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Abstract
We propose an electro-optic tunable optical filter based on sidewall long period waveguide grating (LPWG) in lithium niobate on insolator (LNOI). The operation of our proposed filter is based on the mode coupling, filtering, and absorption achieved, respectively, with two corrugated sidewall LPWGs, a tapered waveguide, and two metal ribbons. Our typical fabricated devices achieved a 16.32-dB rejection band and an EO tuning efficiency of ∼0.344 nm/V. Our proposed LPWG and filter are compact and could be integrated with other LNOI waveguide devices to realize more sophisticated functions for on-chip optical signal processing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Long-period grating (LPG) is an important optical device used to achieve the codirectional coupling between two core modes or between core and cladding modes [1–3]. Because such coupling is highly dependent on the operation wavelength, LPG is a wavelength sensitive device and can be used to achieve the function of sensing [4–6], optical filtering [7–17], and intensity modulation [18]. In addition, its ability to achieve the coupling between the core modes makes it also suitable for mode conversion [3,19–21]. LPGs can be realized based on optical fiber and planar optical waveguide. In comparison, LPG based on planar optical waveguide, also known as long-period waveguide grating (LPWG), attracts more attention. This is because LPWG offers not only the advantage including material, design, and fabrication flexibility and integration capability, but also the platform to incorporate thermo-optic (TO) and electro-optic (EO) effects into the devices to realize tunable or reconfigurable function. Up to now, Dozens of thermo-optically [8–14,21] and electro-optically [15,18–20] tunable [8–15] or reconfigurable [18–21] LPWGs have been demonstrated in polymer [8–12,21], sol-gel material [13], and lithium niobate (LN) [14,15,18–20]. Among them, EO devices based on LN enables high-speed tunable or reconfigurable function, showing a promising prospect in high-speed on-chip optical signal processing. Unfortunately, however, their tuning or reconfiguration voltages are as high as more than 100 V, which hinders the application of the devices. Additionally, their large footprint also limits their integration capability. The reason for the above issues is that the waveguides of these EO devices were fabricated on conventional LN wafer using titanium (Ti) diffusion or annealing proton exchange (APE) process, and these two processes can only achieve low-index-contrast LN waveguide, resulting in a large-size waveguide and a weak EO efficiency.
For the above issues, LN-on-insulator (LNOI), a newly emerging integrated optics platform in recent years, provides a promising solution. LNOI not only keeps the excellent EO, acousto-optic, and nonlinear optical properties of LN, but also enables high-index-contrast and hence high-confinement waveguide, which makes it an excellent platform for developing compact and high-performance EO devices [22,23]. Nowadays, many integrated optical devices fabricated with LNOI have been demonstrated [24–36], including passive and active grating devices [33–36], such as Bragg grating filter [33], leaky-mode LPG [34], EO tunable Bragg grating filter [35], and Bragg grating modulator [36].
In this paper, we propose and demonstrate experimentally an EO tunable LPWG filter in LNOI platform for the first time, to the best of our knowledge. The operation of our proposed filter is based on the mode coupling, filtering, and absorption achieved, respectively, with two corrugated sidewall LPWGs, a tapered waveguide, and two metal ribbons. Our typical fabricated filter achieves a 16.32-dB rejection band and an EO tuning efficiency of ∼0.344 nm/V. Our proposed LPWG filter could find applications in optical signal processing systems.
2. Structure, principle, and design
Our proposed filter is formed on an X-cut LNOI wafer with a 600-nm thick LN film and a 4.7-µm thick buried oxide. Its three-dimensional view and top view are shown schematically in Figs. 1(a) and 1(b), respectively. The device consists of a two-mode rib waveguide (TMRW) along the y direction and two corrugated LPWGs formed asymmetrically on the two sidewalls of the ridge of the TMRW. Further, the TMRW, which has a width wt and a length lt, is adiabatically tapered down at its two ends into two single-mode rib waveguides (SMRWs) with the same width ws to ensure single-mode input and output. The two LPWGs have the same corrugated period Λ, depth d, offset Λ/2, and duty cycle 50%. In addition, a set of chromium gold electrodes with gap we and length lt is located on the TMRW to provide maximum EO tuning and a layer of SiO2 buffer with thickness hc is used to separate the electrodes and the waveguide, as shown in Fig. 1(c). To simplify the device fabrication, both TMRW and SMRW have the same etch depth he. The SMRW supports the E11z and E11x modes while the TMRW supports the E11z, E11x, E21z, and E21x modes.
Fig. 1. (a) Schematic of the proposed LPWG filter, (b) top view, and (c) cross-sectional view of EO tuning region, where the inset in (a) shows the LPWGs formed asymmetrically on the two sidewalls of the ridge of the TMRW (without SiO2 buffer layer and electrodes).
Due to the large birefringence of LN, the period of the LPWGs can not meet the phase matching for both x- and z-polarized modes, simultaneously. Thus, to exploit the maximum EO coefficient γ33 of LN, the two LPWGs are designed to achieve the coupling of the E11z and E21z modes in this work. Thanks to the phase matching provided by the LPWGs, the E11z mode at resonance wavelength of the grating input at the SMRW can be coupled into the E21z mode of the TMRW when it travels along the TMRW, and subsequently, the E21z mode can be filtered out when it travels through the taper due to the leakage into the two side slab waveguides. Further, in view of the fact that the light leaked into the LNOI slab waveguide can also be guided along the y direction and may be collected by the output fiber, thus, to avoid the impact of the leaked light in the two slabs on the isolation of the filter, two metal absorption ribbons with the same length le and separation distance s from the ridge are located, respectively, on the two LNOI slabs of either sides of the output SMRW.
To design our proposed device, we first investigate the rib width ranges of the SMRW and the TMRW by calculating the dispersion characteristics of the E11z and E21z modes at 1550 nm wavelength with a commercial mode solver (COMSOL) at different he = 150, 170, and 200 nm. The effective refractive index N calculated at different he = 150, 170, and 200 nm is shown in Fig. 2, from which we choose wt = 1.7 µm and he = 150 nm to further investigate the electrode-induced absorption losses of the E11z and E21z modes of the TMRW with COMSOL. The calculated results at 1550 nm for different we at different hc = 200, 300, and 400 nm are show in Fig. 3. It can be seen that at the same conditions, the absorption loss of the E21z mode is greater than that of the E11z mode, and a thicker SiO2 buffer layer and a wider electrode gap can help to decrease the absorption loss. However, larger hc and we may result in a lower EO tuning efficiency, thus it is necessary to investigate the impact of both hc and we on the EO tuning efficiency.
Fig. 2. Dispersion characteristics of the E11z and E21z modes at 1550 nm for different rib widths at different etching depths of he = 150, 170, and 200 nm.
Fig. 3. Absorption losses of the E11z and E21z modes of the TMRW at 1550 nm for different we at different SiO2 buffer thicknesses of hc =200, 300, and 400 nm.
To achieve a strong mode coupling at resonance wavelength λ0, the grating pitch Λ must satisfy the phase-matching condition [2]:
where N11z and N21z are the effective refractive indices of the E11z and E21z modes, respectively. Under a certain tuning voltage, to achieve a large EO tuning efficiency, the change ΔN11z of the E11z mode and ΔN21z of the E21z mode introduced by EO effect should be different as far as possible, i.e., a large ΔN (= ΔN11z − ΔN21z). For this purpose, we adopt the structure shown in Fig. 1(c) where the tuning electrodes are partially placed above the TMRW. The tuning electric field distribution and power line calculated with COMSOL is shown in Fig. 4. Below the electrode gap, corresponding to the center of the LN core of the TMRW, the power line is mainly along the z direction, while below the electrodes, corresponding to the edges of the LN core, the power line is mainly along the x direction. Further, in considering that the field distribution of the E11z mode is mainly in the center of the LN core while that of the E21z mode is mainly in the edges of the LN core, it can be deduced that tuning N11z mainly utilize EO coefficient γ33 (= 30.8 pm/V), while tuning N21z mainly utilize EO coefficient γ 13 (= 8.6 pm/V). Obviously, by optimizing the parameters of the electrodes and the TMRW to fully utilize the differences in EO coefficients, the mode field distributions, and the power line mentioned above, a maximum ΔN and, hence, a maximum EO tuning efficiency can be achieved.Based on the above analysis, we next investigate the impact of we, he, and wt on ΔN at 1550 nm wavelength with an applied EO tuning voltage of 20 V for different hc of 200, 300, and 400 nm, respectively. The calculated results are shown in Figs. 5(a)–5(c). From Figs. 5(a) and 5(b), ΔN decrease slowly and linearly with an increase in we or he. While from Fig. 5(c), ΔN decreases rapidly when wt increases from 1.4 µm to 2.1 µm. Additionally, Figs. 5(a)–5(c) indicate that the smaller hc, the larger ΔN. However, from Fig. 4, the smaller hc, the larger the absorption loss. Therefore, to balance the absorption loss and the EO tuning efficiency, we choose hc = 300 nm, wt = 1.7 µm, he = 150 nm, and we = 1.0 µm, correspondingly, ΔN = 0.0002794, and from Fig. 3 the absorption loss of the E21z and E11z modes are 3.78 dB/mm and 1.49 dB/mm, respectively.
Fig. 5. Variations of ΔN with (a) we (wt = 1.7 µm, he =150 nm), (b) he (wt = 1.7 µm, we = 1.0 µm), and (c) wt (we = 1.0 µm, he = 150 nm) at different hc of 200, 300, and 400 nm.
With the above structural parameters of the LNOI TMRW, the field distributions of the E11z and E21z modes of the TMRW are calculated with COMSOL at 1550 nm wavelength and shown in Figs. 6(a) and 6(b), respectively. The corresponding effective refractive indices of the E11z and E21z modes are 1.9485 and 1.9063, respectively. The grating pitch Λ, calculated with Eq. (1), is 36.7 µm at λ0 = 1550 nm. Further, using Δλ0 = ΛΔN and the above ΔN, the EO tuning efficiency of our designed filter can be calculated to be 0.51 nm/V. The identical length le of the two metal absorption ribbons is set to 500 µm, corresponding to an absorption loss of >30 dB, calculated with COMSOL. Meanwhile, to avoid the absorption for the E11z mode in the SMRW induced by the two metal ribbons, the space s is set to 2 µm.
Fig. 6. Calculated field distributions of (a) the E11z and (b) E21z modes of the LNOI TMRW at 1550 nm wavelength superimposed over the waveguide
The grating length is obtained by calculating the transmission spectra of the LPWGs with a grating analysis tool (GratingMOD, Rsoft). To reduce the propagation loss, the grating depth d is set to 150 nm. The grating length is calculated to be 661 µm (18 periods). Figure 7 shows the calculated transmission spectra. The maximum contrast of the rejection band is larger than 28 dB at 1550 nm, indicating that the E11z mode is almost totally converted to the E21z mode. To verify this result, we then simulate the light propagation when the E11z mode at 1550 nm was launched into the designed filter with a three-dimensional finite-difference beam propagation method (3DFD-BPM) (BeamPROP, RSoft). For comparison, we do simulation for two cases of with and without metal absorption ribbons, and the results are shown in Figs. 8(a) and 8(b), respectively. For the case without metal absorption ribbons, the E11z mode converts to the E21z mode in the TMRW, leaks into the two side slabs in the taper, then propagates along the two slabs, as shown in Fig. 8(a). While for the case with metal absorption ribbons, the E11z mode also converts to the E21z mode in the TMRW and leaks into the two side slabs in the taper, but the leaked lights in the two slabs are almost completely absorbed by the two metal ribbons, as shown in Fig. 8(b).
Fig. 8. Simulated light propagation when the E11z mode at 1550 nm was launched into the designed filter (a) without metal absorption ribbons, and (b) with metal absorption ribbons.
3. Device fabrication and characterization
We fabricated the proposed devices following our designed parameters as closely as possible with our in-house microfabrication facilities. Firstly, a 150-nm-thick chromium (Cr) was deposited on a commercial x-cut LNOI wafer. Subsequently, the designed waveguide and the sidewall LPWG patterns were defined on the Cr film by the standard ultraviolet (UV) photolithography and wet etching processes, simultaneously. The patterns were then transferred into the LNOI by the proton-exchange process assisted dry etching method [30]. The measured etch depth was 155 nm, which is close to the design value (150 nm). After the residual Cr film was removed with a dechroming solution, a ∼300-nm-thick SiO2 buffer layer was deposited on the chip by plasma enhanced chemical vapor deposition (PECVD) process. Finally, both input and output facets of the sample were polished carefully, and then the spectral response and the output near-field patterns of the fabricated filters were investigated. For comparison, we did measurement before and after the tuning electrodes and the absorption ribbons were fabricated, respectively. Considering the unavoidable fabrication errors, we designed and fabricated a dozen filters with the same TMRW width but somewhat different grating periods on a single LNOI chip. To facilitate the polishing and subsequent packaging, the chip was cut into several small chips. We measured all filters fabricated successfully on these small chips, and then chose the best one (Λ = 36.0 µm) to fabricate chromium gold tuning electrode and absorption ribbons on it, simultaneously, through electron beam evaporation, lithography with high-precision alignment, and electroplating processes. A microscopic image of the chip with the chromium gold electrodes and absorption ribbons is shown in Fig. 9(a). The total length of the chip was ∼4.0 mm. Microscopic image of partial LPWGs and tuning electrodes are also shown in Figs. 9(b) and 9(c), respectively.
Fig. 9. Microscopic images of (a) our fabricated filter chip with the chromium gold electrodes and absorption ribbons, (b) partial LPWGs, and (c) partial tuning electrodes.
To investigate the spectral response of the fabricated filters, light from an amplified spontaneous emission (ASE) source (B&A, AS4600, 1525-1605 nm) was launched into the input port of the filter under test with an ultra-high numerical aperture fiber (UHNAF). The mode field diameter of the UHNAF used here is about 3.2 µm. The polarization state of the input light was controlled with an in-line fiber polarizer and a polarization controller (PC) so as to excite the E11z mode and E11x mode in the input SMRW, respectively. The output light from the filter was collected with another UHNAF and monitored with an optical spectrum analyzer (OSA, Yokogawa AQ6370D). Before and after the electrodes and the absorption ribbons were fabricated, the transmission spectra of our fabricated best filter, measured for both z- and x-polarized input lights at 25°C and normalized to the direct fiber-to-fiber transmission spectrum, are shown in Figs. 10(a) and 10(b), respectively. Before the electrodes and the absorption ribbons were fabricated, as shown in Fig. 10(a), the transmission spectrum of the z-polarized input light exhibits a distinct rejection band with a maximum contrast of 13.06 dB at the center wavelength 1584.8 nm, while the spectrum of the x-polarized input light exhibits no rejection band. After the electrodes and the absorption ribbons were fabricated, as shown in Fig. 10(b), the transmission spectrum of the z-polarized input light still exhibits a distinct rejection band with a maximum contrast of 16.32 dB at the center wavelength1587.1 nm, while the transmission spectrum of the x-polarized input light still exhibits no any rejection band. Obviously, there is an improvement of ∼ 3.26 dB in contrast of the rejection band due to the absorption of the metal ribbons for the leaked light in the slabs. Meanwhile, the center wavelength of the rejection band redshifts from 1584.8 nm to 1587.1 nm. We speculate this is because the stress induced by the metal electrodes results in a slight change in the refractive indices of the SiO2 buffer and the LN core. In addition, From Figs. 10(a) and 10(b), the minimum insertion loss increases from ∼11.3 dB before fabricating the electrodes to ∼13.8 dB after fabricating the electrodes, an increase of ∼2.5 dB due to the absorption induced by the metal electrodes. It should be pointed out that large insertion loss here is mainly contributed by the fiber-waveguide butt-coupling losses at the two ends, which could be significantly reduced by incorporating edge couplers [23,24] in our design
Fig. 10. Transmission spectra measured for the filer with optimal performance, (a) before and (b) after the tuning electrodes and the metal absorption ribbons were fabricated.
To confirm, the rejection band in Figs. 10(a) and 10(b) are caused by the light coupling from the E11z mode to the E21z mode, we also measured the output near-field patterns of the device with a tunable laser and an infrared camera around the rejection band for the z-polarized input light. We also did measurement before and after the electrodes and the metal ribbons were fabricated, and correspondingly, the captured images for these two cases are shown, respectively, in Figs. 11(a) and 11(b). For the case without the electrodes and the metal ribbons, as shown in Fig. 11(a), the output pattern weakens obviously at 1582.5 nm and disappears almost at the resonance wavelength of 1584.8 nm, indicating a partial and a complete coupling between the E11z and E21z modes. Meanwhile, there are two distinct patterns output from the LNOI slab at the resonance wavelength of 1584.8 nm. While for the case with the electrodes and the metal ribbons, as shown in Fig. 11(b), the output pattern weakens gradually when operation wavelength changes from 1584.7 nm to 1586.5 nm, disappears completely at the resonance wavelength of 1587.1 nm, and reappears faintly at 1588.8 nm. The above changes in the output near-field pattern also indicate that partial or complete coupling between the E11z and E21z modes can be achieved after the electrodes and the metal ribbons were fabricated, even if there is a change in resonance wavelength. Additionally, there is no any output from the LNOI slab due to the existence of the metal absorption ribbons, as expected. The above mode coupling characteristics are completely consistent with the results shown in Figs. 10(a) and 10(b), indicating that the rejection band in the transmission of the filter is caused by the coupling between the E11z and the E21z modes.
Fig. 11. Output near-field patterns taken at the output SMRW of the filter when the E11z mode at different wavelengths is excited at the input SMRW of the filter, (a) before and (b) after the tuning electrodes and the metal absorption ribbons were fabricated.
To facilitate the investigation of the EO and TO tunability of the device, we further packaged the filter with optimal performance. The two ends of the filter were coupled first with the above UHNAF using UV glue. Then, the chip was placed in a simple packaging box and the device electrode pads were bonded with gold wire to the pin of the packaging box. We then investigated the EO tuning characteristics of the packaged filter with the above ASE light source and OSA by applying different positive or negative tuning voltages to the tuning electrodes, where a positive (negative) voltage means that the voltage between the up and the down electrode is positive (negative) (see Fig. 1(b)).
The normalized transmission spectra measured at 25°C are shown in Fig. 12(a). As the tuning voltage increases from 0 V to 20 V, the central wavelength of the rejection band in the transmission spectrum redshifts from 1587.1 nm to 1593.7 nm with the contrast decreasing from 16.32 dB to 13.68 dB. On the other hand, as the tuning voltage decreases from 0 V to −13 V, the center wavelength blueshifts from 1587.1 nm to 1582.8 nm with the contrast decreasing from 16.32 dB to 11.66 dB and then increasing to 15.22 dB. The central wavelengths of the rejection band vary linearly with the tuning voltage at a rate of ∼0.344 nm/V, as shown in Fig. 12(b), but corresponding changes in the contrast are nonlinear, as shown in Fig. 12(a).
Fig. 12. (a) Normalized transmission spectra of the packaged filter measured at different tuning voltages from −13 V- 20 V and (b) variation of the center wavelength with the tuning voltage together with the linear fitting line.
We also measured the effects of temperature variations on the performance of the packaged filter by investigating the shift of the center wavelength of the rejection band with the change of the ambient temperature. The normalized transmission spectra measured at different ambient temperature from 25°C to 55°C are shown in Fig. 13(a). As the ambient temperature changes from 25°C to 55°C, the center wavelength of the rejection band blueshifts linearly from 1587.1 nm to 1582.9 nm at a rate of ∼0.137 nm/°C, as shown in Fig. 13(b).
Fig. 13. (a) Normalized transmission spectra of the packaged filter measured at different temperature from 25°C to 55°C and (b) variation of the center wavelength with the temperature together with the linear fitting line.
4. Conclusion
We have designed, fabricated, and characterized a compact and high-performance EO tunable filter based on sidewall LPWGs in LNOI. Our fabricated filter, which employs two metal ribbons to absorb the leaked light in the LNOI, achieves a contrast of 16.32 dB with a EO tuning of ∼0.344 nm/V. Our proposed LPWG and filter are compact and could be integrated with other LNOI waveguide devices to realize more sophisticated functions for on-chip optical signal processing.
Funding
National Natural Science Foundation of China (62075027, U20A20165); Key Technology R&D Program of Shenzhen (JSGG20210802154413040); National Key Research and Development Program of China (2021YFB2800104); Key R&D Project of Science and Technology Department of Sichuan Province (2023YFS0122); Health Research Project for Cadres of Sichuan Province (2022-204); Sichuan Science and Technology Program (2020JZYZF0001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogab, and J. E. Sipe, “Long-period fiber gratings as band-rejection filters,” J. Lightwave Technol. 14(1), 58–65 (1996). [CrossRef]
2. V. Rastogi and K. S. Chiang, “Long-period gratings in planar optical waveguides,” Appl. Opt. 41(30), 6351–6355 (2002). [CrossRef]
3. W. Wang, J. Wu, K. Chen, W. Jin, and K. S. Chiang, “Ultra-broadband mode converters based on length-apodized long-period waveguide gratings,” Opt. Express 25(13), 14341–14350 (2017). [CrossRef]
4. F. Esposito, A. Srivastava, L. Sansone, M. Giordano, S. Campopiano, and A. Ladicicco, “Label-free biosensors based on long period fiber gratings: a review,” IEEE Sensors J. 21(11), 12692–12705 (2021). [CrossRef]
5. Q. Liu and K. S. Chiang, “Refractive-index sensor based on long-range surface plasmon mode excitation with long-period waveguide grating,” Opt. Express 17(10), 7933–7942 (2009). [CrossRef]
6. C. Deleau, H. C. Seat, O. Bernal, and F. Surre, “High-sensitivity integrated SiN rib-waveguide long period grating refractometer,” Photonics Res. 10(2), 564–573 (2022). [CrossRef]
7. C. Y. Lin and L. A. Wang, “A wavelength- and loss-tunable band-rejection filter based on corrugated long-period fiber grating,” IEEE Photonics Technol. Lett. 13(4), 332–334 (2001). [CrossRef]
8. Y. Chu, K. S. Chiang, and Q. Liu, “Widely tunable optical bandpass filter by use of polymer long-period waveguide gratings,” Appl. Opt. 45(12), 2755–2760 (2006). [CrossRef]
9. K. S. Chiang, C. K. Chow, H. P. Chan, Q. Liu, and K. P. Lor, “Widely tunable polymer long-period waveguide grating with polarization-insensitive resonance wavelength,” Electron. Lett. 40(7), 422–424 (2004). [CrossRef]
10. M. S. Kwon and S. Y. Shin, “Characteristics of polymer waveguide notch filters using thermooptic long-period gratings,” IEEE J. Sel. Top. Quantum Electron 11(1), 190–196 (2005). [CrossRef]
11. M. S. Kwon and S. Y. Shin, “Tunable polymer waveguide notch filter using a thermooptic long-period grating,” IEEE Photonics Technol. Lett. 17(1), 145–147 (2005). [CrossRef]
12. K. S. Chiang, C. K. Chow, Q. Liu, H. P. Chan, and K. P. Lor, “Band-rejection filter with widely tunable center wavelength and contrast using metal long-period grating on polymer waveguide,” IEEE Photonics Technol. Lett. 18(9), 1109–1111 (2006). [CrossRef]
13. A. Moujoud, H. J. Kim, S. H. Kang, G. J. Oh, W. S. Kim, B. S. Bae, and S. Y. Shin, “Double component long period waveguide grating filter in sol-gel material,” Opt. Express 15(23), 15147–15153 (2007). [CrossRef]
14. W. Jin, K. S. Chiang, and Q. Liu, “Thermally tunable lithium-niobate long-period waveguide grating filter fabricated by reactive ion etching,” Opt. Lett. 35(4), 484–486 (2010). [CrossRef]
15. D. Zhang, J. Kang, W. H. Wong, D. Yu, and E. Y. B. Pun, “Electro-optically tunable super-broadband filter based on long period grating in Ti:LiNbO3 waveguide,” Opt. Lett. 40(20), 4715–4718 (2015). [CrossRef]
16. Y. B. Cho, B. K. Yang, J. H. Lee, J. B. Yoon, and S. Y. Shin, “Silicon photonic wire filter using asymmetric sidewall long-period waveguide grating in a two-mode waveguide,” IEEE Photonics Technol. Lett. 20(7), 520–522 (2008). [CrossRef]
17. Q. Liu, Z. Gu, J. S. Kee, and M. K. Park, “Silicon waveguide filter based on cladding modulated anti-symmetric long-period grating,” Opt. Express 22(24), 29954–29963 (2014). [CrossRef]
18. W. Jin, K. S. Chiang, and Q. Liu, “Electro-optic long-period waveguide gratings in lithium niobate,” Opt. Express 16(25), 20409–20417 (2008). [CrossRef]
19. W. Jin and K. S. Chiang, “Mode switch based on electro-optic long-period waveguide grating in lithium niobate,” Opt. Lett. 40(2), 237–240 (2015). [CrossRef]
20. W. Jin and K. S. Chiang, “Reconfigurable three-mode converter based on cascaded electro-optic long-period gratings,” IEEE J. Sel. Top. Quantum Electron. 26(5), 1–6 (2020). [CrossRef]
21. W. Zhao, J. Feng, K. Chen, and K. S. Chiang, “Reconfigurable broadband mode (de)multiplexer based on integrated thermally-induced long-period grating and asymmetric Y-junction,” Opt. Lett. 43(9), 2082–2085 (2018). [CrossRef]
22. A. Boes, B. Corcoran, L. Chang, J. Bowers, and A. Mitchell, “Status and potential of lithium niobate on insulator (LNOI) for photonic integrated circuits,” Laser Photonics Rev. 12(4), 1700256 (2018). [CrossRef]
23. D. Zhu, L. Shao, M. Yu, R. Cheng, B. Desiatov, C. Xin, Y. Hu, J. Holzgrafe, S. Ghosh, A. S. Ansari, E. Puma, N. Sinclair, C. Reimer, M. Zhang, and M. Lončar, “Integrated photonics on thin-film lithium niobate,” Adv. Opt. Photonics 13(2), 242–352 (2021). [CrossRef]
24. M. Zhang, C. Wang, P. Kharel, D. Zhu, and M. Lončar, “Integrated lithium niobate electro-optic modulators: when performance meets scalability,” Optica 8(5), 652–667 (2021). [CrossRef]
25. M. Wang, J. Li, H. Yao, X. Li, J. Wu, K. S. Chiang, and K. Chen, “Thin-film lithium-niobate modulator with a combined passive bias and thermo-optic bias,” Opt. Express 30(22), 39706–39715 (2022). [CrossRef]
26. C. Li, B. Chen, Z. Ruan, H. Wu, Y. Zhou, J. Liu, P. Chen, K. Chen, C. Guo, and A. L. Liu, “High modulation efficiency and large bandwidth thin-film lithium niobate modulator for visible light,” Opt. Express 30(20), 36394–36402 (2022). [CrossRef]
27. M. Xu and X. Cai, “Advances in ultra-wideband LiNbO3 thin-film modulators,” in Optical Fiber Communication Conference (Optica Publishing Group, 2023), paper Tu3C.1.
28. M. Zhang, C. Wang, R. Cheng, A. S. Ansari, and M. Lončar, “Monolithic ultra-high-Q lithium niobate microring resonator,” Optica 4(12), 1536–1537 (2017). [CrossRef]
29. T. J. Wang, G. L. Peng, M. Y. Chan, and C. H. Chen, “On-Chip optical microresonators with high electro-optic tuning efficiency,” J. Lightwave Technol. 38(7), 1851–1857 (2020). [CrossRef]
30. X. Li, K. Chen, and L. Wang, “Compact and electro-optic tunable interleaver in lithium niobate thin film,” Opt. Lett. 43(15), 3610–3613 (2018). [CrossRef]
31. M. Zhang, K. Chen, M. Wang, J. Wu, and K. S. Chiang, “Electro-optic reconfigurable two-mode (de)multiplexer on thin-film lithium niobate,” Opt. Lett. 46(5), 1001–1004 (2021). [CrossRef]
32. L. Song, J. Chen, R. Wu, Y. Zheng, Z. Liu, G. Wang, C. Sun, M. Wang, and Y. Cheng, “Electro-optically tunable optical delay line with a continuous tuning range of ∼220 fs in thin-film lithium niobate,” Opt. Lett. 48(9), 2261–2264 (2023). [CrossRef]
33. M. A. Baghban, J. Schollhammer, C. E. Herranz, K. B. Gylfason, and K. Gallo, “Bragg gratings in thin-film LiNbO3 waveguides,” Opt. Express 25(26), 32323–32332 (2017). [CrossRef]
34. W. Jin and K. S. Chiang, “Leaky-mode long-period grating on a lithium-niobate-on-insulator waveguide,” Optica 8(12), 1624–1631 (2021). [CrossRef]
35. K. Abdelsalam, E. Ordouie, M. G. Vazimali, F. A. Juneghani, P. Kumar, G. S. Kanter, and S. Fathpour, “Tunable dual-channel ultra-narrowband Bragg grating filter on thin-film lithium niobate,” Opt. Lett. 46(11), 2730–2733 (2021). [CrossRef]
36. D. Pohl, A. Messner, M. R. Escalé, J. Holzer, J. Leuthold, and R. Grange, “100-GBd Waveguide bragg grating modulator in thin-film lithium niobate,” IEEE Photonics Technol. Lett. 33(2), 85–88 (2021). [CrossRef]
