In this work, to the best of our knowledge, the first demonstration of 1047-nm Q-switched mode-locked Nd:YLF cladding waveguide lasers fabricated by femtosecond laser direct writing (FsLDW) is reported. Modulated by Gr-ReS2-Gr heterostructure film, the fabricated waveguide laser delivers laser pulses with a pulse duration of as short as 31 ps at a fundamental repetition rate of up to 9.55 GHz. The maximum output power under the pulsed regime is determined to be 300 mW with a slope efficiency of 31.77%. The result in this work indicates promising applications of Gr-ReS2-Gr heterostructure film for modulation of ultrafast pulsed laser and compact Nd:YLF waveguide lasers for integrated photonics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optical waveguides are basic components in integrated photonics and they feature high intracavity intensities because of their compact geometries [1,2]. Some of the original optical properties, such as lasing performance and nonlinear optical properties, in the bulk material can be well improved within the waveguide volume. Therefore, waveguide-based devices have aroused increasingly more attention due to their functionalities and miniaturization, opening up exciting possibilities and opportunities for modern integrated photonics research [3,4]. One typical example is waveguide lasers, which are compact laser sources possessing enhanced optical gain, reduced lasing threshold, and small footprint based on waveguide structures defined in laser gain media. Such small laser sources are well compatible with multi-functional photonic integrated circuits (PICs), enabling on-chip photonic applications operating in both classical and quantum regimes [3–6].
A number of micro-/nano-machining techniques have been applied to introduce the desired refractive index contrast for waveguide formation in many dielectrics. Femtosecond laser direct writing (FsLDW), in particular, is considered to be one of the most flexible approaches for definition of waveguide structures with diverse geometries in two-dimensional (2D) and even three-dimensional (3D) configurations [7–9]. Due to its unique simplicity in micro-/nano-structure fabrication and general applicability for dielectric materials, the study of FsLDW dielectric waveguide technology has won an important place in modern integrated photonics research since the pioneer study of Davis et al. in 1996 . Until now, FsLDW has been applied for the fabrication of waveguide structures in dozens of dielectric materials [6,9,11–16].
As one of the most promising laser gain media, rare-earth-doped yttrium lithium fluoride (YLiF4 or YLF) crystal features weak thermal lensing (19 times lower than that of Nd:YAG in case of, e.g., Nd:YLF, detailed information can be referenced in Refs. [17,18]), large fluorescence line width (around 1.5 nm at 1047 nm), long lifetime of 4F3/2 Nd3+ energy level (485 μs in case of Nd:YLF), and naturally polarized oscillation, making it an ideal candidate for both continuous-wave (CW) and pulsed laser operation [3,17–24]. Up to date, waveguide lasers based on rare-earth-doped YLF, such as Pr:YLF , Nd:YLF , Tm:YLF [21–23], and Tm,Ho:YLF , have been demonstrated by several fabrication techniques, including liquid phase epitaxy (LPE), diamond blade dicing, and FsLFW. However, these reported compact lasers are all operated in the CW operation. The very first demonstration of rare-earth-doped YLF waveguide lasers operating in the pulsed regime is still missing.
When it comes to the study of waveguide pulsed lasers, the design of waveguide cavity has to be well studied with a view to maintaining the intrinsic compact structure and miniaturized footprint of a waveguide laser. In contrast to the active techniques that require active cavity length stabilization and bulky acousto-optic or electro-optic modulators, the passive schemes using saturable absorber (SA) elements are considered to be more compatible to compact lasers due to the ease of SA integration . This is one of the main reasons that 2D layered materials are regarded as the most promising SA materials for on-chip photonic applications since their flexibility for on-chip integration. One of the unique features of such a material family is the possibility to assemble different 2D materials into a vertical heterostructure, which combines the respective merits of each material and offers high flexibility on the collective optical and electric properties, such as SA performance [26–29]. Rhenium disulphide (ReS2), as a new member of the transition metal dichalcogenides (TMDs), has the unique distorted 1T structure with weak interlayer coupling and characteristics of layer-insensitive direct bandgap, which is different from the other members of TMDs family . Employing ReS2 as SAs in Q-switched and mode-locked regimes have been reported in fiber lasers and solid-state lasers [30,31]. Furthermore, inspired by those enhanced nonlinear optical properties of 2D materials realized by heterostructures [27,29], we combine ReS2 and graphene in this work [32,33], achieving the assembled Gr-ReS2-Gr heterostructure film, which has never been reported to be used as SA for modulation of waveguide lasers.
In this work, we report on 9.55-GHz Q-switched mode-locked FsLDW Nd:YLF waveguide lasers modulated by Gr-ReS2-Gr heterostructure film for the first time, to the best of our knowledge. The maximum output laser power is determined to be 300 mW and the slope efficiency is 31.77%. The pulse duration is measured to be as short as 31 ps. The μ-PL properties of Nd:YLF waveguide is investigated and the linear and nonlinear optical response of Gr-ReS2-Gr heterostructure film are explored.
2. Sample preparation
2.1 Waveguide fabrication by FsLDW
We employed the FsLDW technique to fabricate the depressed cladding waveguide with a circular geometry buried inside the Nd:YLF crystal (1 at%, c-cut with dimensions of 10(a) mm × 10(b) mm × 2(c) mm, two 10(a) mm × 10(b) mm facets and two 10(a) mm × 2(c) mm facets were polished to optical-grade quality and kept uncoated). The femtosecond laser pulses (with a pulse width of 50 fs at the central wavelength of 800 nm and polarization paralleled to the b-axis of Nd:YLF) were delivered by a femtosecond laser system (Astrella, Coherent Inc., USA) and focused by a microscope objective lens (40 ×, N.A. = 0.65) with a maximum depth of 100 μm beneath the largest crystal surface (10(a) mm × 2(c) mm facet). The pulse energy was set to be 0.2 μJ for the definition of the bottom half of the cladding while 0.12 μJ for the top half. The main reason of applying different pulse energies is to ensure the laser-induced damage tracks at the top and bottom semicircles are identical in length and width. During the laser-writing process, the Nd:YLF crystal was placed on a PC-controlled 3D micro-positioning stage for a precise translation along the b-axis direction at a constant velocity of 1 mm/s (avoiding crystal cracking by minimizing the stress effect induced by laser pulses). After finishing one laser-track writing, the femtosecond laser beam will be refocused at a new position just next to the last laser track with a lateral separation of 3 μm, and initiating a new track writing. In this manner, 56 parallel laser-induced damage tracks were fabricated and constitute a circle geometry with a radius of 35 μm in the cross section. We chose 35-μm radius in cross-section to make the compromise between low waveguide loss and single-mode operation since small geometry will result in high waveguide losses while too large diameter will cause multi-mode operation. Figure 1(b) exhibits the optical microscope image of the cladding waveguide fabricated in Nd: YLF crystal. The laser-induced damage tracks were quite clean and no crystal cracking was observed, showing excellent FsLDW fabrication and potential of this waveguide for good light confinement. To explore the effects of FsLDW on the lattice changes and fluorescence properties, the micro-spectroscopy experiment was performed by employing a fiber confocal microscope (Alpha300 R, WITec GmBH) at room temperature. Under the excitation of a CW 488-nm laser source focused through the cladding waveguide cross section with a depth of 10 μm by a microscope objective (100×, N.A. = 0.9), the luminescence emission was detected via a spectrometer (UHTS 300 SMFC VIS). Under this regime, confocal μ-PL spectra and 2D mapping images were obtained. Confocal μ-PL emission spectra (see Fig. 1(a)) were collected from three critical regions, namely, filament, guiding region and bulk. It is clear that there is an obvious decrease of luminescence intensity in the filament, which can be confirmed by the spatial 2D distribution of μ-PL intensity (Fig. 1(c)). The intensity reduction from the modified area manifests the partial lattice distortion and damage in the laser-modified region. The slight blue shift and the broadening of the emission line from filament further indicate the appearance of laser-induced lattice damages in the modified region. In contrast, the μ-PL emission intensity and the peak features in the waveguide (unmodified core) region are almost identical to those in the bulk material region, suggesting good preservation of the original crystal network and luminescence properties in the waveguiding area .
2.2 Gr-ReS2-Gr heterostructure characterization
Gr-ReS2-Gr few-layer film, a novel 2D heterostructure material severed as SA element in this work, was prepared by chemical vapor deposition (CVD) on a polished sapphire substrate (with dimensions of 10(x) mm × 10(y) mm × 0.25(z) mm), which is supplied by 6Carbon Technology, China. To characterize the Gr-ReS2-Gr few-layer film sample, a series of experimental tests have been implemented. Firstly, the atomic force microscopy (AFM) measurement (tapping mode, by the Bruker Dimension Icon AFM system) was performed to investigate the surface morphology and thickness of the film sample. The height profile (see Fig. 2) of the CVD-prepared thin film determines the thin film thickness of around 24.3 nm. Then, the linear optical transmittance of the sample was measured by a UV-Vis-NIR Spectrophotometer (UV-1800, Shimadzu). The smooth optical transmittance spectrum of Gr-ReS2-Gr heterostructure film from visible to infrared indicates the potential broadband optical modulation properties of Gr-ReS2-Gr heterostructure film. And the transmittance value is determined to be 78.03% at the wavelength of 1064 nm, which is lower than that of the pure ReS2 (around 89%) . The higher linear absorption value is actually beneficial for saturable absorption as it usually offers a higher modulation depth . Furthermore, I-scan measurement is performed to investigate the nonlinear optical response of the Gr-ReS2-Gr few-layer film sample. The excitation laser source is a 1030-nm femtosecond fiber laser (FemtoYL-10, YSL Photonics, China). Figure 3(b) indicates the optical transmittance as a function of the laser intensity focused on the sample (the beam is viewed as a Gaussian beam with a beam waist radius of around 52 μm at the focal position). By fitting the following formula [14,35]:31]. Moreover, the saturable intensity is determined to be around 19.98 μJ/cm2.
3. Waveguide laser characterization
3.1 CW waveguide laser
To estimate the guiding performance of FsLDW Nd:YLF cladding waveguide under the passive regime, we employed a typical end-face coupling arrangement. A solid-state laser with a wavelength of 1064 nm served as the laser source and was focused by an objective lens (20×, N.A. = 0.4). The polarization of the incident laser beam was adjusted by a half-wave plate, enabling measuring light transmittance of Nd:YLF cladding waveguide for all-angle polarizations. The output laser guided by the cladding waveguide was collected by an identical objective lens (20×, N.A. = 0.4) and detected by a power meter sensor. Figure 5(b) shows the light transmittance under different polarizations. It is clear that the optical transmittance is polarization-dependent. The maximum transmittance is along TM polarization (π-polarization) while the minimum is along TE polarization (σ-polarization). This result is most likely due to the optical birefringence of Nd:YLF crystal since the optical absorption coefficients as well as FsLDW-induced refractive index change along different crystal axis is different, resulting in different transmission along TM and TE polarizations. To investigate the lasing performance of fabricated cladding waveguide, we replaced the laser source by a tunable CW Ti:Sapphire laser (Coherent MBR) to be used as the optical pumping source. The experimental schematic illustration is shown in Fig. 4 (there is no Gr-ReS2-Gr heterostructure film inserted in the cavity under the CW operation regime). Based on the end-face coupling system, a plano-convex lens (f = 25 mm) and a microscope objective (20×, N.A. = 0.4) are used for laser in- and out-coupling. The laser cavity consists of a pump mirror (M1 with a transmittance of 99.8% at 808 nm and a reflectivity of >99.9% at 1064 nm) and an output mirror (M2 with a reflectivity of approximately 60% at 1064 nm) butt-adhered to the two end-facets of Nd:YLF crystal.
The maximum output power is measured to be 354 mW (with a slope efficiency of 38.13%, see Fig. 5(a)), which is higher than the previous reported values (195.4 mW) obtained in Nd:YLF channel waveguide lasers . The inset of Fig. 5(a) is the near-field mode profile of the output waveguide laser imaged by a CCD camera. Despite the beam divergence is removed by the waveguide structure, the experimental determined lasing modal profile in Fig. 5(a) is more like a multi-mode instead of a good single-mode operation. The main cause of this effect is the gain-oriented modal selection due to the non-optimized FsLDW fabrication parameters and the resultant polarization-dependent guiding properties. The central lasing wavelength is measured to be 1047 nm with a full wave at half maximum (FWHM) value of 0.2 nm (see Fig. 5(c), spectrometer resolution is 0.2 nm). Please note that the polarization of the output laser is π-polarization which is selected naturally by the gain anisotropy of the Nd:YLF crystal. However, lasing along two light polarizations can be switched even coexist in optically birefringent laser crystals like YLF crystal. One factor to affect polarization-switching is the intracavity loss, which can be controlled by the output coupling condition . Therefore, dual-wavelength waveguide lasing based optically anisotropic laser crystals can be realized with the proper control of cavity loss. Figure 5(d) exhibits the relative intensity of output 1047-nm laser as a function of pump laser with tunable wavelengths but a fixed pump power. It can be summarized that, the cladding waveguide used in this work, is able to support efficient lasing at the wavelength of 1047 nm in the whole absorption range of Nd:YLF (from 795 nm to 837 nm), peaking at around 808-nm pumping.
3.2 Q-switched mode-locked waveguide laser
To further investigate the lasing performance of the fabricated FsLDW Nd:YLF cladding waveguide under the pulsed operation, the Gr-ReS2-Gr heterostructure film was inserted into the cavity and used as an SA element for laser modulation (schematic illustration of the experimental setup is shown in Fig. 4). In this way, a Q-switched mode-locked waveguide laser was obtained. The output pulsed laser was detected by a High-Speed InGaAs Photodetector (New focus, 1414 model) coupled by a single mode fiber after a long-pass filter and analyzed by a digital oscilloscope (Tektronix, MSO 72504DX, 25 GHz bandwidth, 14 ps rise time). The maximum output is determined to be 300 mW (with a slope efficiency of 31.77%, see Fig. 6(a)), which is relatively lower than that under the CW regime. The reductions of maximum output power and slope efficiency are caused by additional absorption/scattering loss introduced by the SA element. However, it is noted that similar lasing threshold values have been achieved experimentally for both CW and Q-switched mode-locked operation in this work. We believe this is because the insertion of SA elements also has non-negligible impacts on both the waveguide and lasing modal profiles and thus spot sizes, which are strongly linked to the lasing threshold values . Besides, the total transmission of the output coupler in the CW regime is reduced in the pulsed regime because of the SA element, which also tends to slightly reduce the lasing threshold . And these effects will balance the increase of lasing threshold in the pulsed regime due to the additional loss introduced by the SA element, resulting in a similar value to that in the CW regime in this work. The Q-switched envelopes and the mode-locked pulse trains are exhibited in Figs. 6(b) and 6(c). The pulse duration is determined to be 31 ps, which is, to the best of our knowledge, the shortest for Nd:YLF waveguide lasers . According to the mode-locked pulse trains, the repetition rate can be determined to be 9.55 GHz, which is in good agreement with the theoretical value, i.e., 9.96 GHz (the theoretical fundamental repetition rate is calculated according to the formula frep = c/2nl, where c is the light speed, n is the effective refractive index of the waveguide, and l is the effective length of the cavity). In this case, the n is determined to be 1.47 which is the refractive index of Nd:YLF crystal under 1047 nm. The l is determined to be 10.25 mm, which consists of the waveguide length of 10 mm and the SA thickness of 0.25 mm. Please note that the slightly lower repetition rate of the experimental result compared to the theoretical value is likely because the actual cavity length in experiments is slightly longer than the determined value of l caused by the small air gaps existing among the elements in the cavity). Nonetheless, the value of 9.55 GHz is, to the best of our knowledge, up to now the highest repetition rate for any type of Nd:YLF lasers . The stability of the Q-switched and mode-locked pulse train waveforms have been testified for around 1-hour time scale, and only <7% difference in pulse intensity can be identified. At high pump powers, the waveform has slight disturbance as a result of thermal effect by heating SA. While no damage of SA was observed in the experiment, which suggests that the SA used in this work possesses relatively high damage threshold. The wavelength of the output laser under the Q-switched mode-locked operation is measured to be 1047 nm (π-polarization), which is identical to the laser emission under the CW operation.
In the future work, we will attempt to realize dual-wavelength laser (i.e., 1047 and 1053 nm) operating in both CW and pulsed regimes employing Nd:YLF cladding waveguides by further optimizing fabrication parameters and reducing the waveguide loss along the TE polarization. In addition, in order to achieve CW mode-locked Nd:YLF waveguide lasers, we will research on employing multifunctional ultra-fast SA element, such as metal nanoparticles, and efficient dispersion management techniques .
In this work, a 1047-nm Q-switched mode-locked laser, with a fundamental repetition rate up to 9.55 GHz based on a monolithic Nd:YLF waveguide platform fabricated by FsLDW, is demonstrated for the first time, to the best of our knowledge. The SA element for modulating pulses generation is provided by the novel Gr-ReS2-Gr heterostructure film, whose linear and nonlinear optical properties have been investigated. As expected, the Gr-ReS2-Gr heterostructure film exhibits the enhanced nonlinear optical responses compared with pure materials. This work indicates the promising applications of Gr-ReS2-Gr heterostructure film in broadband optical modulation and of monolithic waveguide platform defined by FsLDW in miniaturized laser sources for both integrated and ultrafast photonics.
National Natural Science Foundation of China (12074223, 61775120); Taishan Scholar Foundation of Shandong Province; China Postdoctoral Science Foundation (2020M682155).
Y. Jia acknowledges the support from “Taishan Scholars Youth Expert Program” of Shandong Province and “Qilu Young Scholar Program” of Shandong University, China. F. Chen thanks the support from “Taishan Scholars Climbing Program” of Shandong Province. The authors gratefully acknowledge Mr. Q. Lu from Shandong University, Prof. H. Yu from Shandong University, and Ms. L. Sun from Shandong Normal University for their kind help on crystal processing, optical characterization, and µ-PL analysis.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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