Large-Mode-Field Low-Loss Trident Edge Coupler Based on Thin-Film Lithium Niobate Platform

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.

In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.

Fiber-to-chip optical interconnects are critical for enabling efficient signal transmission in photonic integrated circuits, especially in data centers and optical communication systems. However, the large mismatch in mode field diameter between standard single-mode fibers and integrated waveguides leads to significant coupling losses. In this work, a compact, broadband, and polarization-insensitive subwavelength grating (SWG) edge coupler is proposed for silicon nitride (Si₃N₄) waveguides. The coupling performance is evaluated using the finite-difference time-domain (FDTD) method and eigenmode expansion (EME). The designed coupler exhibits a compact length of 55μm with low coupling loss and wide bandwidth. When coupled with a standard single-mode fiber, the minimum coupling losses for the TE0 and TM0 modes are 0.18dB and 0.35dB, respectively. At 1550nm, the corresponding coupling losses are 0.24dB (TE₀) and 0.35dB (TM₀). The 0.4dB bandwidths are 490nm for TE₀ and 350nm for TM₀, demonstrating excellent broadband performance. The alignment tolerance analysis shows that the coupling loss fluctuation of the TE₀/TM₀ mode is 0.5dB/1.4dB in the horizontal deviation of 2μm, and the coupling loss fluctuation of the TE₀/TM₀ mode is 0.8dB/0.7dB in the vertical deviation of 2μm. Compared to conventional single-taper edge couplers, the proposed design achieves higher broadband coupling efficiency and improved tolerance to fiber misalignment, making it highly suitable for practical packaging and integration.

I will discuss progress on the development of ultra-low loss thin film lithium niobate (TFLN) photonic platform and its application in classical and quantum information processing. Examples of devices to be presented include low power consumption and high bandwidth electro-optic (EO) modulators and EO frequency comb sources, as well as their integration with high power lasers, of interest for applications in optical communications and microwave photonics. TFLN is also promising for spectral and temporal control of non-classical light. In particular, I will discuss the development of quantum transducers and photonics needed for realization of multiplexed quantum repeaters.

Low loss ridge waveguides on the hybrid single crystalline silicon and lithium niobate thin films

Thin-film lithium niobate (TFLN) has a proven record of building high-performance electro-optic (EO) modulators. However, it has consistently posed challenges in securing low driving voltage, wide electro-optic bandwidth, low insertion loss, and high modulation efficiency simultaneously. Here, we demonstrate a telecom-wavelength EO modulator on the TFLN platform incorporating multifunctional benzocyclobutene (BCB) material. The low dielectric constant (low-k) BCB effectively reduces RF loss of the modulator and enables perfect velocity matching with a narrow electrode gap, thereby overcoming the conventional voltage–bandwidth trade-off. Meanwhile, in combination with a bilayer inversely tapered waveguide, it also facilitates the realization of high-efficiency edge couplers, significantly reducing the coupling loss of the modulator. In addition, the underlying TFLN slab is selectively removed to eliminate dielectric relaxation, ensuring a stable low-frequency EO response and bias-drift-free operation. The fabricated 13-mm-long modulator exhibits low half-wave voltages Vπ of 1.5 V in the C-band and 1.19 V in the O-band, corresponding to half-wave voltage-length products of 1.95 V·cm and 1.55 V·cm, respectively. Thanks to the BCB-clad edge coupler, an ultra-low coupling loss of 0.54 dB per facet is obtained. Ultra-wide EO bandwidths exceeding 110 GHz across the C + O-bands are demonstrated, and high-speed PAM8 data transmission with data rates up to 390 Gbit/s is successfully recorded in both C- and O-bands. The proposed modulator architecture not only delivers excellent overall performance, but also simplifies the fabrication process and expands the application potential.

We propose an efficient and polarization-insensitive edge coupler (EC) constructed principally with two cascaded vertical waveguide tapers. The proposed edge coupler only requires ordinary 365 nm (i-line) ultraviolet source for lithography process. We experimentally demonstrate the proposed EC on two kinds of photonic integrated circuit (PIC) platforms: silicon nitride (Si3N4) and lithium niobate thin film. Both achieve polarization-insensitive fiber chip coupling efficiency of >70% in the C-band. Our proposed EC have the advantages of efficient, cost-saving, and easy to implement and could serve as an effective solution to facilitate low-loss chip-fiber coupling.

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.

Despite the growing interest in thin film lithium niobate (TFLN) as a material platform for photonic integrated circuits (PIC), its moderate refractive index, CMOS-incompatible fabrication processes and inherent material anisotropy still raise questions about achieving dense integration comparable to mature platforms like silicon photonics while preserving the superior properties of lithium niobate. Here we show a photonic inverse design method to enable miniaturization and dense integration of lithium niobate PIC components. As proofs-of-concept, we experimentally demonstrate ultra-compact mode-division (de)multiplexer (19 × 25 μm²), multimode waveguide crossing (15 × 15 μm2), and waveguide bends (30 μm bending radius). The fabricated components are used to construct multimode photonic circuits for large-capacity data communications, demonstrating dense integration of over 10 waveguide elements within a 0.06 mm² chip area. By integrating electro-optic modulators on the same chip, high-speed data modulation is demonstrated with 120 Gbps data rate per channel alongside multimode signal transmission. This work is expected to advance 10-fold higher area density of passive components and optical path design in TFLN.

Thin-film lithium niobate (TFLN) has been widely used in electro-optic modulators, acoustic--optic modulators, electro-optic frequency combs, and nonlinear wavelength converters owing to the excellent optical properties of lithium niobate. The performance of these devices is highly dependent on the fabrication quality of TFLN. Although state-of-the-art TFLN microrings with an intrinsic quality factor (Q-factor) exceeding 1 × 107 have been realized by inductively coupled plasma-reactive ion etching (ICP-RIE) and chemical mechanical polishing (CMP), ICP-RIE has moderate throughput, moderate reproducibility, and high cost in etching TFLN, while CMP features moderate throughput and low cost in etching TFLN. Here, a wet etching method for TFLN, leading to the fabrication of a micro-racetrack with an intrinsic Q-factor of over 9.27 × 106 is developed. The suitability of this method to fabricate a narrow coupling gap between the bus waveguide and microring enables the coupling conditions of the microring to be customized. This method features a high throughput, a high reproducibility, and a low cost in etching TFLN, showing the potential to boost the mass production of integrated LN photonic devices with high fidelity and affordabilitydramatically.

Lithium niobate has a number of attractive piezoelectric and ferroelectric properties that have been exploited for many applications. The main purpose of our work was to grow lithium niobate thin films on silicon substrates and make surface acoustic wave (SAW) devices. Theoretical curves of SAW properties of bulk crystals and films lithium niobate on silicon with a coating of silicon oxide were also obtained. The theoretical results include elastic wave properties (velocities and power flow angle) in lithium niobate bulk crystals and SAW properties (velocities and coupling coefficients). Thin films of lithium niobate have been deposited on Si (100) substrates (with SiO2 coatings) by rf magnetron sputtering. Lithium niobate film samples with very good transparency and uniformity were obtained. The film samples were structurally analyzed using Bragg x-ray diffraction (XRD). Lithium niobate XRD peaks for specific sputtering conditions corresponding to thin film orientation of (012), (104) and (110) were identified. Highly textured films showing only one orientation of the lithium niobate could be grown for all those of these above orientation on identical amorphous substrates.

In recent years, thin-film lithium niobate on insulator (LNOI) has emerged as a promising and efficient platform for photonic integrated devices. In this letter, we present an edge-coupled fiber-to-chip spot-size (SSC) based on LNOI double trident layer structure. Simulation shows that the coupling efficiency of SSC is 93% at 1550 nm, which realizes the efficient coupling between single-mode fiber and lithium niobate thin film waveguide. The SSC features a large mode spot size of up to 4 µm and an alignment tolerance of 0.75 µm, indicating high tolerance to optical deviations and manufacturing variances.

Driven by the growing demands in wireless communication, remote sensing and emerging 6G networks, research on microwave signal measurement techniques has attached intensive attention. Unlike conventional electronic‐based approaches, photonics chip‐based microwave signal measurement systems offer significant advantages, including broad operation bandwidth, reduced weight, and enhanced resistance to unwanted electromagnetic interference. Despite notable progress in integrated microwave photonic measurement systems, the majority remains constrained by bandwidth below 30 GHz, primarily due to the limitation of modulators. Furthermore, most previous studies focus on the measurement of one single parameter, typically the frequency, limiting their applications in more complex, real‐world situations. Here, an on‐chip photonic microwave multi‐parameter measurement system is presented on the thin‐film lithium niobate (TFLN) platform. The system enables measurement of microwave frequency, phase, and amplitude, offering an ultra‐high bandwidth (up to 60 GHz) with low root‐mean‐squared errors: 450 MHz for frequency, 3.43° for phase, and 1.64% for amplitude. Additionally, the system is validated by the time‐domain reconstruction of unknown sinusoidal microwave signals based on measurement results. This demonstration further broadens the scope of integrated TFLN photonic devices for microwave signal measurement, providing a viable solution to the bandwidth limitations of existing microwave networks and addressing the increasing demands of future information‐driven technologies.

Electro-optic phase modulation is a core technology for signal processing in optical communication and microwave photonics, yet the current multifunctional integrated devices for high-bandwidth phase modulation and efficient frequency doubling suffer from critical bottlenecks including large crosstalk, excessive chip area, bandwidth-voltage trade-off and single material performance limitations. To address these issues, this study proposes a Si₃N₄-TFLN heterogeneous integrated design scheme by synergizing the low-loss property of silicon nitride and the strong electro-optic effect of thin-film lithium niobate. A dual-drive Mach-Zehnder modulator (DD-MZM) and a racetrack micro-ring resonator (MRR) with high quality-factor and large free spectral range are cascaded to realize the monolithic integration of high-precision phase modulation and microwave signal frequency doubling. Co-simulation results show that the designed device achieves an electro-optic modulation bandwidth of 72 GHz and a half-wave voltage-length product of 2.8 V·cm, which well reconciles the bandwidth-voltage trade-off. The device exhibits a frequency doubling efficiency of 15%, 30.4% higher than that of the single TFLN-based MRR. The cascaded coupling structure suppresses the functional crosstalk to -38 dB, and the total chip layout area is only 3.648 mm², over 60% smaller than that of the existing similar devices. This work provides a high-performance on-chip solution for high-density integrated systems in optical communication, microwave photonics and quantum information processing.

Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin-film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro-optic effect and second-order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x-cut TFLN are constrained to minimum period sizes on the order of microns, restricting access to highly phase-mismatched interactions such as counter- and backward-propagating frequency conversion. In this work, we demonstrate scalable periodic poling of x-cut TFLN with domains periods as short as 215 nm and realize devices that support both counter- and back-propagating phase matching. We estimate conversion efficiencies of 1,474 %/W/cm2 and 45 %/W/cm2 for the two interaction types, respectively. Sum frequency generation measurements confirm that the nonlinear generation takes place in the desired direction. Furthermore, we report spontaneous parametric down conversion for the counter-propagating configuration and, for the first time, for a backward propagating device. This breakthrough provides unprecedented control over engineering of ferroelectric domain geometries in TFLN, leading into the generation of photon pairs with precisely tailored spatial and spectral characteristics. Such capabilities hold strong potential for advancing quantum signal processing, scalable quantum computing architectures, and precision quantum metrology.