Proposal for an ultra-broadband polarization beam splitter using an anisotropy-engineered Mach-Zehnder interferometer on the x-cut lithium-niobate-on-insulator

Polarization beam splitter based on 2D transmissive grating

This paper introduces a two-dimensional transmissive grating polarization beam splitter (PBS) exhibiting exceptional polarization-sensitive properties with high diffraction efficiency. The optimized grating structure can concentrate the energy of TE-polarized light at the (0, ±1) orders and the energy of TM-polarized light at the (±1, 0) orders under normal incidence with a wavelength of 550nm. The polarization splitting diffraction efficiency (DE) of the grating can reach 40.17%, and the extinction ratio (ER) exceeds 18dB. This proposal marks the pioneering use of two-dimensional transmissive grating to achieve a polarization beam splitter in two perpendicular diffraction planes, presenting an innovative approach to the development of such devices. The proposed grating structure is simple, high-performing, tolerant, and applicable in a wide range of applications such as polarization imaging and high-precision two-dimensional displacement measurement.

High performance polarizer on thin-film lithium niobate with width-tapered Euler bending

In this Letter, we propose and demonstrate an integrated polarizer on thin film lithium niobate (TFLN). The polarizer consists of a width-tapered 180° Euler bending waveguide featuring thin thickness and bilevel mode convertors with silica cladding. Notably, the TE0 mode is efficiently confined in the waveguide, while the TM0 mode confronts significant bending losses. The measurements reveal that the excess loss remains below 1.5 dB, and the extinction ratio surpasses 19 dB within a working bandwidth spanning from 1480 to 1578 nm. The proposed polarizer holds considerable promise for enhancing polarization handling within TFLN photonic circuits.

Design of a broadband polarization controller based on silicon nitride-loaded thin-film lithium niobate

A novel design of a polarization controller based on "etch-less" Si3N4-loaded thin film LiNbO3 is described. Broadband operation in the spectral range between 1.45 and 1.65 µm is achieved by using a mode evolution TM/TE splitter/converter, two mode evolution 3-dB couplers, and two electro-optic phase shifters. Numerical simulations show that the on-chip insertion loss should not exceed 1 dB. A single TE-mode output can be adjusted by applying control voltages lower than 10 V for an arbitrary input polarization state.

Highly efficient and tunable polarization rotator based on lithium niobate on an insulator

The lithium niobate on an insulator (LNOI) platform has greatly advanced the development of integrated photonics recently, where efficient polarization management components are indispensable. In this work, we propose a highly efficient and tunable polarization rotator based on the LNOI platform and the low-loss optical phase change material antimony triselenide (S b ₂ S e ₃). The key polarization rotation region is formed by a LNOI waveguide with a cross section of the double trapezoidal shape and a S b ₂ S e ₃ layer deposited atop the LNOI waveguide in an asymmetrical way, where an isolating layer of silicon dioxide is sandwiched between them to reduce the material absorption loss. Based on such a structure, we have achieved the efficient polarization rotation in a length of only 17.7 µm, where the polarization conversion efficiency and insertion loss are 99.6% (99.2%) and 0.38 dB (0.4 dB) for the trans-electric (TE)-to-trans-magnetic (TM) rotation. If we further change the phase state of the S b ₂ S e ₃ layer, other polarization rotation angles besides 90° can also be obtained for the same device, revealing a tunable function. We believe that the proposed device and design scheme could offer an efficient method for realizing the polarization management on the LNOI platform.

Integrated lithium niobate polarization beam splitter based on a photonic-crystal-assisted multimode interference coupler

Lithium niobate on insulator (LNOI) is a promising platform for high-speed photonic integrated circuits (PICs) that are used for communication systems due to the excellent electro-optic properties of lithium niobate (LN). In such circuits, the high-speed electro-optical modulators and switches need to be integrated with passive circuit components that are used for routing the optical signals. Polarization beam splitters (PBSs) are one of the fundamental passive circuit components for high-speed PICs that can be used to (de)multiplex two orthogonal polarization optical modes, enabling on-chip polarization division multiplexing (PDM) systems, which are suitable for enhancing the data capacity of PICs. In this Letter, we design and experimentally demonstrate a high-performance PBS constructed by a photonic crystal (PC)-assisted multimode interference (MMI) coupler. The measured polarization extinction ratio (ER) of the fabricated device is 15 dB in the wavelength range from 1525 to 1565 nm, which makes them suitable for the high-speed and large data capacity PICs required for future communication systems.

Polarization beam splitter based on the asymmetric directional coupler of lithium niobate film

A polarization beam splitter (PBS) based on a lithium niobate film asymmetric directional coupler is proposed. The PBS is located on a lithium niobate platform on an insulator consisting of a silicon nitride-lithium niobate waveguide (SLW) and a lithium niobate waveguide (LNW). By rationally designing the SLW and LNW sizes, TE polarization satisfies the phase matching condition and TM polarization phase mismatch. The numerical simulation results show that the extinction ratio (ER) and insertion loss (IL) of PBS for TE mode are 30.57 and 0.66 dB, respectively, and the ER and IL of PBS for TM mode are 28.15 and 0.11 dB, respectively, at an operating wavelength of 1.55 µm.

High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings

A high-performance optical filter is proposed and realized with multimode waveguide grating (MWG) and two-mode multiplexers on the x-cut lithium-niobate-on-insulator (LNOI) platform for the first time, to the best of our knowledge. The present optical filter is designed appropriately to avoid material anisotropy as well as mode hybridness, and has a low excess loss of 0.05 dB and a high sidelobe suppression ratio (SLSR) of 32 dB in theory with Gaussian apodization. The fabricated filters show a box-like response with 1-dB bandwidth of 6-23 nm, excess loss of ∼0.15 dB, sidelobe suppression ratio of >26 dB. The device performance is further improved with a sidelobe suppression ratio as high as 48 dB and a low excess loss of ∼0.25 dB by cascading two identical MWGs.

Microring resonator based on polarization multiplexing for simultaneous sensing of refractive index and temperature on silicon platform

Silicon photonic integrated sensors based on microring resonators are a promising candidate to achieve high-performance on-chip sensing. In this work, a novel dual-parameters sensor based on polarization multiplexing on silicon-on-insulator (SOI) platform is proposed and demonstrated experimentally, simultaneously achieving refractive index (RI) and temperature sensing with high sensitivity and large detection range (DR). The experimental results show that the RI sensitivity and temperature sensitivity of the TM-operated sensor are 489.3 nm/RIU and 20.0 pm/°C, respectively, and that of the TE-operated sensor are 102.6 nm/RIU and 43.3 pm/°C, respectively. Moreover, the DR of the fabricated sensor is 0.0296 RIU, which is 4.2 times that of the conventional TM-operated sensor based on the microring resonator. The dual-parameters sensor based on polarization multiplexing can successfully realize the simultaneous measurement of the RI and the temperature, showing potential applications of silicon photonic on-chip sensors in reality.

Plasmonic loss-mitigating broadband adiabatic polarizing beam splitter

The intriguing analogy between quantum physics and optics has inspired the design of unconventional integrated photonics devices. In this paper, we numerically demonstrate a broadband integrated polarization beam splitter (PBS) by implementing the stimulated Raman adiabatic passage (STIRAP) technique in a three-waveguide plasmonic system. Our proposed PBS exhibits >250 nm transverse-magnetic (TM) bandwidth with <-40 dB extinction and >150 nm transverse-electric (TE) bandwidth with <-20 dB extinction, covering the entire S-, C-, and L-bands and part of the E-band. Moreover, near-lossless light transfer is achieved in our system despite the incorporation of a plasmonic hybrid waveguide because of the unique loss mitigating feature of the STIRAP scheme. Through this approach, various broadband integrated devices that were previously impossible can be realized, which will allow innovation in integrated optics.

Broadband and compact polarization beam splitter in LNOI hetero-anisotropic metamaterials

In this paper, theoretical modeling and numerical simulations of a high-performance polarization beam splitter (PBS) based on hetero-anisotropic metamaterials are proposed on the lithium-niobate-on-insulator (LNOI) platform. The hetero-anisotropic metamaterials constructed by sub-wavelength gratings (SWGs) can be regarded as effective anisotropy medium, which exhibits strong birefringence without breaking the geometrical symmetry, contributing to the formation of PBS. Rather than the principle of PBS based on beat-length difference of transverse electric (TE) polarization and transverse magnetic (TM) polarization, the device can realize polarization beam splitting in single beat length, and the footprint of the proposed PBS can be reduced to 8 µm × 160 µm (with S-bend). The simulation results show that the bandwidth is 185 nm (1450∼1634 nm) for TE polarization while the bandwidth is 85 nm (1490∼1575 nm) for TM polarization when the polarization extinction ratio is >20 dB. Furthermore, the insertion loss is less than 1 dB in the range of 1450 to 1650 nm, for both TE and TM polarization. Additionally, the proposed device proves strong robustness of the fabrication tolerance.

Polarization-independent, lithium-niobate-on-insulator directional coupler based on a combined coupling-sections design

We have proposed a polarization-independent directional coupler (DC) by using sections with optimal coupling strength designs on the lithium-niobate-on-insulator platform. With this design, arbitrary polarization-independent coupling ratio ranging from 0% to 100% can be achieved by tuning the length of the identical coupling region. The DC exhibits ultralow excess losses (<0.1dB) and polarization-dependent losses (<0.05dB for the complete coupling) over a bandwidth of 100 nm. Moreover, the proposed DC is compact in size, simple in structure, and easy to fabricate.