The polarizer is a key component for integrated photonics to deal with the strong waveguide birefringence, especially for silicon photonics. A high-performance silicon TE-pass polarizer covering all optical communication bands with low insertion loss (IL) and high polarization extinction ratio (PER) is proposed here. This polarizer is based on anisotropic subwavelength grating (SWG) metamaterials, which maintain the fundamental TE mode as a guided mode but make the fundamental TM mode leaky. Furthermore, based on this working mechanism, the proposed polarizer can work well for any upper cladding material, including air and silicon dioxide (SiO2). The numerical results show that our proposed TE-pass polarizer has a remarkable performance with IL < 0.34 dB over 420 nm (PER > 23.5 dB) or 380 nm (PER > 30 dB) for the air cladding, and IL < 0.3 dB over 420 nm (PER > 25 dB) or 320 nm (PER > 30 dB) for the SiO2 cladding. The fabricated polarizer shows IL < 0.8 dB and PER > 23 dB for the bandwidths of 1.26-1.36 µm and 1.52-1.58 µm (other bandwidths were not measured due to the limited instrument in our research center, but it still covers the most important O-band and C-band).
Generation of Kerr soliton microcombs on microresonators enables power-efficient, phase-coherent, and broadband frequency teeth generation, which has revolutionized a wide range of scientific areas such as astronomy, metrology, spectroscopy, communications, etc. However, compared with a conventional frequency scanning method that requires a complex start-up and feedback control, turnkey generation of soliton microcombs remains challenging and a more promising solution is desired. Here, we propose for the first time and numerically demonstrate that turnkey generation of soliton microcombs can be achieved on thin-film lithium niobate on insulator (LNOI) microresonators for polarization along the ordinary axis of lithium niobate (LN) for which the photorefractive (PR) effect dominates. The PR effect shows power-dependent refractive index change, which is strong and opposite to that of the Kerr effect and thermal effect, thus enables the self-routing and converge of the total pump-resonator detuning into the existence region of soliton. Our results show that initiated with a certain amount of initial pump-resonator detuning on either blue- or red-detuned side, generation of soliton microcombs can self-start, self-route, and finally get stable without any artificial frequency scanning. Moreover, we show that deterministic and turnkey generation of single soliton microcombs can be achieved by leveraging a phase-modulated pump laser. Thanks to the inherent electro-optic effect of LNOI, a lab-on-a-chip device with monolithically integrated high-speed phase modulators and high-Q microresonators is feasible.
Lithium niobate (LiNbO3, LN) is a promising material for integrated photonics due to its natural advantages. The commercialization of thin-film LN technology has revitalized this platform, enabling low-loss waveguides, micro-rings, and compact electro-optical modulators. However, the anisotropic birefringent nature of X-cut LN leads to mode hybridization of TE and TM modes, which is detrimental to most polarization-sensitive integrated optical waveguide devices. A novel structure, to the best of our knowldege, utilizing a densely packed bent waveguide array is presented in this paper to eliminate mode hybridization. The refractive index is modulated in a manner that eliminates the avoided crossing of the refractive index curves of the TE and TM fundamental modes; thus, mode hybridization is prevented. The structures are readily accessible in the full range of commercially available LN film thicknesses from 400 to 720 nm and in any etching depth. The proposed structures give a polarization extinction ratio of −30dB across all bend radii, while simultaneously maintaining low excess loss of less than −1dB after reaching a 100 µm bend radius.
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