32 dB modal gain in Er:LiNbO3 waveguide amplifiers with Er,Yb:Al2O3 cladding layer via bidirectional pumping scheme

<p indent="0mm">Lithium niobate on insulator (LNOI) is considered one of the most competitive integrated photonics platforms due to the excellent optical properties of lithium niobate crystal and their compatibility with semiconductor micro/nano processing. Based on the continuous improvement of micro-nano fabrication technology, various transmission and control devices, such as low-loss waveguides and high-quality factor micro-cavities, showing better performance than traditional bulk materials devices, have been realized on LNOI. However, as an essential part of integrated photonics, the research of LNOI laser has made a breakthrough recently. Lithium niobate is an indirect bandgap material and thus tough for electroluminescence. It is a feasible and straightforward scheme to realize optically pumped laser based on rare-earth ion doping. This paper summarizes our research group’s recent research progress on realizing microdisk, microring, and single-mode lasers based on rare-earth-ion-doped LNOI and related works from other groups. We hope this review could benefit the related research on the active devices on LNOI.

Thin‐film lithium niobate on insulator (LNOI) emerges as a promising platform for integrated quantum photon source, enabling scalable on‐chip quantum information processing. The most popular technique to overcome the phase mismatching between interacting waves in waveguide is periodic poling, which is intrinsically sensitive to poling uniformity. Here, an alternative strategy to offset the phase mismatching of spontaneous parametric down‐conversion (SPDC) process, so‐called modal phase matching, in a straight waveguide fabricated on a dual‐layer LNOI is reported. The dual‐layer LNOI consists of two 300 nm lithium niobates with opposite directions, which significantly enhances the spatial overlap between fundamental and high‐order modes and thus enables efficient SPDC. This dual‐layer waveguide generates photon pairs with pair generation rate of 41.77 GHz , which exhibits excellent signal‐to‐noise performance with coincidence‐to‐accidental ratio up to 58298 1297. Moreover, a heralded single‐photon source with second‐order autocorrelation and heralded rate exceeding 100 kHz is observed. The results provide an experiment‐friendly approach for efficient generation of quantum photon sources and benefit the on‐chip quantum information processing based on LNOI.

Mode hybridization phenomenon in air-cladded X-cut Y-propagating and Z-propagating thin film lithium niobate strip multimode waveguides is numerically studied and a mathematical relation between structural parameters leading to hybrid modes is formulated. Dependence of hybrid modes on waveguide dimensions, sidewall angles and wavelength is also analyzed. The results obtained are used to design lithium niobate on insulator (LNOI) taper for converting fundamental TM mode to higher order TE mode, and an optimum length for achieving a high conversion efficiency of 99.5% is evaluated. Birefringent Y-propagating LN and isotropic Z-propagating LN tapers are compared in terms of length, figures of merit, and fabrication tolerance. Tapers exhibit a broad bandwidth of 200 nm with an extinction ratio less than − 18 dB. The results of mode hybridization analysis are useful in design optimization of adiabatic tapers, tunable time delays, optical interconnects, mode converters and demultiplexers for mode division multiplexing (MDM) applications.

New effective technique to produce waveguides in lithium niobate on insulator (LNOI)

In this contribution, we investigate second harmonic generation (SHG) in periodically poled lithium niobate (LN) on insulator waveguides and examine under what conditions such waveguides suffer from undesirable loss due to lateral leakage. We investigate the lateral leakage losses in X-cut and Z-cut LN for the fundamental (1550 nm) and second harmonic (775 nm) wavelengths. Our findings show that Z-cut lithium niobate on insulator (LNOI) is more likely to suffer from lateral leakage and has a lower SHG efficiency. We further provide design guidelines for highly efficient nonlinear optical waveguides in LNOI and show how lateral leakage can be avoided.

We report the fabrication of crystalline microresonators of high quality (Q) factors with a controllable wedge angle on lithium niobate on insulator (LNOI). Our technique relies on a femtosecond laser assisted chemo-mechanical polish, which allows us to achieve ultrahigh surface smoothness as critically demanded by high Q microresonator applications. We show that by refining the polish parameters, Q factors as high as 4.7 × 107 can be obtained and the wedge angle of the LNOI can be continuously tuned from 9° to 51°.

Compact and low loss magneto-optical waveguide isolator for TE mode based on lithium niobate on insulator

In this Letter, we propose and demonstrate a low-loss and polarization-insensitive edge coupler based on an etched facet double-tip inverse taper structure in lithium niobate on insulator (LNOI) across the C- and L-bands. Using a simple dry etching process, we have obtained a flat edge facet for fiber coupling, eliminating the need for chip edge polishing. The fabricated edge coupler exhibits a fiber-to-chip coupling loss of 1.28/0.67 dB per facet for TE/TM mode at 1550 nm. The measured coupling loss is lower than 1.5/1.0 dB per facet for TE/TM mode, and the polarization-dependent loss is below 0.85 dB over the 1530-1630 nm wavelength range. This scheme is suitable for wafer-scale production and provides a cost-effective and efficient solution for fiber-to-chip coupling, which will find applications in the LNOI photonic integrated circuits.

(De)Multiplexing Technologies In article number 2100529, Yonghui Tian, Arnan Mitchell, Yikai Su, and co-workers experimentally demonstrated mode and polarization-division multiplexing on a thin-film lithium niobate on insulator (LNOI) platform. By introducing silicon nitride as a loading material atop the LNOI photonics chip, the devices can be integrated with high-speed electro-optic modulators to achieve high-capacity and low-cost photonic integrated circuits suitable for data communication applications, while avoiding the direct etching of lithium niobate.

We numerically demonstrate that lithium niobate on insulator (LNOI) waveguides may support confined short-wavelength surface acoustic waves that interact strongly with optical fields through backward-stimulated Brillouin scattering in both Z- and X-cut orientation. We conduct fully anisotropic simulations that consider not only moving boundary and photoelastic forces, but also roto-optic forces for the Brillouin interaction. Our results indicate that photoelasticity dominates the Brillouin gain and can reach as high as G B /Q m =0.43W−1m−1 in standard ridge waveguides.

We propose a hybrid photonic platform based on chalcogenide glass (GeSbSe) integrated with lithium niobate on insulator (LNOI), providing enhanced performance and compactness for integrated photonics. Key photonic components, including grating couplers (GCs), micro-ring resonators, multimode interference (MMI), and the Mach–Zehnder interferometer (MZI), are designed and fabricated on this platform. The micro-ring resonator achieves an intrinsic quality factor of 72,000 and a propagation loss of ∼3 dB/cm. The multimode interference couplers demonstrate minimal excess loss, while the MZI exhibits an extinction ratio exceeding 30 dB over a bandwidth of 50 nm. This hybrid platform, leveraging the unique optical properties of GeSbSe and LNOI, offers scalability and low-loss photonic integrated circuits, with promising applications in high-speed optical communications and signal processing.

Cooperative upconversion effects on the characteristics of Er3+ doped and Er3+/Yb3+ co-doped waveguide amplifiers

Lithium niobate on insulator (LNOI) electro-optic modulator has broad application prospects in rapidly developing largecapacity and high-density coherent optical communication systems, due to its small size, ultra-high bandwidth and low voltage. However, the high coupling loss currently limits its practical application. In this paper, in order to realize efficient coupling property, we propose a high-coupling-efficiency bilayer inverse tapered spot-size converter (SSC) that can be integrated with an LNOI modulator. The spot-size converter consists of three parts: a double-layer inverse tapered waveguide in LNOI, a straight LNOI-based waveguide and a SiON waveguide. We mainly consider end-surface coupling efficiency, abrupt loss and transmission efficiency. The simulation results show that the total coupling efficiency of the chip to the lensed fiber with a mode-spot diameter of 3.2 µm can achieve to 97.51%/96.3% for TE/TM light at a wavelength of 1550 nm. The coupler shows high coupling efficiency and good polarization independence. The 3dB alignment tolerance between fiber and coupler is estimated as &plusmn;1.4 &mu;m/&plusmn;1.4 &mu;m in X/Z direction for TE light, and &plusmn;1.45 &mu;m/&plusmn;1.3 &mu;m in X/Z direction for TM light. Our coupler shows relatively large alignment tolerances that makes the optical packaging of the LNOI modulator more reliable. The proposed high-efficiency spot-size converter in this work is of great significance for realizing the practical application of LNOI modulators.

Lithium niobate on insulator (LNOI), with its excellent electro-optic properties, high transparency, and stability, has been widely used in high-speed photonic integrated circuits (PICs). Waveguide crossings are one of the most fundamental and critical interconnect structures in PICs. They enable efficient, low-loss, and low-crosstalk transmission of optical signals at waveguide intersections, thereby facilitating complex optical routing in two-dimensional planes. We demonstrate an improved broadband, low-loss, compact LNOI waveguide crossing covering portions of the S-band and fully spanning the C, L, and U bands. Using the adjoint algorithm, the design achieves <0.12 dB insertion loss and < −40 dB crosstalk over 180 nm bandwidth, with best-case values of 0.05 dB and −50 dB. Its 16 × 16 μm2 footprint is smaller than comparable designs. This work demonstrates the viability of high-performance, compact inverse-designed devices on the LNOI platform, advancing possibilities for high-density photonic applications in data centers and quantum photonics.

We present lithography and argon plasma etching of lithium niobate on insulator (LNOI) rib waveguides using reflowed photoresist etch masks and 405 nm photolithography. Melting the photoresist at temperatures greatly exceeding its glass transition temperature while minimizing feature distortion through photoresist adhesion control reduces sidewall surface roughness and allows the photoresist to be used both as the pattern mask and the hard etch mask. Waveguide sidewall surfaces exhibiting sub-nm root mean square roughness are fabricated. Dependence of sidewall roughness and angle on feature width, and propagation loss on thermal annealing of the fabricated devices is characterized. Measured quality factors on fabricated microresonators exceed one million. LNOI rib waveguides and resonators with low propagation loss increase nonlinear optical conversion efficiencies and are useful for efficient electro-optic modulation. Photolithography compatible fabrication of low loss LNOI photonic integrated circuits facilitates scalable commercialization.