Low-loss and polarization-insensitive double-tip edge coupler with etched facet on a lithium niobate on insulator platform.

(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.

<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.

Parametric comparison of ultra-compact few-mode waveguides of three types (strip, rib, and buried) on thin-film Lithium Niobate On Insulator (LNOI) and SOI platforms is presented. Performance of waveguides is compared in terms of waveguide cross-sectional area, mode loss, dispersion, mode-hybridization and power confinement for both quasi-transverse electric (qTE) and quasi-transverse magnetic (qTM) modes. It is found that LNOI waveguides exhibit lower dispersion with physical dimensions comparable to that of SOI waveguides. The results are vital in choosing an optimum configuration of few-mode waveguides which is crucial for designing few-mode devices in mode-multiplexing schemes.

Thin film lithium niobate has been widely considered as a promising photonic integration platform. However, due to the non-vertical sidewall of lithium niobate etching, it is challenging to design an efficient fiber coupler for the submicron-sized lithium niobate waveguide. Here, a high-performance edge coupler to a cleaved fiber is introduced on the thin film lithium niobate platform using a sharp three-dimensional taper. The proposed taper structure, which ensures a nearly adiabatical transition of both transverse-electric (TE) and transverse-magnetic (TM) modes, is formed by two standard patterning steps for lithium niobate. A silica ridge waveguide is used as the intermediate transition structure between the taper and a high-numerical-aperture fiber. The simulated coupling losses are as low as −0.1 dB per facet, while the experimental mean values reach −0.29 and −0.24 dB for TE and TM modes, respectively. The coupling spectra exhibit a flat wavelength response, which almost coincides for both polarizations. The 0.5-dB coupling bandwidth is beyond 180 nm. Within this bandwidth, the worst coupling losses are −0.61 and −0.56 dB for TE and TM modes, respectively, and the polarization-dependent loss is below 0.17 dB. The present edge coupler also exhibits a good fabrication tolerance, and its fabrication can also facilitate wafer-scale processing without wafer dicing and end-facet polishing.

Lithium niobate on insulator (LNOI) is an important high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-\chi^{(2)}$</tex> material platform for applications in nonlinear and quantum photonic integrated circuits [1]. Submicron-waveguide structures of periodically poled LNOI (PPLNOI) are particularly attractive, since they enable enhanced functionality through strong mode confinement and quasi-phase matching between different wavelengths, as well as improved scaling. To fully capitalize on these advantages, it is essential to fabricate waveguides with well-defined periodically poled domains and low optical losses, both of which remain technological challenges.

Lithium niobate (LN) devices have been used in optical communication and nonlinear optics widely due to its impressive optical properties. Thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices further. But a high-efficient fiber-chip optical coupler for the LNOI-based devices for practical applications is still absent. In this article, we demonstrate a wide-band, highly efficient and polarization independent edge coupler based on LNOI fabricated by planar semiconductor process. The measured ultra-high numerical aperture fiber (UHNAF)-to-chip optical coupling loss at 1550 nm is 0.54 dB/facet (0.59 dB/facet) for TE(TM). The coupler has the coupling loss lower than 1dB/facet for both TE and TM light at wavelengths longer than 1527nm.

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

A three-dimensional (3D) electric field sensing scheme is proposed and experimentally demonstrated based on an integrated lithium niobate on insulator (LNOI) platform. The 3D measurement is realized by packing three LNOI-based sensor chips in a triangular-prism-type clamp. For each sensor chip, the optical waveguide has an asymmetrical Michelson interferometer architecture, and the tapered dipole antenna is inclined to the optical waveguide. By finely placing the three sensor chips in the clamp, the three pairs of inclined tapered dipole antennas are mutually orthogonal and can be applied to measure the electric field in three orthogonal polarization directions. The volume of the packaged 3D sensor is 9.5 cm3. In the experiment, a flat response in the frequency range of 10 MHz to 3 GHz is demonstrated. In addition, a 3 × 3 response calibration matrix is obtained and utilized to reduce the measurement error. After calibration, the relative measurement error of the electric field amplitude is smaller than 5.1% for every polarization direction.

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.

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.

Polarization management is a key technique in integrated photonic circuits. In this paper, a low loss and compact TE-pass polarizer based on lithium niobate on insulator (LNOI) platform is presented. By utilizing subwavelength grating (SWG) metamaterials and inverse design algorithm, the TE0 mode propagates through the SWG region with minimal loss, while the TM0 mode is efficiently coupled out and suppressed through shape-optimized algorithm, thereby achieving an expanded bandwidth of the polarization extinction ratio (PER). With a footprint of 66 μm, the polarizer exhibits low insertion loss (IL) &lt; 0.174 dB and a PER &gt; 10 dB over 176 nm (1465–1641 nm), reaching 33.2 dB at 1550 nm. Furthermore, the proposed polarizer demonstrates superior overall performance, along with promising potential for polarization management and mode conversion in high-performance LNOI-based integrated photonic systems.

Optical switches that effectively isolate outputs are crucial for precisely and efficiently directing and controlling optical signals. This study presents the design and characterization of a 1×2 electro-optic (EO) switch utilizing few-mode waveguides on a lithium niobate on insulator (LNOI) platform. Simulations conducted with the beam propagation method revealed equal splitting between outputs at 0 volts. When 1 volt was applied, the output switched to the bar port of the multimode interferometers (MMI), and at 3.5 volts, the output switched to the cross port of the MMI at a wavelength of 1.55 μm. The device, measuring 6.56 mm in length, exhibits a voltage efficiency of 1.66 V·cm. The switch shows minimal insertion loss of 1.85 dB for output 1 and 1.87 dB for output 2, ensuring efficient signal transmission. Additionally, it maintains low crosstalk levels of −30 dB and −27.72 dB for output 1 and output 2, respectively, indicating effective isolation between output channels. Received: 8 September 2024 | Revised: 7 November 2024 | Accepted: 7 March 2025 Conflicts of Interest The authors declare that they have no conflicts of interest to this work. Data Availability Statement Data are available from the corresponding author upon reasonable request. Author Contribution Statement Md Koushik Alam: Conceptualization, Methodology, Formal analysis, Resources, Data curation, Writing – original draft, Visualization. Noor Afsary: Software, Formal analysis, Writing – original draft. Md Nasir Uddin: Formal analysis, Investigation, Writing – review &amp; editing. A. K. M. Naim Ishtiaq: Formal analysis, Data curation, Writing – review &amp; editing. Md Omar Faruk Rasel: Conceptualization, Writing – review &amp; editing, Supervision.