Rotating twisted templates for imprinting polarization gratings with sub- to dozens-micron period

Xiangyu Jiang, xianglin ye, Kuangdi Xue, Yingjie Zhou, Fan Fan, and Shuangchun Wen

DOI: 10.1364/OL.528047 Received 24 Apr 2024; Accepted 06 May 2024; Posted 08 May 2024  View: PDF

Abstract: In this Letter, we report and experimentally demonstrate a novel scheme for imprinting polarization gratings (PGs) with a pair of templates. Compared with the traditional method that single template can only imprint PG with a single period, cascading two templates can control the period of imprinted PG at will. However, the low diffraction efficiency is inevitably caused by cascading two templates. Therefore, rigorous coupled wave analysis (RCWA) is adopted to design a multi-twisted template to address this challenge. As a proof of concept, two multi-twisted templates with a period of 1.6 μm were fabricated, and PGs with a large period range from 0.4 to 48.6 μm were successfully imprinted. The proposed scheme is expected to enable rapid, robust, and high-quality mass production of beam steering, large-angle deflectors, and diffractive optical couplers.

Mask structure optimization for beyond EUV lithography

Ziqi Li, Lisong Dong, Xu Ma, and Yayi Wei

DOI: 10.1364/OL.523596 Received 12 Mar 2024; Accepted 06 May 2024; Posted 09 May 2024  View: PDF

Abstract: Beyond extreme ultraviolet (BEUV) lithography with 6.x nm wavelength is regarded as a future technique to continue the pattern shirking in integrated circuit (IC) manufacturing. This work proposes an optimization method for the mask structure to improve the imaging quality of BEUV lithography. Firstly, the structure of mask multilayers is optimized to maximize its reflection coefficient. Then, a mask diffraction near-field (DNF) model is established based on the Born series algorithm, and the aerial image of BEUV lithography system can be further calculated. Additionally, the mask absorber structure is inversely designed using the particle swarm optimization (PSO) algorithm. Simulation results show a significant improvement of the BEUV lithography imaging obtained by the proposed optimization methods.

Engineering dynamical photon blockade with Liouville exceptional points

Zhuang Geng, Yongjian Chen, Yongyuan Jiang, Yan Xia, and Jie Song

DOI: 10.1364/OL.523210 Received 06 Mar 2024; Accepted 06 May 2024; Posted 08 May 2024  View: PDF

Abstract: We investigate the dynamical blockade in a nonlinear cavity and demonstrate the connection between the correlation function g$^{(2)}$(t) and system parameters in the entire nonlinear region. Utilizing the Liouville exceptional points (LEP$_s$) and quantum dynamics, near-perfect single-photon blockade (1PB) can be achieved. By fine-tuning system parameters to approach the second-order LEP (LEP$_{2}$), we improved single-photon statistics in both weak and strong nonlinearity regimes, including a significant reduction of g$^{(2)}$(t) and a pronounced increase in the single-photon occupation number. In the strong nonlinearity region, the maximum photon population may correspond to stronger antibunching effect. Simultaneously, time window and period of blockade can be controlled by selecting detuning based on the LEP$_{2}$. Furthermore, the 1PB exhibits robustness against parameter fluctuations, and this feature can be generalized to systems for implementing single-photon sources with nonharmonic energy levels.

Establishing equivalent circuits of mounted, high-power VCSEL arrays for iToF cameras

Kangning Liu, Yubing Wang, Jian Zhang, Yuqing Chen, Mingshi Zhang, Xing Zhang, Qin li, Yongqiang Ning, and Lijun Wang

DOI: 10.1364/OL.526010 Received 10 Apr 2024; Accepted 06 May 2024; Posted 06 May 2024  View: PDF

Abstract: Solid-state indirect time-of-flight (iToF) cameras are crucial to numerous short-to-medium range applications, owing to their advantages in terms of system integrability and long-term reliability. However, due to the low light intensity, the sensing range of iToF cameras is generally limited to few meters, which hinders their wide applications. Further increasing the sensing range requires not only higher-power laser diodes but also well-designed driver circuits, which are based on prior knowledge of the laser diodes’ equivalent circuits (ECs). However, experimental studies on ECs of mounted, high-power vertical-cavity surface-emitting laser (VCSEL) array that comprehensive incorporates all parasitic, especially parasitic stemming from printed-circuit boards, remain absent. In this letter, 850-nm VCSEL array with 15.3 W peak power and 581 MHz bandwidth are fabricated, and more importantly, their ECs are experimentally established. Leveraging the accurate ECs, a compact iToF camera with sensing range up to 11.50 m is designed. In addition, a modified precision model is proposed to better evaluate the iToF camera’s performance.

Phase-Shaping of Dual-Pumped Kerr Frequency Combs in Bichromatic Brillouin Lasers

Anastasiia Sheveleva, Moise Deroh, Bertrand Kibler, Christophe Finot, and Erwan Lucas

DOI: 10.1364/OL.522008 Received 20 Feb 2024; Accepted 05 May 2024; Posted 06 May 2024  View: PDF

Abstract: We investigate the spectral phase characteristics of dual-pumped Kerr frequency combs generated in a bichromatic Brillouin fiber laser architecture with normal dispersion, producing square-like pulse profiles. Using a pulse shaper, we measured the relative phase between the pump Stokes and adjacent lines, revealing a symmetric phase relationship. Our results highlight the absence of the abrupt phase difference typically observed with single continuous-wave pumping. By manipulating spectral amplitudes and phases, we demonstrate the transformation into various optical waveforms. The stability of our low-noise frequency comb ensures reliable performance in practical settings

3D Printing of Fresnel Lens on the Facet of Single Mode Fiber for Trapping, Manipulation and Spectrum Measurement

yirui wang, Minglong Li, Haodong Zhu, qiuhong min, Yuanhao Lou, dan wu, jian ma, Zhen Yu Yang, Ming Zhao, and Yuanjie Pang

DOI: 10.1364/OL.524889 Received 27 Mar 2024; Accepted 05 May 2024; Posted 06 May 2024  View: PDF

Abstract: Fiber optical tweezer (FOT) provides a functionality for micro-/nanoparticle manipulation with a slim and flexible optical fiber setup. An added in-situ spectroscopic functionality can achieve characterization of the trapped particle, potentially useful for endoscopic, in-vivo studies in an inherently heterogeneous environment if the applicator end is all-fiber-built. Here, we demonstrate an all-fiber optical tweezers (a-FOT) for the trapping and in-situ spectral measurement of a single, cell-sized microparticle. The key to ensure the simultaneous bifunctionality is a high numerical aperture (NA) Fresnel lens fabricated by two-photon direct laser writing (DLW) corrected by grid-correction methods. We demonstrate trapping and time-resolved, in-situ spectroscopy of a single upconversion particle (UCP), a common fluorescent bio-marker in biophotonics. The system achieves a 0.5-s time resolution in the in-situ spectral measurement of a trapped UCP. The all-fiber designed system preserves the advantages of flexibility and robustness of fiber, potentially useful for in-vivo biomedical studies such as cell-to-cell interactions, pH and temperature detection and nucleic acids detection.

Time-domain Vernier-effect based optical fiber sensor

Han Chunyang, Kechuang Han, and song mengzhen

DOI: 10.1364/OL.527913 Received 24 Apr 2024; Accepted 05 May 2024; Posted 06 May 2024  View: PDF

Abstract: In this letter, we demonstrate an easy-to-fabricate time-domain Vernier-effect based sensor. An all-fiber variable optical delay line is utilized to drive OPD scan of two interferometers simultaneously, fiber Bragg gratings are used to filter out two slightly detuned time-domain interferometric signals. Then two normalized interferograms with different spatial frequencies can be achieved and utilized to generate an envelope modulation, viz., Vernier envelope, with enhanced sensitivity in comparison to the native state of the interferometers used. The sensitivity magnification factor of our structure can be regulated simply via altering the resonant wavelength difference of FBGs rather than optimizing the OPDs of the interferometers. The proposed sensor is independent of the precise and complicated fabrication procedures, the Vernier signal can be demodulated without broadband light source and spectrometer. We argue that the proposed structure may inspire a new concept for constructing simple and cheap Vernier-effect based sensors that is well suited for practical applications.

GHz-rate 57-fs optoacoustic mode-locking fiber laser based on cascaded all-fiber pulse compression

Xintong Zhang, Wenbin He, Xiaocong wang, Benhai Wang, Qi Huang, yu zheng, Ruochen Yin, Zhi Yuan Huang, Jin Xie, Kaihui Liu, Xin Jiang, Lixin Xu, Yuxin Leng, and Meng Pang

DOI: 10.1364/OL.520119 Received 30 Jan 2024; Accepted 05 May 2024; Posted 08 May 2024  View: PDF

Abstract: We demonstrate a compact ultrafast fiber laser system that can deliver 1.87 GHz pulse train at 1550 nm with a pulse energy of 54 pJ and an ultrashort pulse duration of 57 fs. While an optoacoustic mode-locking soliton fiber laser was used as the seed light source at GHz rate, a stage of Er-doped fiber amplifier boosted the laser average power to ~320 mW, corresponding to a pulse energy of ~170 pJ. Then, an all-fiber pulse compression set-up was constructed, providing a high compression ratio of ~10 with a total transmission efficiency of ~32%. In the cascaded pulse compression configuration, multiple fiber samples with alternately normal and anomalous dispersion were fused together, providing efficient nonlinear spectral broadening whereas suppressing excessive pulse temporal broadening over propagation. This GHz-rate ultrafast fiber laser, with compact configuration, broadband spectral coverage and high time-resolving ability, could be used as the seed light source for constructing high-repetition-rate, high-average-power ultrafast laser system and may also find a few applications in optical measurements and microwave photonics.

Few-cycle, mJ-level, mid-wave infrared pulses generated via post compression of chirped pulse amplifier

Alphonse Marra, Yi Wu, Nicholas Belden, and Zenghu Chang

DOI: 10.1364/OL.524212 Received 29 Mar 2024; Accepted 05 May 2024; Posted 08 May 2024  View: PDF

Abstract: Few-cycle pulses were generated by passing a beam from a cryogenically cooled Fe:ZnSe chirped pulse amplifier at a repetition rate of 400 Hz through a gas-filled hollow core fiber followed by dispersion compensating bulk CaF2. The krypton-filled fiber at 53 psi yielded 1.14-mJ, 42-fs pulses centered at 4.07-µm, while the oxygen-filled fiber at 45 psi delivered 0.78-mJ, 39-fs pulses spanning 3 to 5.5 µm. This work is a step in the direction toward a high repetition rate mid-wave infrared driver of isolated attosecond keV X-ray pulses.

A fast image-free autofocus method for passive Fourier single-pixel imaging microscopy

Mingyang Ni, Yu Cai, Yihao Xue, Huaxia Deng, and xinglong gong

DOI: 10.1364/OL.516755 Received 21 Dec 2023; Accepted 05 May 2024; Posted 06 May 2024  View: PDF

Abstract: Autofocus is crucial for capturing sharp images with imaging devices for information acquisition. Traditional autofocus strategies based on post-processing become less efficient for Passive Fourier single-pixel imaging microscopy of yet low temporal resolution. In this paper, a fast and image-free autofocus system is proposed for passive Fourier single-pixel imaging microscopy. Based on complementary design of optical path , the system can measure the focus degree at 5000 fps while maintaining a high light efficiency for imaging. The proposed system can be easily combined with existing trinocular microscopes, which provides a welcomed boost to the practicability of passive Fourier single-pixel imaging microscopy.

Deep-UV 2 .8 nm laser by six-harmonic generation of cavity dumped laser source

jinyan wang, Zheng Quan, Xiaohua Wang, Xi Chen, Yi Yao, li shijie, and Li Qi

DOI: 10.1364/OL.523452 Received 11 Mar 2024; Accepted 04 May 2024; Posted 06 May 2024  View: PDF

Abstract: We presented the first demonstration of an ultra-violet laser at 2 .8 nm by six-harmonic generation of an electro-optic Q-switched cavity dumping 1342nm Nd: YVO4 laser. It offers high power, constant short pulse duration, and adjustable pulse repetition rate. The pulse duration is independent of pump power and repetition rate compared to classical Q-switched oscillators. The output efficiency of the UV laser is optimized by adjusting the focusing lens. With the incident pump power of 30W, the maximum average output power of 249 mW was obtained at 13 kHz. The pulse width remained 3.4–3.5 ns from 5 to 20 kHz. The maximum pulse energy of 28.1 μJ was obtained at 5 kHz and the corresponding peak power was up to 8.1 kW.

Multi-wavelength second-harmonic generation in double-clad high nonlinear fiber induced by multiple phase-matching

XIAOYU CHEN, Xue Zhou, xin yan, Xuenan Zhang, Takenobu Suzuki, Yasutake Ohishi, and Tonglei Cheng

DOI: 10.1364/OL.525099 Received 04 Apr 2024; Accepted 04 May 2024; Posted 08 May 2024  View: PDF

Abstract: In this study, multi-wavelength second harmonic generation (SHG) based on self-phase modulation (SPM) broadband supercontinuum (SC) was observed by employing a double-clad high nonlinear optical fiber (HNLF) in conjunction with a femtosecond laser. At a wavelength of 1050 nm and an average pump power of 320 mW, multiple phase-matching conditions were achieved and SH signals of produced central wavelengths at ~530.7 nm, ~525.1 nm, ~503.5 nm, and ~478.7 nm were observed, with the SHG efficiency reaching ~1.34×10-4. The SHG in this experiment can be attributed to the utilization of doped optical fiber. Dopants introduced within the optical fiber created defect states, facilitating optical-chemical transformations induced by the high optical power of femtosecond laser. Specifically, the core of this optical fiber was doped with germanium (Ge), while the cladding was doped with fluorine (F). This combination of dopants ultimately resulted in an enhanced second-order polarization susceptibility. Additionally, theoretical simulations were conducted, aligning closely with the experimental findings. To the best of our knowledge, this work marks the first demonstration of multi-wavelength SHG in optical fibers and offers a distinctive avenue for customizing multi-wavelength ultrafast light sources, with great potential applications in the fields of ultrafast laser sources, medical diagnostics, and optical sensing fields.

Speckle-free coherent imaging through deep turbulence

Casey Pellizzari, Timothy Bate, Maya Mandyam, Cameron Radosevich, Samuel Horst, and Mark Spencer

DOI: 10.1364/OL.520540 Received 01 Feb 2024; Accepted 04 May 2024; Posted 06 May 2024  View: PDF

Abstract: We develop and validate a model-based iterative reconstructionframework for digitally correcting coherentimages corrupted by deep turbulence. In general,this framework is applicable to coherent-imaging approachesthat gain access to the complex-optical field;however, we demonstrate our approach with multishotdigital holography data. To test our image correctionframework, we generate calibrated deep-turbulenceconditions from our laboratory testbed. Using the resultingdata, we demonstrate ground-breaking performancein terms of speckle-free image correction in deepturbulenceconditions.

High-peak-power long-wave infrared BaGa4Se7 optical parametric oscillator with 6.7-13.9 μm widely tunable range

Yu Liu, Zhiyong Li, Jiyong Yao, Hai Wang, Zi Zhu, Jinzhou Bai, Jing FU, Yue Lu, yijun zheng, Rongqing Tan, Ke ChangJun, and HONGRUI YUAN

DOI: 10.1364/OL.522496 Received 28 Feb 2024; Accepted 04 May 2024; Posted 15 May 2024  View: PDF

Abstract: A high-peak-power, widely tunable range long-wave infrared optical parametric oscillator (OPO) based on the BaGa4Se7 (BGSe) crystal is demonstrated in this Letter. Pumped by a 1064 nm Nd: YAG laser, a high-peak-power of 0.15 MW was achieved at 9.8 μm with a pulse width of 5.0 ns. At 11.0 μm, a high beam quality of M²x = 4.1 and M²y = 3.3 was achieved. By rotating the BGSe crystal, a broad tuning range of 6.7-13.9 μm was realized. Furthermore, theoretical analysis was conducted to elucidate the reasons behind the improvement in beam quality in the x-direction as the wavelength of the idler light increases.

Effect of pre-plasma on terahertz radiation from laser plasma filaments in a collinear geometry

LiLi Sun, Yanping Chen, Zhelin Zhang, Tianhao Xia, Linzheng Wang, Chen Wang, Liming Chen, and Zheng-Ming Sheng

DOI: 10.1364/OL.524884 Received 27 Mar 2024; Accepted 02 May 2024; Posted 06 May 2024  View: PDF

Abstract: Terahertz (THz) radiation from air plasma in the presence of pre-plasma in a collinear geometry is investigated experimentally, where the pre-plasma is formed by the pre-pulse with a Gaussian beam profile and the measured THz radiation is driven by a main laser pulse. The pre-plasma has a de-focusing effect for the main pulse passing through it, which reduces the effective length of the plasma filament formed by the main laser pulse for THz radiation. It is found that only the part not overlapped by the pre-plasma can actually produce THz radiation. Thus, the amplitude of the THz pulse driven by the main pulse can be modified by changing the spatial separation between two plasma filaments. The experimental observations are qualitatively in agreement with our numerical simulation results. It is also found that the change of the time delay between the pre-pulse and the main pulse does not change the THz radiation amplitude for a given spatial separation. This study suggests a practical way for the manipulation of THz waves through an interaction between laser plasma filaments.

Ultra-slow-light and dynamically quantitative optical storage modulation via quasi-BICs

Dongwen Zeng, Zeyu Wu, Guiqiang Liu, Mengyu Yu, Xiaoshan Liu, Jing Chen, Chaojun Tang, Wei Du, and Zhengqi Liu

DOI: 10.1364/OL.524580 Received 25 Mar 2024; Accepted 02 May 2024; Posted 03 May 2024  View: PDF

Abstract: We achieve dynamically tunable dual quasi-bound states in the continuum (quasi-BICs) by implementing them in a silicon-graphene multilayer composite structure and utilize the quasi-BIC modes to achieve ultra-large group delays (velocity of light slows down 105 times), showing 2-3 orders of magnitude higher than the group delays of previous electromagnetically induced transparency modes. The double-layer graphene holds great tuning capability and leads to the dramatically reduced group delay from 1929.82 ps to 1.58 ps with only 100 meV. In addition, the log-linear variation rule of group delay with Fermi level (Ef) in the range of 0-10 meV is analyzed in detail, and the double logarithmic function relationship between group delay and quality factor (Q-factor) is theoretically verified. Finally, the quantitative modulation of optical storage is further realized in this basis. Our research provides ideas for the reform and upgrading of slow optical devices.

Determination of Plasma Properties in Liquid Jets Through Time-Resolved Experiments on Third Harmonic Reflection Dynamics

Shireen Hilal, Maksim Melnik, Azat Ismagilov, Anton Tsypkin, and Sergey Kozlov

DOI: 10.1364/OL.524582 Received 22 Mar 2024; Accepted 02 May 2024; Posted 03 May 2024  View: PDF

Abstract: The study of plasma in liquid jets represents a significant area of research encompassing plasma science, dynamics, and properties. This paper presents experimental studies on plasma formation processes in liquid jets of water, ethanol, and isopropyl based on the dynamics of the third harmonic reflection from the induced plasma. Through time-resolved experiments, and theoretical estimations using Keldysh theory, plasma properties including density, and frequency for all three media are evaluated. The isopropyl demonstrates the highest values of the characteristics mentioned. These findings hold significant potential for advancing our understanding of plasma-based radiation sources e.g. terahertz generation.

Single-etched fiber-chip coupler with a metal mirror on 220-nm SOI platform for perfectly vertical coupling

Lihang Wang, Jifang Qiu, Zhenli Dong, YuChen Chen, Lan Wu, Hongxiang Guo, and Jian Wu

DOI: 10.1364/OL.524717 Received 25 Mar 2024; Accepted 02 May 2024; Posted 03 May 2024  View: PDF

Abstract: Vertical couplers play a pivotal role as essential components supporting interconnections between fibers and photonic integrated circuits. In this study, we propose and demonstrate a high-performance perfectly vertical coupler based on a three-stage inverse design method, realized through a single full etching process on a 220-nm silicon-on-insulator platform with a backside metal mirror. Under surface-normal fiber placement, experimental results indicate a remarkable 3-dB bandwidth of 99 nm with a peak coupling efficiency of -1.44 dB at the wavelength of 1549 nm. To the best of our knowledge, this achievement represents the best record to date for a perfectly vertical coupler fabricated under similar process conditions.

Sub2Full: split spectrum to boost OCT despeckling without clean data

Lingyun Wang, JOSÉ-ALAIN SAHEL, and Shaohua Pi

DOI: 10.1364/OL.518906 Received 19 Jan 2024; Accepted 02 May 2024; Posted 02 May 2024  View: PDF

Abstract: Optical coherence tomography (OCT) suffers from speckle noise, causing the deterioration of image quality, especially in high-resolution modalities like visible light OCT (vis-OCT). The potential of conventional supervised deep learning denoising methods is limited by the difficulty of obtaining clean data. Here, we proposed an innovative self-supervised strategy called Sub2Full (S2F) for OCT despeckling without clean data. This approach works by acquiring two repeated B-scans, splitting the spectrum of the first repeat as a low-resolution input, and utilizing the full spectrum of the second repeat as the high-resolution target. The proposed method was validated on vis-OCT retinal images visualizing sublaminar structures in outer retina and demonstrated superior performance over conventional Noise2Noise and Noise2Void schemes. The code is available at https://github.com/PittOCT/Sub2Full-OCT-Denoising.

The Hermite-Gaussian-Talbot-Carpets

Abdelhalim Bencheikh and Dongmei Deng

DOI: 10.1364/OL.521442 Received 20 Feb 2024; Accepted 01 May 2024; Posted 02 May 2024  View: PDF

Abstract: In this paper, we demonstrate the generation of theHermite–Gaussian-Talbot-Carpets (HGTC) based on theinterference of a Hermite-Gaussian beam array withconstant successive separation (shift). Even-though HGbeams accelerate during propagation, but due to its sym-metric structure (profile), the self-imaged carpets aregenerated at straight lines perpendicular to the propa-gation direction, at particular distances, multiple of thefamous Talbot distance z_T . Due to the symmetric struc-ture of the HG beams unit cell, the calculated distanceis a half of the usual known Talbot distance. The samecarpets are also observed in planes situated at differentfractions of z_T , but with different frequency apparitions.An interesting feature of these carpets, is that the dimen-sion of one cell of the beam array is constant in eachperiod (period fraction). We believe that such novel Car-pets will be useful in photonics for creating lattices andoptical potentials.

Ultra-low threshold deep-ultraviolet generation in hollow-core fiber

Mohammed Sabbah, Kerrianne Harrington, Leah Murphy, Christian Brahms, Stephanos Yerolatsitis, James Stone, Tim Birks, and John Travers

DOI: 10.1364/OL.523673 Received 14 Mar 2024; Accepted 01 May 2024; Posted 02 May 2024  View: PDF

Abstract: Tunable ultrashort pulses in the ultraviolet spectral regionare in great demand for a wide range of applications, including spectroscopy and pump-probe experiments. While laser sources capable of producing such pulses exist, they are typically very complex. Notably, resonant dispersive-wave (RDW) emission has emerged as a simple technique for generating such pulses. However, the required pulse energy used to drive the RDW emission, so far, is mostly at the microjoule level, requiring complicated and expensive pump sources. Here, we present our work in lowering the pump energy threshold for generating tuneable deep-ultraviolet pulses to the level of tens of nanojoules. We fabricated a record small-core anti-resonant fiber with a hollow core-diameter of just 6 μm . When filled with argon, the smallmode area enables higher-order soliton propagation and deep-ultraviolet (220 nm to 270 nm) RDW emission from 36 fs pump pulses at 515 nm with the lowest pump energy reported to date. This approach will allow the use of low-cost and compact laser oscillators to drive nonlinear optics in gas-filled fibers for the first time.

Quantum beam splitter based on free charged particles

Dmitry Makarov and Ksenya Makarova

DOI: 10.1364/OL.525697 Received 05 Apr 2024; Accepted 01 May 2024; Posted 01 May 2024  View: PDF

Abstract: It is well known that the beam splitter is an integral part of many classical and quantum devices. The use of beam splitters in quantum technologies is currently particularly relevant. The emergence of new types of beam splitters provides new statistical characteristics of the separated photon beam, their control and new possibilities for use in various devices. This article presents a new type of beam splitter based on free charged particles. This type of beam splitter has all the properties of a linear beam splitter with its reflection coefficient R, transmission coefficient T, and phase shift, which are presented in a simple analytical form. This type of beam splitter has interesting application prospects.

Temporal compressive edge imaging enabled by a lensless diffuser camera

Ze Zheng, Bao-Lei Liu, Jiaqi Song, Lei Ding, Xiaolan ZHONG, Lingqian Chang, Xiaojun Wu, David McGloin, and Fan Wang

DOI: 10.1364/OL.515429 Received 21 Dec 2023; Accepted 01 May 2024; Posted 02 May 2024  View: PDF

Abstract: Lensless imagers based on diffusers or encoding masks enable high-dimensional imaging from a single shot measurement and have been applied in various applications. However, to further extract image information such as edge detection, conventional post-processing filtering operations are needed after the reconstruction of the original object images in diffuser imaging systems. Here, we present the concept of a temporal compressive edge detection method based on a lensless diffuser camera, which can directly recover a time sequence of edge images of a moving object from a single-shot measurement without further post-processing steps. Our approach provides higher image quality during edge detection, compared with the conventional post-processing method. We demonstrate the effectiveness of this approach by both numerical simulation and experiments. The proof-of-concept approach can be further developed with other image post-process operations or versatile computer vision assignments toward task-oriented intelligent lensless imaging systems.

Spectroscopy and efficient dual-wavelength laser performances of a Nd:GYSAG crystal

Huang Conghui, WenFang Lin, Qiannan Fang, Min Xu, shulong zhang, Siliang Tao, Shanming Li, Chengchun Zhao, and Yin Hang

DOI: 10.1364/OL.525380 Received 02 Apr 2024; Accepted 01 May 2024; Posted 02 May 2024  View: PDF

Abstract: We reported on the spectral properties and dual-wavelength laser performances of a novel, to the best of our knowledge, Nd:Gd1.8Y1.2ScAl4O12 (Nd:GYSAG) crystal for the first time. The absorption spectra, emission spectra, and fluorescence lifetime were systematically investigated. Further, a continuous-wavelength (CW) laser output power up to 5.02 W was obtained under an absorbed pump power of 9.45 W with slope and optical-to-optical efficiencies of 59.4% and 53.1%, respectively, at 1061.2 nm and 1063.2 nm. A stable passively Q-switched (PQS) laser employing Cr:YAG as a saturable absorber (SA) was realized. The maximum average output power of 0.756 W with slope of near 34.4% was obtained with the pulse width, pulse energy, and peak power of 14.0 ns, 128.1 µJ, and 9.15 kW, respectively. The results indicate that the Nd:GYSAG crystal is an excellent laser medium for generating high-efficiency dual-wavelength laser and has a potential in terahertz (THz) laser generation.

Monolithic tunable dual-wavelength laser utilizing erbium-doped lithium niobate on insulator

minglu cai, Xujia Zhang, Tianyi Li, Shi Hao, Tieying Li, Hao Li, Yuanlin Zheng, Xianfeng Chen, Jianping Chen, and Kan Wu

DOI: 10.1364/OL.522774 Received 01 Mar 2024; Accepted 30 Apr 2024; Posted 01 May 2024  View: PDF

Abstract: We demonstrate a monolithic tunable dual-wavelength laser fabricated on erbium-doped lithium niobate on an insulator (Er: LNOI). The dual-wavelength laser enables independent tuning, with a continuously linear electro-optic (EO)-modulated tuning range of 11.875 GHz at a tuning efficiency of 0.63 pm/V. Tunable microwave generation within 50 GHz with a maximum side-mode suppression ratio of 35 dB is experimentally demonstrated by further exploring the charge accumulation effect in LNOI. The monolithic design of this work paves the way for microscale integration of laser devices, presenting significant prospects in photonics research and applications.

Dual-wavelength synchronously mode-locked Cr:LiSAF laser with tunable beating frequency and central wavelength

Zekican Erturk, MUHARREM KILINC, Serdar Okuyucu, Yusuf OZTURK, Mikhail Pergament, Franz Kaertner, and Umit Demirbas

DOI: 10.1364/OL.523994 Received 18 Mar 2024; Accepted 30 Apr 2024; Posted 02 May 2024  View: PDF

Abstract: We demonstrate versatile dual-wavelength synchronous mode-locking of a diode-pumped Cr:LiSAF laser for the first time. Two-color mode-locked operation is achieved by using intracavity birefringent filters (BRFs) or etalons as frequency-selective elements. Using filters with different thicknesses and hence different free spectral ranges (FSRs), wavelength separation in two-color mode-locking could be adjusted between 1 and 9 nm, with corresponding beating frequencies in the 0.4-3.5 THz range. Moreover, the central wavelength of the two-color output could be tuned smoothly between 840 nm and 875 nm, only limited by the bandwidth of the semiconductor saturable absorber mirror (SESAM) used for mode-locking. The method, which enables easy adjustment of central wavelength and beating frequency of dual-wavelength operation, is suitable for use in other laser gain media as well.

Robust, wide-range, and precise transmitter IQ impairments monitoring scheme for coherent DSCM systems

Linsheng Fan, Yucheng Jia, Muqi Liu, Jiexing Lin, Qun Zhang, siyu gong, Yongchao Jin, Pengxi Yang, chen cheng, and Yanfu Yang

DOI: 10.1364/OL.523049 Received 05 Mar 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: In this letter, we present a robust, wide-range, and precise monitoring scheme for transmitter impairments in coherent digital subcarrier multiplexing (DSCM) systems. The proposed scheme employs frequency-domain pilot tones to compensate for frequency offset, polarization aliasing, and carrier phase noise, thus isolating transmitter impairments from channel distortions. It then implements widely linear filter to compensate for transmitter impairments by equalizing symmetric subcarriers. Transmitter impairments monitoring are derived from the equalizer coefficients. By mitigating the interference from constant phase shifts in advance, wide-range and precise monitoring of transmitter impairments including IQ skew, IQ phase and gain imbalances is achieved. We experimentally validated our approach using a 48 GBaud, four-subcarrier, dual-polarization coherent DSCM system. The results confirm the method's capability for wide-range, robust, and precise transmitter impairments monitoring in coherent DSCM systems, maintaining performance even in presence of phase noise and rapid polarization variation.

Design of optically transparent and broadband absorber based on multi-objective optimization algorithm

Yulin Zhao, Jia-Hui Fu, Qunhao Zhang, Hao Feng, Wei Wei, Wan Chen, Kuang Zhang, and Qun Wu

DOI: 10.1364/OL.524371 Received 29 Mar 2024; Accepted 29 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: In this letter, an optically transparent and broadband absorber designed using a multi-objective genetic algorithm (MOGA) is proposed. The absorption of the multilayer lossy frequency selective surface based absorber is calculated by multilayer absorption equations and equivalent circuit models. To solve the problem of the unbalanced structure absorption bandwidth and thickness, an algorithm is used for optimizing the geometric and sheet resistance parameters of the structure. A multilayer and optically transparent absorber with 90% absorption bandwidth covering a frequency range of 2-18 GHz (S-band to Ku-band) is developed based on the MOGA design method with optical transmittance of 60%. Its total thickness consists of a wavelength of only 0.095 and it has high oblique incidence stability, which makes it useful in the stealth technology and transparent electromagnetic shielding applications.

High sensitivity humidity sensor based on the U-shaped tapered no-core fiber coated with MXene

Jiayi Cong, Mengxin Yang, Daozi Zhou, Lei Meng, Shuai Feng, and Min Lv

DOI: 10.1364/OL.524913 Received 27 Mar 2024; Accepted 29 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: Ti3C2Tx MXene is an emerging two-dimensional material, which has good potential in relative humidity (RH) measurement because of its unique layer structure, strong hydrophilic nature and large specific surface ar-ea. Here, a high-performance RH sensor integrating Ti3C2TX MXene nanosheets and U-shaped tapered no-core fiber (UTNCF) is proposed. The sensing principle is based on mode interference. The change of ambient RH leads to the change of the refractive index (RI) of Ti3C2Tx MXene, which eventually leads to the shift of the trans-mission spectrum of the sensing probe. The average sensitivity is 1.11 nm/%RH in the RH range of 45% to 80% and the response time is 25 ms. The proposed mi-cro-nano fiber RH sensor has the advantages of high sensitivity, fast response, good repeatability and stabil-ity. In addition, the proposed sensor has a broad appli-cation prospect in human respiratory monitoring, in-dustrial and agricultural production and environmental monitoring.

Improved performance of fiber optic hydrogen sensor based on controllable optical heating technology

Jixiang Dai, Kai Yin, Zhangning Chen, Wenbin Hu, Minghong Yang, Jinghua Fu, Xuxu Sun, and Xianfeng Chen

DOI: 10.1364/OL.522375 Received 26 Feb 2024; Accepted 29 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: A novel and compact fiber optic hydrogen sensor based on light intensity demodulation and controllable optical heating technology is proposed and experimentally investigated. This system employs three photodetectors for optic signal transformation. The first PD is used to receive a little fraction of amplified spontaneous emission (ASE) for calibration, and the second PD is utilized to detect optic signal reflected by single mode fiber deposited with WO3-Pd2Pt-Pt composite film. The last PD is utilized to receive optical power reflected by short fiber Bragg grating (SFBG) with central wavelength located in steep wavelength range (the intensity decreases approximately linearly with increase of wavelength) of ASE light source. A 980 nm laser and proportion integration differentiation (PID) controller were employed to ensure the hydrogen sensitive film working at operating temperature of 60°C. This sensing system can display a quick response time of 0.4 s towards 10000 ppm hydrogen in air. In addition, detection limit of 5 ppm in air can be achieved with this sensing system. The stability of this sensor can be greatly enhanced with controllable optical heating system, which can greatly promote its potential application in various fields.

Spatial and Mode Selective Switch for Orbital Angular Momentum Mode Division Multiplexing

Weihang Zhong, Zhongzheng Lin, Lixun Wu, Zituo Wu, HONGJIA CHEN, Yujie Chen, and Siyuan Yu

DOI: 10.1364/OL.515916 Received 14 Dec 2023; Accepted 29 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: In analogy to wavelength selective switch in wavelength division multiplexed (WDM) optical fiber communications systems, a spatial and optical mode selective switch (SMSS) would be an important component in future ultra-high capacity optical fiber communication systems based on space and mode division multiplexing. In this work, an orbital angular momentum (OAM) mode division multiplexing (MDM) device is proposed and experimentally demonstrated. The SMSS consists of a separating part for transforming OAM modes to spatial modes and a recombining part for selecting and recombining the modes to any spatial channel. The SMSS is able to implement strictly non-blocking switching between a total of 36 SDM / MDM channels configured as 4 spatial channels each supporting 9 OAM mode channels.

Enhancement of silicon vacancy fluorescence intensity in SiC using dielectric cavity

Qi-Cheng Hu, Ji Xu, Qin-Yue Luo, Hai-Bo Hu, Pei-Jie Guo, Cheng-Ying Liu, Shuang Zhao, Yu Zhou, and Junfeng Wang

DOI: 10.1364/OL.522770 Received 04 Mar 2024; Accepted 29 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Over past decades, spin qubits in silicon carbide (SiC)have emerged as promising platforms for a wide rangeof quantum technologies. The fluorescence intensityholds significant importance in the performance of quantum photonics, quantum information process, and sensitivity of quantum sensing. In this work, a dual-layerAu/SiO2 dielectric cavity is employed to enhance thefluorescence intensity of the shallow silicon vacancy in4H-SiC. Experimental results demonstrate a significant5-fold augmentation in fluorescence count, corroborating our theoretical predictions. Based on this, we further investigate the influence of dielectric cavities onthe contrast and linewidth of optically detected magnetic resonance (ODMR). There is a 2-fold improvementin magnetic field sensitivity. In spin echo experiments,coherence times remain constant regardless of the thickness of dielectric cavities. These experiments pave theway for broader applications of dielectric cavities inSiC-based quantum technologies.

Effect of temperature on GaN integrated optical transceiver chip

Jiabin Yan, Li Fang, Yiqun Yan, Zhihang Sun, Fan Shi, zheng shi, and Yongjin Wang

DOI: 10.1364/OL.525315 Received 01 Apr 2024; Accepted 29 Apr 2024; Posted 06 May 2024  View: PDF

Abstract: The GaN integrated optical transceiver chip based on multiple quantum wells (MQW) structure exhibits great promise in the fields of communication and sensing. In this letter, the effect of ambient temperature on the performance of GaN integrated optical transceiver chips including a blue MQW light-emitting diode (LED) and a MQW photodiode (PD) is comprehensively studied. Temperature-dependent light-emitting and current-voltage characteristics of the blue MQW LEDs are measured with the ambient temperature ranging from -70 ℃ to 120 ℃. The experimental results reveal a decline in the electroluminescent (EL) intensity and an obvious redshift in the emission peak wavelength of the LED with increasing ambient temperature. The light detection performance of MQW PD under different temperatures is also measured with the illumination of an external blue MQW LED, indicating an enhancement in the PD sensitivity as the temperature rises. Finally, the temperature effect on the MQW PD under the illumination of the MQW LED on the GaN integrated optical transceiver chip is characterized and the PD photocurrent increases with higher ambient temperature. Furthermore, the measured temperature characteristics indicate that the GaN integrated optical transceiver chip offers a promising application potential for optoelectronic temperature sensor.

Observation of Single-Photon Spin Hall Effect

Yinghang Jiang, Jiguo Wu, Rongchun Ge, and Zhiyou Zhang

DOI: 10.1364/OL.522132 Received 23 Feb 2024; Accepted 29 Apr 2024; Posted 06 May 2024  View: PDF

Abstract: The single-photon spin Hall effect (SPSHE) is explored in this paper. The physics of the spin Hall effect and its quantum weak measurement method with a dimensionless pointer are elucidated through particle number representation. Our weak measurement scheme obviates the necessity of high-resolution single-photon array detectors. Consequently, we have successfully observed the spin Hall effect within a 20ns temporal window using a position-resolution-independent single-photon detector with remarkably low noise levels. The weak measurement of the dimensionless pointer presented in this paper boosts both the detection accuracy and response speed of the photonics spin Hall effect, thereby contributing significantly to fundamental theoretical research in spin photonics and precise measurements of physical property parameters.

Demonstration of 4H silicon carbide on aluminum nitride integrated photonics platform

Ruixuan Wang, Jingwei Li, Lutong Cai, and Qing Li

DOI: 10.1364/OL.521157 Received 07 Feb 2024; Accepted 28 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: The existing silicon-carbide-on-insulator photonic platform utilizes a thin layer of silicon dioxide under silicon carbide to provide optical confinement and mode isolation. Here, we replace the underneath silicon dioxide layer with a 1-µm-thick aluminum nitride and demonstrate a 4H-silicon-carbide-on-aluminum-nitride integrated photonics platform for the first time. Efficient grating couplers, low-loss waveguides, and compact microring resonators with intrinsic quality factors up to 210,000 are fabricated. In addition, by undercutting the aluminum nitride layer, the intrinsic quality factor of the silicon carbide microring is improved by nearly one order of magnitude (1.8 million). Finally, an optical pump-probe method is developed to measure the thermal conductivity of the aluminum nitride layer, which is estimated to be over 30 times of that of silicon dioxide.

Symmetry of constellation diagram-based Intelligent SNR estimation for visible light communications

MAOREN WANG, Zhen Zhang, Huixin Zhang, Zabih Ghassemlooy, and Tian Zhang

DOI: 10.1364/OL.525115 Received 28 Mar 2024; Accepted 28 Apr 2024; Posted 02 May 2024  View: PDF

Abstract: Visible light communication (VLC) technology with rich spectrum resources is thought of as an essential component in the future ubiquitous communication networks. Accurately monitoring its transmission impairments is important for improving the stability of high-speed communication networks. Existing research on intelligently monitoring the signal-to-noise ratio (SNR) performance of VLC focuses primarily on the application of neural networks but neglects the physical nature of communication systems. In this work, we propose an intelligent SNR estimation scheme for VLC systems, which is based on the symmetry of constellation diagrams with classical deep learning frameworks. In order to increase the accuracy of the SNR estimation scheme, we introduce two data augmentation methods (DA): point normalization and quadrant normalization. The results of extensive simulations demonstrate that the proposed point normalization method is capable of improving accuracy by about 5, 10, 14, and 26% respectively for 16-, 64-, 256-, and 1024-quadrature amplitude modulation compared with the same network frameworks without DA. The effect of accuracy improvement can be further superimposed with traditional DA methods. Additionally, the extensive number of constellation points (e.g., 32, 64, 128, 256, 512, 1024, and 2048) on the accuracy of SNR estimation is also investigated.

Non-line of sight optical wireless communication system enabled by wavefront shaping for multi-user indoor access

Huiyi Weng, Wei Wang, Zhiwei Chen, Bowen Zhu, Weihao Ni, Mingzhu Yin, rongguo Lu, Zizheng Cao, Zhaohui Li, and Fan Li

DOI: 10.1364/OL.523233 Received 06 Mar 2024; Accepted 26 Apr 2024; Posted 03 May 2024  View: PDF

Abstract: In this letter, we experimentally investigate a non-line-of-sight (NLOS) optical wireless communication (OWC) system that utilizes wavefront shaping techniques to realize simultaneous data transmission for multiple users. Wavefront shaping techniques are employed to address the issue of low intensity of diffusely reflected light at the receiver in NLOS scenarios for indoor high-speed access. To achieve communication path planning and tracing for two different users in free-space optical communication, the pixels of the spatial light modulator (SLM) are divided into two halves to separately manipulate the wavefront of two independent data carriers centered at different wavelengths. The maximum received optical power can be effectively improved by more than 15 dB with the wavefront shaping technique. To avoid power enhancement of non-target wavelength, the wavelength interval of two different users is experimentally studied. The difference in power enhancement ratio (DPER) is increased with the wavelength interval, and 14.95dB DPER is obtained with a 10nm wavelength interval. Under the aforementioned wavelength planning strategy, successful transmission and reception of 2×160 Gbit/s 16-QAM signals for two users with coherent detection is achieved using wavelengths of 1550nm and 1560nm in indoor access scenario.

High-speed forward-viewing OCT probe based on Lissajous sampling and sparse reconstruction

Xinyu Wu, Ragib Ishrak, Reza Reihanisaransari, Yogesh Verma, Bryan Spring, Kanwarpal Singh, and Rohith Reddy

DOI: 10.1364/OL.521595 Received 15 Feb 2024; Accepted 26 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: We present a novel endoscopy probe using optical coherence tomography (OCT) that combines sparse Lissajous scanning and compressed sensing for faster data collection. This compact probe is only 4 mm in diameter and achieves a large field of view of 2.25 mm² and an 11 mm working distance. Unlike traditional OCT systems that use bulky raster scanning, our design features a dual-axis piezoelectric mechanism for efficient Lissajous pattern scanning. It employs compressive data reconstruction algorithms that minimize data collection requirements for efficient, high-speed imaging. This approach significantly enhances imaging speed by over 40\%, substantially improving miniaturization and performance for endoscopic applications.

Customizable polarization-selective narrow-band meta-filters using printable UV-curing nanocomposite

Yang Li, Yan Li, Shujing Liu, Jingru Wang, Zejia Zhao, Adnan Khan, Ming Feng, and Feng Song

DOI: 10.1364/OL.520934 Received 06 Feb 2024; Accepted 26 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Low-cost nanocomposite metasurfaces have demonstrated attractive potential to replace the equivalent dielectric metasurfaces for light engineering. However, the resonance characteristics of embedded structures in nanocomposite metasurfaces have not been further analyzed beyond the effective refractive index. Herein, we have proposed customizable polarization-selective narrow-band meta-filters using ultraviolet-curable (UV) nanocomposites. As an additional degree of freedom, near-field effects between highly concentrated doped nanoparticles can enhance the Mie resonance of low aspect ratio (AR = 0.2) meta-units. The surface lattice resonances (SLR) of meta-filters can be coupled with enhanced Mie resonances of individual meta-units to realize tunable narrow-band (FWHM ~ 0.007λ) reflections with intensities near unity. Meanwhile, the polarization-selective properties of the reflection peaks can be tuned by optimizing the asymmetric lattice. Such proposed new-generation customizable meta-filters will offer novel strategies for specific near-infrared fluorescence filtering in integrated imaging systems.

Compensating birefringence of test-mass substrates for gravitational-wave detectors with arbitrary polarization states

Marc Eisenmann, Shalika Singh, and Matteo Leonardi

DOI: 10.1364/OL.523753 Received 18 Mar 2024; Accepted 25 Apr 2024; Posted 26 Apr 2024  View: PDF

Abstract: Gravitational waves detectors are ultimately limitedby thermal noise in their most sensitive region. Cryo-genic operation combined with crystalline substratesand coatings is a promising approach to reduce thisnoise, thereby increasing their sensitivity and detectionrate. However, crystalline materials can exibit birefrin-gent behaviours which will degrade the detector sensi-tivity or duty-cycle. Currently, no available substrate orcoating meets the stringent birefringence requirement.Here, we demonstrate the use of a pair of identical elec-tro polarization retarders to generate arbitrary polariza-tion states and compensate birefringence of a KAGRAtest-mass substrate.

Compact Mode Converters in Thin-film Lithium Niobate Integrated Platforms

JingJing Zhang, pengfei qiu, runyu He, XIAOXIAN SONG, ZiJie Dai, Yang Liu, Dong Pan, Junbo Yang, and Kai Guo

DOI: 10.1364/OL.524739 Received 25 Mar 2024; Accepted 25 Apr 2024; Posted 26 Apr 2024  View: PDF

Abstract: Mode converters, crucial elements within photonic integrated circuits (PICs) designed for multimode optical transmission and switching systems, present a challenge due to their bulky structures in thin-film lithium niobate (TFLN) integrated platforms, which are incompatible with the compact and efficient nature desired for dense PICs.In this work, we propose TE$_1$-TE$_0$, TE$_2$-TE$_0$, and TE$_3$-TE$_0$ mode converters in shallowly etched TFLN, within small footpints.The experimental results show that the insertion loss is 0.4 dB, 0.6 dB, and 0.5 dB for the compact TE$_1$-TE$_0$, TE$_2$-TE$_0$, and TE$_3$-TE$_0$ mode converters, respectively, and these devices can be operated within a wide 1 dB bandwidth over 100 nm.This work facilitates the development of low-loss, broadband, and compact monolithically integrated photonic devices for future multimode communication networks in TFLN integrated platforms.

Phase unwrapping algorithm based on phase diversity wavefront reconstruction and virtual Hartmann-Shack technology

Ying Zhang, Hua Bao, Naiting Gu, Shuqi Li, Yiqun Zhang, and Changhui Rao

DOI: 10.1364/OL.515821 Received 13 Dec 2023; Accepted 24 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: Phase unwrapping algorithms play a crucial role in various phase measurement techniques. Traditional algorithms cannot work well in strong noise environments, which makes it very difficult to obtain the accurate absolute phase from the noisy wrapped phase. In this paper, we proposed a novel phase unwrapping algorithm based on phase diversity (PD) wavefront reconstruction and virtual Hartmann-Shack (VHS) technology, called PD-VHS, which coped well with the large-scale noise in the wrapped phase. In simulation experiments, hundreds of random noise wrapped phases, containing the first 45 Zernike polynomials and the wavefront RMS= 0.5λ, 1λ,are used to compare the classical quality-map guided algorithm, the VHS algorithm with decent noise immunity and our PD-VHS algorithm. When SNR drops to just 2dB, the mean RMSEs of residual wavefront between the unwrapped result and the absolute phase of quality-map guided algorithm and VHS algorithm are up to 3.99λ, 0.44λ and 4.29λ, 0.85λ respectively, but our algorithm's are low to 0.11λ and 0.17λ. Simulation results demonstrated that the PD-VHS algorithm significantly outperforms the quality-map guided algorithm and the VHS algorithm under strong noise conditions.

Miniaturized Preamplifire Integration in Ultrasound Transducer Design for Enhanced Photoacoustic Imaging

Mohsin Zafar, Rayyan Manwar, and Kamran Avanaki

DOI: 10.1364/OL.512445 Received 29 Nov 2023; Accepted 24 Apr 2024; Posted 06 May 2024  View: PDF

Abstract: Photoacoustic imaging utilizes the photoacoustic effect to record both vascular and functional characteristics of biological tissue. Photoacoustic signals have typically a low amplitude which cannot be read efficiently by data acquisition systems. This necessitates the use of one or more amplifiers. These amplifiers are somewhat bulky (for example, the ZFL-500LN+, Mini-Circuits, USA, or 351A-3-50-NI, Analog Modules Inc., USA). Here, we describe the fabrication and development process of a transducer with a built-in low noise preamplifier that is encased within the transducer housing. This new design could be advantageous for applications where a compact transducer + preamp is required. We demonstrate the performance of this compact detection unit in a laser scanning photoacoustic microscopy system by imaging a rat ear ex-vivo and rat brain vasculature in-vivo.

Simulation of diffraction and scattering using the Wigner Distribution Function

Emilie Pietersoone, Jean Michel Letang, Simon Rit, and Max Langer

DOI: 10.1364/OL.523608 Received 19 Mar 2024; Accepted 23 Apr 2024; Posted 23 Apr 2024  View: PDF

Abstract: X-ray phase-contrast imaging enhances soft tissue visualization by leveraging the phase shift of X-rays passing through materials. It permits to minimize radiation exposure due to high contrast, as well as high resolution imaging limited by the wavelength of the X-rays. Phase retrieval extracts the phase shift computationally, but simulated images fail to recreate low-frequency noise observed in experimental images. To this end, we propose a new method to simulate phase contrast images using the Wigner Distribution Function. This permits the simulation of wave and particle effects simultaneously and simulates images photon by photon. Here, we give a first demonstration of the method by simulating the Gaussian double-slit experiment. It has the potential for realistic simulation of low-dose imaging.

Enabling Endogenous Distributed Acoustic Sensing in Digital Subcarrier Coherent Transmission System

Zihe Hu, Ming-Ming Zhang, Yuqi Li, Junda Chen, Weihao Li, Yuxuan Xiong, Luming Zhao, Can Zhao, and Ming Tang

DOI: 10.1364/OL.524132 Received 19 Mar 2024; Accepted 22 Apr 2024; Posted 23 Apr 2024  View: PDF

Abstract: To monitor the health of the fiber network and its ambient environment in densely populated access/metro network areas, in this letter, an endogenous distributed acoustic sensing (DAS) has been proposed and achieved in a coherent digital subcarrier multiplexing (DSCM) system. Rather than specially-allocating sensing probe in general integrated communication and sensing schemes, the fractional Fourier transformed (FrFT) training sequence (TS) designated for time/frequency synchronization in DSCM coherent communications has been repurposed for sensing. While achieving excellent synchronization performance of communication, the FrFT-based TS can also be concurrently utilized to perform distributed vibration sensing. Experimental results demonstrate that the FrFT-based timing/frequency synchronization sequence is repurposed to achieve a DAS sensitivity of 70 pε·Hz-½ at a spatial resolution of 5 m, along with 100-Gb/s 16 quadrature amplitude modulation (QAM) DSCM transmission, without a loss of spectral efficiency.

RF E-field enhanced sensing based on Rydberg-atom-based superheterodyne receiver

Wenguang yang, Mingyong Jing, Hao Zhang, Linjie Zhang, Liantuan Xiao, and Suotang Jia

DOI: 10.1364/OL.522466 Received 26 Feb 2024; Accepted 21 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: We present enhanced sensing of radio frequency (RF) electric field (E-field) by the combined polarizability of Rydberg atoms and the optimized local oscillator (LO) field of superheterodyne receiving. Our modified theoretical model reveals the dependencies of the sensitivity of E-field amplitude measurement on the polarizability of Rydberg states and the strength of the LO field. The enhanced sensitivities of megahertz (MHz) E-field are demonstrated at the optimal LO field for three different Rydberg states 43D5/2, 60S1/2, and 90S1/2. The sensitivity of 63 MHz for the 90S1/2 state reaches 9.6×10^-5 V/m/Hz½, which is approximately an order of magnitude higher than those already published. This result closely approaches the theoretical sensitivity limit of RF dipole antennas and indicates the potential for exceeding the limit in measuring sub-MHz E-field. This atomic sensor based on the Rydberg Stark effect with heterodyne technique is expected to boost an alternative solution to electric dipole antennas.

On-chip integrated metasystem for spin-dependent multi-channel colour holography

Zhanying Ma, Xianjin Liu, Yuqi Peng, Dasen Zhang, Zhenzhen Liu, and Jun Jun Xiao

DOI: 10.1364/OL.520289 Received 30 Jan 2024; Accepted 19 Apr 2024; Posted 08 May 2024  View: PDF

Abstract: On-chip integrated metasurface driven by in-plane guided waves is of great interests in various light field manipulation applications such as colorful augmented reality and holographic display. However, it remains a challenge to design colorful multichannel holography by a single on-chip metasurface. Here we present metasurfaces integrated on top of guided-wave photonic slab that achieves multi-channel colorful holographic light display. An end-to-end scheme is used to inverse design the metasurface for projecting off-chip preset multiple patterns. Particular examples are presented for customized patterns that were encoded into the metasurface with a single-cell meta-atom, working simultaneously at RGB color channels and for several different diffractive distance, with polarization dependence. Holographic images are generated at 18 independent channels with such a single-cell metasurface. The proposed design scheme is easy to implement and the resulting device is viable to fabrication, promising a plenty of applications in nanophotonics.

Spin injection enhancement in CW VECSEL at room temperature using push-pull optical pumping

Alexandre Joly, Ghaya Baili, Jean-marie George, Isabelle Sagnes, Daniel Dolfi, and Mehdi Alouini

DOI: 10.1364/OL.522095 Received 22 Feb 2024; Accepted 17 Apr 2024; Posted 18 Apr 2024  View: PDF

Abstract: We report the enhancement of spin injection efficiency in an external cavity VCSEL based on a non-resonant pumping coupled with a polarized optical resonant illumination. This double pumping scheme allows both the injection of spin polarized electrons in the conduction band and the selection of the spin orientation for the electron/hole recombination laser process. Experimentally, a flip of the polarization state of the laser is achieved with an ellipticity of +31° (spin down) and -33° (spin up), so an increase of about 50% of the ellipticity in comparison to an optical non-resonant pumping alone.

Design of an ultrafast plasmonic nanolaser for high-intensity broadband emission operating at room temperature

peng zhou, Lei Jin, Kun Liang, Xiongyu Liang, Li Junqiang, Xuyan Deng, Wang yilin, Guo Jiaqi, Li Yu, and Jiasen Zhang

DOI: 10.1364/OL.518240 Received 09 Jan 2024; Accepted 12 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: We propose a plasmonic nanolaser based on Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) structure, which effectively confines light on a subwavelength scale (~λ/14). As the pump power increases, the proposed plasmonic nanolaser exhibits broadband output characteristics of 20nm and the maximum output power can reach 20uW. Furthermore, the carrier lifetime at the upper energy level in our proposed structure is measured to be about 400 fs using a double pump-probe excitation. The ultrafast characteristic is attributed to the inherent Purcell effect of plasmon systems. Our work paves the way toward deep-subwavelength mode confinement and ultrafast femtoseconds plasmonic laser in Spaser-based interconnects,eigenmode engineering of plasmonic nanolasers, Nano-LEDs, spontaneous emission control.

Realization of the Pascal based on Argon using a Fabry-Pérot refractometer

Isak Silander, Johan Zakrisson, Ove Axner, and Martin Zelan

DOI: 10.1364/OL.523293 Received 11 Mar 2024; Accepted 11 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: Based on a recent experimental determination of the static polarizability and a first-principles calculation of the frequency-dependent dipole polarizability of argon, this work presents, by use of a Fabry-Pérot refractometer operated at 1550 nm, a realization of the SI unit of pressure, the pascal, for pressures up to 100 kPa, with an uncertainty of [(0.98 mPa)^2+(5.8x10 ^(−6)P)^2+(26×10^(−12)P^2)^2]^(1/2). The work also presents a value of the molar polarizability of N_2 at 1550 nm of 4.396572(26) × 10^-6 m^3/mol, which agrees well with previously determined ones in the literature.

Polarization switching in a mid-infrared Er:YAlO3 laser

Florent Cassouret, Ahmed Nady, Pavel Loiko, Simone Normani, Alain BRAUD, Weidong Chen, Valentin Petrov, Dunlu Sun, Peixiong Zhang, Bruno viana, Ammar Hideur, and Patrice Camy

DOI: 10.1364/OL.523010 Received 13 Mar 2024; Accepted 10 Apr 2024; Posted 10 Apr 2024  View: PDF

Abstract: We report on a polarization-resolved study of mid-infrared emission properties of Er3+-doped orthorhombic yttrium aluminum perovskite YAlO3 single crystal. For the 4I11/2→4I13/2 Er3+ transition, the stimulated emission cross-section is 0.20×10-20 cm² at 2919 nm for light polarization E ‖ c. Pumped by an Yb-fiber laser at 976 nm, the 10 at.% Er:YAlO3 laser delivered 1.36 W at 2919 nm with a slope efficiency of 31.4%, very close to the Stokes limit, a laser threshold as low as 33 mW and a linear polarization. Pump-induced polarization switching between E || b and E || c eigen states was observed and explained by excited-state absorption from the terminal laser level.

Non-resonant Bragg scattering four-wave-mixing at near visible wavelengths in low-confinement silicon nitride waveguides

Nicholas Jaber, Scott Madaras, Andrew Starbuck, Andrew Pomerene, Christina Dallo, Douglas Trotter, Michael Gehl, and Nils Otterstrom

DOI: 10.1364/OL.519793 Received 25 Jan 2024; Accepted 10 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: Quantum state coherent frequency conversion processes—such as Bragg scattering four wave mixing (BSFWM)—hold promise as a flexible technique for networking heterogeneous and distant quantum systems. In this letter, we demonstrate BSFWM within an extended (1.2-m) low-confinement silicon nitride waveguide and show that this system has the potential for near unity quantum coherent frequency conversion in visible and near-visible wavelength ranges. Using sensitive heterodyne laser spectroscopy at low optical powers, we characterize the Kerr coefficient (∼1.55 W^{−1}m^{−1}) and linear propagation loss (∼0.0175 dB/cm) of this non-resonant waveguide system, revealing a record-high nonlinear figure of merit (NFM = γ/α ≈ 3.85 W^{−1}) for BSFWM of near visible light in non-resonant silicon nitride waveguides. We demonstrate how, at high yet achievable on-chip optical powers, this NFM would yield a comparatively large frequency conversion efficiency, opening the door to near-unity flexible frequency conversion without cavity enhancement and resulting bandwidth constraints.

Off-plane quartz-enhanced photoacoustic spectroscopy

Huijian Luo, Junming Li, Haohua Lv, Jiabao Xie, Chenglong Wang, Haoyang Lin, Ruobin Zhuang, Wenguo Zhu, Yongchun Zhong, Ruifeng Kan, JianHui Yu, and Huadan Zheng

DOI: 10.1364/OL.506650 Received 25 Sep 2023; Accepted 01 Jan 2024; Posted 31 Jan 2024  View: PDF

Abstract: In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam neither went through the prong spacing of the quartz tuning fork (QTF), nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as excitation source, the noise level was suppressed by ~2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.