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2021, 41(8): 0823013
Nonlinear Optics

We use the nonlinear coupled-mode theory to theoretically investigate second-harmonic generation (SHG) in subwavelength x-cut and z-cut lithium niobate (LN) thin-film waveguides and derive the analytical formula to calculate SHG efficiency in x-cut and z-cut LN thin-film waveguides explicitly. Under the scheme of optimal modal phase matching (MPM), two types of LN thin films can achieve highly efficient frequency doubling of a 1064 nm laser with a comparable conversion efficiency due to very consistent modal field distribution of the fundamental wave and second-harmonic wave with efficient overlap between them. Such a robust MPM for high-efficiency SHG in both the subwavelength x-cut and z-cut LN thin-film waveguides is further confirmed in a broad wavelength range, which might facilitate design and application of micro–nano nonlinear optical devices based on the subwavelength LN thin film.
Chinese Optics Letters
2021, 19(7): 071902
Special Issue on Lithium Niobate Based Photonic Devices

We theoretically propose a hybrid lithium niobate (LN) thin-film waveguide that consists of an amorphous silicon stripe and etch-free z-cut LN for highly efficient wavelength conversion, circumventing the challenging etching on LN material. Profiting from the spatial symmetry breaking of the waveguide, the asymmetric hybrid modes can spontaneously achieve phase matching with small modal area and large spatial mode overlap, enabling enhanced second harmonic generation with a normalized conversion efficiency over $3900% W-1·cm-2$ (0.5-mm-long propagation distance). The choice of integrating silicon with LN alleviates the fabrication challenge, making the platform potentially compatible with silicon photonics.
Chinese Optics Letters
2021, 19(6): 060004
Early Posting

High-<i>Q</i> lithium niobate (LN) optical micro-resonators are excellent platform for future applications in optical communications, nonlinear optics and quantum optics. To date, high <i>Q</i> factors are typically achieved in LN using either dielectric masks or femtosecond laser ablation, while the more standard and commonly available lift-off metallic masks are often believed to lead to rough sidewalls and lowered <i>Q</i> factors. Here we show that LN microring resonators with strong light confinement and intrinsic <i>Q</i> factors over 1 million can be fabricated using optimized lift-off metallic masks and dry etching processes, corresponding to a waveguide propagation loss of ~ 0.3 dB/cm. The entire process is fully compatible with wafer-scale production and could be transferred to other photonic materials.
Chinese Optics Letters
2021, 19(6):
Special Issue on Lithium Niobate Based Photonic Devices

We demonstrate high-quality (intrinsic Q factor ～2.8 × 106) racetrack microresonators fabricated on lithium niobate thin film with a free spectral range (FSR) of ～86 pm. By integrating microelectrodes alongside the two straight arms of the racetrack resonator, the resonance wavelength around 1550 nm can be red shifted by 92 pm when the electric voltage is raised from -100 V to 100 V. The microresonators with the tuning range spanning over a full FSR are fabricated using photolithography assisted chemo-mechanical etching.
Chinese Optics Letters
2021, 19(6): 060002
Special Issue on Lithium Niobate Based Photonic Devices

Integrated traveling-wave lithium niobate modulators need relatively large device lengths to achieve low drive voltage. To increase modulation efficiency within a compact footprint, we report an integrated Fabry–Perot-type electro-optic thin film lithium niobate on insulator modulator comprising a phase modulation region sandwiched between two distributed Bragg reflectors. The device exhibits low optical loss and a high tuning efficiency of 15.7 pm/V. We also confirm the modulator’s high-speed modulation performance by non-return-to-zero modulation with a data rate up to 56 Gbit/s.
Chinese Optics Letters
2021, 19(6): 060003
Early Posting

We report on the fabrication and optimum of lithium niobate planar and ridge waveguide at the wavelength of 633 nm. To obtain a planar waveguide, oxygen ions at the energy of 3.0 MeV with a fluence of 1.5×1015 ions/cm2 are implanted in the polished face of LiNbO3 crystals. For planar waveguide, a loss of 0.5 dB/cm is obtained after annealing at 300 oC for 30minutes. The ridge waveguide is fabricated by diamond blade dicing method on optimized planar waveguide. The guiding properties are investigated by prism coupling and end-face coupling method.
Chinese Optics Letters
2021, 19(6):
Early Posting

Periodically poled lithium niobate on insulator (PPLNOI) ridge waveguides are desirable for high-efficiency nonlinear frequency conversions, and the fabrication process of such waveguides is crucial for the device performance. In this work, we report fabrication and characterization of local periodically poled ridge waveguide. Ridge waveguides were fabricated by dry etching, then the high-voltage pulses were applied to locally poled the ridge waveguides. Second harmonic generation with a normalized conversion efficiency of 435.5%W-1cm-2 was obtained in the PPLNOI ridge waveguide, which was consistent with the triangular domain structure revealed by the confocal microscopy.
Chinese Optics Letters
2021, 19(6):
Special Issue on Lithium Noibate Based Photonic Devices

We propose and demonstrate a polarization diversity two-dimensional grating coupler based on the lithium niobate on insulator platform, for the first time, to the best of our knowledge. The optimization design, performance characteristics, and fabrication tolerance of the two-dimensional grating coupler are thoroughly analyzed utilizing the three-dimensional finite-difference time-domain method. Experimentally, $-7.2 dB$ of coupling efficiency is achieved with 1 dB bandwidth of 64 nm. The polarization-dependent loss is about 0.4 dB around 1550 nm. Our work provides new polarization multiplexing approaches for the lithium niobate on insulator platform, paving the way for critical applications such as high-speed polarization multiplexed electro-optical modulators.
Chinese Optics Letters
2021, 19(6): 060006
Research Articles

Dual-chirped difference frequency generation (DFG) is an advantageous technique for generating the broadband mid-infrared (IR) idler wave, which is inaccessible by a population-inversion-based laser system. In principle, the generated idler wave may even suffer a spectrum broadening compared with the driving pulsed lasers if the pump and signal waves are oppositely chirped. However, broadband phase-matching is always the determining factor for the resulting efficiency and the bandwidth of the generated idler wave. In this study, specific to an oppositely dual-chirped DFG scheme, we derive the precondition to realize broadband frequency conversion, wherein a negative $(1/\unicode[STIX]{x1D710}_{p}-1/\unicode[STIX]{x1D710}_{i})/(1/\unicode[STIX]{x1D710}_{s}-1/\unicode[STIX]{x1D710}_{i})$, in terms of the correlation coefficient of the group velocity ($\unicode[STIX]{x1D70E}$), is necessary. However, most birefringence bulk crystals can only provide the required material dispersions in limited spectral regions. We show that the periodically poled lithium niobate crystal that satisfies an inactive Type-II (eo-o) quasi-phase-matching condition has a stable negative $\unicode[STIX]{x1D70E}$ and exerts the expected broadband gain characteristic across an ultra-broad idler spectral region $(1.7{-}4.0~\unicode[STIX]{x03BC}\text{m})$. Finally, we propose and numerically verify a promising DFG configuration to construct a tunable mid-IR spectrum broader based on the broadband phase-matched oppositely dual-chirped DFG scheme.
High Power Laser Science and Engineering
2020, 8(2): e27
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