Lithium Niobate Thin Films Research Articles

Polarization division multiplexing link using a high-speed lithium niobate automatic polarization demultiplexer

We propose and experimentally demonstrate a polarization division multiplexing (PDM) link employing an integrated dual-polarization thin-film lithium niobate (TFLN) modulator and an on-chip automatic polarization demultiplexer. The tracking speed of an 80-Gb/s PDM link achieves a maximum of 600 rad/s with a bit error rate (BER) below 2.4 × 10−2. Furthermore, at a polarization scrambling rate (SR) of 100 rad/s, BERs maintain around 1.7 × 10−3 over a period of half an hour. This configuration takes advantage of the TFLN platform, which includes high-speed modulation, fast polarization control, and a compact footprint, effectively doubling capacity per wavelength using simple direct detection. The demonstration highlights the effectiveness of on-chip polarization multiplexing and demultiplexing, particularly in scenarios involving polarization fluctuations, offering promising prospects for applications in commercial short-reach data centers.

Thin-film lithium niobate polarization-independent modulators for mode and polarization multiplexing

We demonstrated a high-order mode modulator based on thin-film lithium niobate (TFLN). By introducing a mode converter and a polarization splitter-rotator (PSR), the modulator can achieve dual-mode modulation. Additionally, we designed a periodic capacitively loaded traveling-wave electrode on TFLN to enhance modulation efficiency and bandwidth. The modulator exhibits a polarization extinction ratio greater than 20 dB at a wavelength of 1550 nm, a modulation efficiency of V π LTE0=2.541V⋅cm and V π LTM0=2.570V⋅cm, and a 3 dB bandwidth exceeding 60 GHz. This not only meets the requirements for polarization-independent modulators but also supports on-chip multimode applications such as mode and polarization multiplexing.

Gain Dynamics in Integrated Waveguide Amplifier Based on Erbium-Doped Thin-Film Lithium Niobate

Gain Dynamics in Integrated Waveguide Amplifier Based on Erbium-Doped Thin-Film Lithium Niobate

Highly efficient second-harmonic generation in a double-layer thin-film lithium niobate waveguide

Highly efficient second-harmonic generation in a double-layer thin-film lithium niobate waveguide

Optimization and multiphysics analysis of high-performance lithium niobate modulators

Thin-film lithium niobate electro-optic modulators (TFLN EOMs) are widely utilized across various fields due to their exceptional performance. In this theoretical study, we optimized a TFLN EOM featuring a multimode interference coupler, cosine-curved waveguides, and a modulating arm straight waveguide structure. By tuning the etching depth, waveguide width, and geometric parameters of the traveling wave electrodes, we achieved a half-wave voltage-length product of 1.6V⋅cm and a 3 dB bandwidth of approximately 140 GHz. Additionally, we performed a multiphysics analysis to investigate the temperature characteristics of the modulator. The results indicate that increased temperature induces thermal-optic effects in the material, leading to changes in the waveguide’s refractive index and the optical beam phase, which results in increased insertion loss. Moreover, the stress generated by thermal expansion is concentrated in the modulator’s waveguide, representing a weak point that is prone to failure.

Thermo-optic tuning of quasi bound state in continuum in lithium niobate thin film hetero-nanograting

Lithium niobate (LN) is difficult to etch precisely due to its stable physical and chemical properties. In recent years, more and more research has focused on etchless thin film lithium niobate (TFLN). Here, a one-dimensional SiO2 nanograting structure fabricated on lithium niobate on insulator (LNOI) platform is designed in this study. This nanograting can generate one-dimensional diffraction waves. Then quasi-bound state in the continuum (q-BIC) can be introduced by aligning with the waveguide mode supported by the TFLN, which can achieve a high-quality factor and strong field enhancement, thus improving the interaction between light and matter. Furthermore, we validate the polarization characteristics of the nanograting structure, and measure the thermo-optical tuning sensitivity of the device to be 26.67 pm/K. This finding opens up potential avenues for realizing multi-dimensional tunable and dynamic photonic devices.

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators

Microstructure and characteristics of Zn and Mg-doped LiNbO3 materials for electrochromic devices

In this investigation, polycrystalline targets of lithium niobate (LN), zinc-doped lithium niobate (Zn:LN), and magnesium-doped lithium niobate (Mg:LN) were synthesized utilizing the Hot Isostatic Pressing (HIP) and Cold Isostatic Pressing (CIP) sintering technique. Subsequently, amorphous thin films of LN, Zn:LN, and Mg:LN and electrochromic devices were fabricated via radio frequency (RF) magnetron sputtering. Comprehensive characterization of the microstructure and optoelectronic properties of the targets, films and devices was conducted employing scanning electron microscopy, X-ray diffraction, spectrophotometry, atomic force microscopy, and an electrochemical workstation. Studies have shown that doping with zinc and magnesium enhances the sintering quality of ceramics, reducing lithium vacancies in polycrystalline lithium niobate ceramics. Zinc doping increases the resistance of lithium niobate, whereas magnesium doping introduces a secondary phase that decreases the resistance. The resistances of LN, Zn:LN, and Mg:LN polycrystalline ceramics are 8.35 GΩ, 9.80 GΩ, and 7.61 GΩ, respectively. The doping process refines the growth of the target and film particles, enhancing the microstructural quality. Notably, the ionic conductivity of the Mg:LN film escalates to 1.7 × 10−5 S/cm. Using doped lithium niobate thin films as electrolytes, the electrochromic devices showed a 38 % increase in coloration efficiency and a 46 % improvement in bleaching efficiency, significantly enhancing device performance.

Thin film lithium niobate electro-optic modulator with reduced half-wave voltage length product through design

This paper aims at shortening electrode spacing in a thin film lithium niobate (TFLN) electro-optic modulator (EOM) while avoiding an increase in metal absorption loss, thereby reducing the half-wave voltage length product (V π ⋅L). Through numerical simulations, we find that metal absorption loss reaches its peak values when the optical modes of the metal-clad dielectric waveguide and ridged waveguide hybridize. This negative effect can be mitigated by adjusting the electrode width to modify the optical mode of the metal-clad dielectric waveguide. In addition, we raise the vertical position of the electrodes to further mitigate metal absorption loss and reduce the electrode spacing. By calculating the optimal buffer layer thickness for two crystal axis orientations, our findings reveal a 19% reduction in V π ⋅L at conventional crystal axis orientation (θ=±90∘) and a 16% decrease at unconventional crystal axis orientation (θ=54∘). Notably, V π ⋅L at unconventional crystal axis orientation is 5% lower than at conventional crystal axis orientation. These findings demonstrate the effectiveness of geometric configuration optimization toward enhancing the efficiency and performance of the TFLN EOM.

Robust and broadband tap couplers using misaligned waveguides based on thin-film lithium niobate.

Robust and high-performance power splitters with an ability to partition the input light power into different proportions are vital for various on-chip optical systems. Here we propose and experimentally demonstrate several tap couplers with unbalanced power splitting ratios (PSRs) from 4%: 96% to 40%: 60% using misaligned waveguides on thin-film lithium niobate (TFLN). The device utilizes misaligned waveguide width differences to obtain a flat power splitting ratio over a broadband optical wavelength. The measured results indicate that all the proposed devices exhibit less than 2% PSR variation over a 120 nm optical bandwidth from 1500 nm to 1620 nm and over ±100 nm planar fabrication tolerance. The demonstrated tap couplers offer a promising application prospect in high-density photonic integrated circuits.

High-efficiency and easy-processing thin-film lithium niobate edge coupler

Fiber-to-chip coupling with ultralow loss and broadband operation wavelength range is essential in the practical applications of thin-film lithium niobate (TFLN) integrated photonic devices. However, the existing edge couplers often require electron beam lithography overlaying and multiple etching processes, which are expensive and complex. In this Letter, we demonstrate an edge coupler that includes only a vertically tapered TFLN waveguide and silicon dioxide cladding. The coupling efficiency between a lensed fiber and an on-chip LN waveguide is down to 1.43 dB/facet, while the 3-dB bandwidth exceeds the range from 1510 to 1630 nm. These edge couplers also show reliable fiber misalignment tolerance. Furthermore, the fabrication complexity is greatly reduced since only a single etching step for the TFLN waveguide is needed. The minimum waveguide width of 1.3 μm guarantees compatibility with i-line photolithography, providing a potential for massive production of TFLN devices.

Modal phase-matching in thin-film lithium niobate waveguides for efficient generation of entangled photon pairs

Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device’s capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.

High-efficiency second-harmonic generation in reverse-polarization dual-layer lithium niobate waveguides.

Second-harmonic (SH) generation is an important way to generate short-wavelength light sources. Thin-film lithium niobate (TFLN) resonators and waveguides featuring tight light confinement are flexible to realize mode- or quasi-phase-matching (MPM or QPM) and thus efficient SH generation. Here, we report an efficient SH with an absolute conversion efficiency of 45% in a reverse-polarized double-layer TFLN waveguide. Such a high conversion efficiency without periodic poling was realized with the pump of 67 mW in the 1550 nm band, benefiting from the employment of the largest nonlinear coefficient d33 and the significant nonlinear interaction of the fundamental transverse electric (TE) mode and the first-order TE mode in the vertical direction. This work paves the way toward on-chip integrated nonlinear optics in terms of harmonic generation and quantum light sources across the telecom and visible bands.

120 GOPS Photonic tensor core in thin-film lithium niobate for inference and in situ training

Photonics offers a transformative approach to artificial intelligence (AI) and neuromorphic computing by enabling low-latency, high-speed, and energy-efficient computations. However, conventional photonic tensor cores face significant challenges in constructing large-scale photonic neuromorphic networks. Here, we propose a fully integrated photonic tensor core, consisting of only two thin-film lithium niobate (TFLN) modulators, a III-V laser, and a charge-integration photoreceiver. Despite its simple architecture, it is capable of implementing an entire layer of a neural network with a computational speed of 120 GOPS, while also allowing flexible adjustment of the number of inputs (fan-in) and outputs (fan-out). Our tensor core supports rapid in-situ training with a weight update speed of 60 GHz. Furthermore, it successfully classifies (supervised learning) and clusters (unsupervised learning) 112 × 112-pixel images through in-situ training. To enable in-situ training for clustering AI tasks, we offer a solution for performing multiplications between two negative numbers.

Topological parametric gain and two-color edge states in equidistant lithium niobate waveguide arrays.

We analyze parametric χ2 processes in equidistant finite-size arrays of thin-film lithium niobate waveguides, where the fundamental harmonic (FH) field supports topological edge states due to the specific interplay between inter- and intra-modal couplings of two families of guided modes, while the second-harmonic (SH) field only supports bulk modes. Regimes of topological parametric gain are identified, where the gain only occurs in the edge states of the FH field, regardless of the spatial distribution of the pump SH field. The topological gain of the FH component generally triggers localization of the SH field near an edge of the array in the optical parametric oscillation dynamics. In small-size arrays, parametric gain at both edges can be observed even when pumped at one side. This process can lead to an anomalous "tunneling" of the SH field to the opposite edge. We also analyze the existence and stability of two-color nonlinear edge states (solitons), in which both FH and SH fields are localized at an edge of the array. Depending on the phase-matching condition, such solitons either emerge from the linear FH edge state without a power threshold or exist above a certain power threshold dictated by the coupling strength in the SH field.

High-Linearity Dual-Parallel Mach–Zehnder Modulators in Thin-Film Lithium Niobate

Microwave photonic (MWP) systems are inseparable from conversions of microwave electrical signals into optical signals, and their performances highly depend on the linearity of electro-optic modulators. Thin-film lithium niobate (TFLN) is expected to be an ideal platform for future microwave photonic systems due to its compact size, low optical loss, linear electro-optic effect, and high bandwidth. In this paper, we propose a TFLN modulator with a low voltage–length product (VπL) of 1.97 V·cm and an ultra-high-linearity carrier-to-distortion ratio (CDR) of 112.33 dB, using a dual-parallel Mach–Zehnder interferometer configuration. It provides an effective approach to fully suppress the third-order intermodulation distortions (IMD3), leading to 76 dB improvement over a single Mach–Zehnder modulator (MZM) in TFLN. The proposed TFLN modulator would enable a wide variety of applications in integrated MWP systems with large-scale integration, low power consumption, low optical loss, and high bandwidth.

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Lithium niobate waveguide-integrated Bi2Te3/graphene heterojunction photodetector.

We demonstrate an on-chip photodetector by integrating a graphene and topological insulator Bi2Te3 heterostructure on a thin-film lithium niobate waveguide. Lithium niobate on insulator (LNOI) waveguides are fabricated by the photolithography-assisted chemical mechanical etching method. The bismuth telluride (Bi2Te3) and graphene heterostructure design provides enhanced photocurrent due to the effective photocarrier generation. The lithium niobate waveguide-integrated Bi2Te3/graphene heterojunction presents a high absorption coefficient of 2.1 dB/µm. The Bi2Te3/graphene heterojunction photodetector exhibits a responsivity of 2.54 mA/W without external bias at a 1.55 µm wavelength, which is enhancement of sevenfold as compared to the pure graphene-based photodetector. The photodetector has a 3 dB bandwidth of over 4.7 GHz. This work provides a potentially viable method for a self-powered, high responsivity, and fast response of the photodetector integrated with the LNOI photonic platform.

Highly Efficient Slow‐Light Mach–Zehnder Modulator Achieving 0.21 V cm Efficiency with Bandwidth Surpassing 110 GHz

AbstractHigh‐speed electro‐optic modulators are key components in modern communication networks and various applications that require chip‐scale modulation with large bandwidth, high modulation efficiency, and compact footprint. However, fundamental trade‐offs make it challenging to achieve these metrics simultaneously, and thus new methodologies must be explored. To this end, a Mach–Zehnder modulator harnessing slow‐light waveguides and capacitively loaded slow‐wave electrodes are presented on silicon‐nitride‐loaded lithium niobate on an insulator platform. The increased group index and reduced microwave loss significantly improve the modulation efficiency. With the 1‐mm‐length modulation section, a low half‐wave voltage length product Vπ·L of 0.21 V cm is obtained, which is one order of magnitude smaller than that of conventional thin film lithium niobate Mach–Zehnder modulators, and a modulation bandwidth of surpassing 110 GHz is achieved. The digital signal processor‐free non‐return‐to‐zero signal and eight‐level pulse amplitude modulation signal of up to 180 and 300 Gbps, respectively, are generated by the modulator, which provides ultra‐large bandwidth, ultra‐high efficiency, and a compact solution for next‐generation electro‐optic systems.

Design and analysis of single-electrode integrated lithium niobate optical phased array for two-dimensional beam steering

The realization of high-speed, low-power optical phased array (OPA) on thin-film lithium niobate on insulator (LNOI) is considered an ideal solution for the next generation of solid-state beam steering. Most reported on-chip two-dimensional optical phased arrays suffer from issues such as large antenna spacing, high power consumption and complex wiring due to independent control of array elements. To address these challenges while fully utilizing the benefits of the LNOI platform, we propose a two-dimensional beam-scanning OPA based on lithium niobate (LN) waveguides. We design a multi-layer cascaded domain engineering structure inside the LN waveguide, combined with wavelength tuning, to enable two-dimensional beam scanning with single electrode controlling the OPA. Through simulation, we achieve a 42°×9.2° two-dimensional beam steering. Compared to existing on-chip integrated OPAs, this work offers significant advantages in increasing integration, simplifying control units and reducing power consumption.