Volume 14, Issue 26 - Special Issue: Lithium Niobate Photonics

Thin-film lithium niobate (TFLN) has played a pivotal role in the advancement of integrated photonics, by supporting a diverse range of applications including nonlinear optics, electro-optics, and piezo-optomechanics. The effective realization and enhancement of these interactions rely heavily on the implementation of high quality photonic microresonators. The pursuit of novel resonator architectures with optimized properties thus represents a central research area in TFLN photonics. In this work, we design and fabricate TFLN Fabry–Perot microresonators, by placing a straight section of waveguide between a pair of tapered photonic crystal mirrors. The resonator features a high quality factor of 6 × 10 5 at 1,530 nm and a compact length of 100 µm. The functionality of the device is further demonstrated by integrating on-chip electrodes for high-frequency piezo-optomechanical modulation. Our device can serve as an appealing candidate for developing high-performance photonic components on the TFLN platform.

Tailoring the interaction between acoustic waves and optical waves in integrated photonic circuits has emerged as a promising avenue for quantum optics and high-speed information processing. Among various approaches, on-chip acousto-optic modulation (AOM) has been extensively explored in the past decade and successfully demonstrated across many integrated photonics platforms. In this paper, we design and fabricate metallic gratings with periods varying from 0.4 μm to 3 μm deposited on a half-etched thin film lithium niobate (TFLN) device cladded by 1-μm thick silica. By illuminating the metallic grating with an intensity-modulated pump beam, we can generate thermoelastically driven SAWs with frequencies up to 6.58 GHz, and a linewidth of 1.8 MHz at a center frequency of 1.41 GHz. This thermoelastic approach eliminates reliance on a piezoelectric material response, offering broader compatibility for integrated photonics applications.

Nonlinear signal generation requires precise control of the input polarization to satisfy phase-matching conditions. Conventional polarization management using external fiber polarization controllers or bulk wave plates increases coupling complexity and can degrade polarization fidelity and conversion efficiency in nonlinear photonic systems. Here, we demonstrate on-chip polarization control in thin-film lithium niobate nonlinear photonic circuits. Integrated polarization modulators enable real-time tuning of arbitrary input polarization states and thus provide on-demand control of nonlinear conversion in a periodically poled lithium niobate waveguide. A closed-loop feedback system, which integrates auto-compensation and automatic fiber-chip alignment routines, automatically optimizes the second-harmonic generation intensity and maintains performance over extended periods despite polarization scrambling and environmental perturbations. This integrated approach reduces coupling complexity and offers a scalable route toward fully reconfigurable nonlinear photonic systems.

Erbium-doped waveguide lasers have attracted great interests in recent years due to their compact footprint, high scalability and low phase noise. In this work, by using a high-external-gain erbium-doped thin-film lithium niobate waveguide as the gain medium, a fiber-Bragg-grating based Fabry–Perot-type laser is demonstrated with the low pump threshold of few-milliwatts, narrow bandwidth of 0.1 nm, high external output power above 2 mW and maximum optical signal-to-noise ratios above 50 dB. Laser linewidth measurements by self-delayed homodyne and heterodyne detections reveal the underlying multi-longitudinal-mode laser structure and the average intrinsic linewidth as low as ∼50 kHz for the individual longitudinal-modes. Theoretical modeling of the waveguide laser is also conducted with high consistence with the experimental measurements. The demonstrated high-power erbium-doped waveguide laser on thin-film lithium niobate can find diverse applications in optical communication and laser sensing.

Erbium-doped thin-film lithium niobate (Er 3+ :TFLN) enables integrated photonic devices through its efficient photoluminescence. However, the fixed transition energies of erbium ions intrinsically restrict emission to the telecommunications C-band (1530–1565 nm), limiting spectral versatility. To transcend this constraint, we engineered periodically poled Er 3+ :TFLN waveguides that concurrently integrate optical amplification and nonlinear frequency conversion. Within this platform, we harnessed erbium ions stimulated emission under 980 nm pumping to achieve net optical gain (0.8 dB) at 1538.2 nm. Simultaneously, we exploited the quasi-phase-matching (QPM) capability of the poled structure to perform sum-frequency generation (SFG) between the 976.0 nm pump and the amplified 1538.2 nm signal. This dual-process yielded visible emission at 597.1 nm with 84 nW output power and a normalized conversion efficiency of 68 % W −1  cm −2 . Critically, this work demonstrates-for the first time in Er 3+ :TFLN-spectral extension beyond the C-band through synergistic pump amplification and nonlinear mixing. Our monolithic architecture establishes a new paradigm for broadband on-chip photonics, enabling applications including multi-wavelength laser sources, quantum entangled photon pair generators, and on-chip biophotonic sensing systems.

Squeezed states of light play a key role in quantum-enhanced sensing and continuous-variable quantum information processing. Realizing integrated squeezed light sources is crucial for developing compact and scalable photonic quantum systems. In this work, we demonstrate on-chip broadband vacuum squeezing at telecommunication wavelengths on the thin-film lithium niobate (TFLN) platform. Our device integrates periodically poled lithium niobate (PPLN) nanophotonic waveguides with low-loss edge couplers, comprising bilayer inverse tapers and an SU-8 polymer waveguide. This configuration achieves a fiber-to-chip coupling loss of 1.4 dB and a total homodyne detection loss of 4.0 dB, enabling a measured squeezing level of 1.4 dB. Additional measurements in a more efficient PPLN waveguide (without low-loss couplers) infer an on-chip squeezing level of approximately 10 dB at a pump power of 62 mW. These results underscore the potential of TFLN platform for efficient and scalable squeezed light generation.

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/cm 2 and 45 %/W/cm 2 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.

We demonstrate a versatile heterogeneous integration platform unifying III–V gain with thin-film lithium niobate (TFLN) photonic circuits to create high-performance lasers with integrated functionality. This breakthrough overcomes the critical barrier to fully integrated photonic systems by combining optical gain, low-loss cavities, and phase control on a single chip. We present two distinct laser architectures: a distributed feedback laser achieving 11.0 kHz intrinsic linewidth and 4.0 mW in-fiber power through self-injection locking to a high- Q TFLN resonator, and a Vernier ring laser exhibiting 44 nm continuous tuning range with > ${ >} $ 40 dB side-mode suppression ratio. Crucially, the heterogeneous integration of the gain section with TFLN’s components provides a promising path to implementing direct intracavity modulation, which is a functionality that typically requires discrete components. This inherent capability makes our platform a foundational advancement for future compact, robust systems in coherent communications, ultrafast optical metrology, quantum photonic processors, and microwave photonic systems operating at GHz bandwidths, marking a significant advancement toward complete photonic system integration.

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 V π 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.

Periodically poled thin-film lithium niobate (TFLN) crystals are the fundamental building block for highly-efficient quantum light sources and frequency converters. The efficiency of these devices is strongly dependent on the interaction length between the light and the nonlinear material, scaling quadratically with this parameter. Nevertheless, the fabrication of long, continuously poled areas in TFLN remains challenging, the length of continuously poled areas rarely exceeds 10 mm. In this work, we demonstrate a significant progress in this field achieving the periodic poling of continuous poled areas of 70 mm length with a 3 μm poling period and a close to 50 % duty cycle. We compare two poling electrode design approaches to fabricate long, continuous poled areas. The first approach involves the poling of a single, continuous 70 mm long electrode. The second utilize a segmented approach including the poling of more than 20 individual sections forming together a 70 mm long poling area with no stitching errors. While the continuous electrode allows for faster fabrication, the segmented approach allows to individually optimize the poling resulting in less duty cycle variation. A detailed analysis of the periodic poling results reveals that the results of both are consistent with previously reported poling outcomes for shorter devices. Thus, we demonstrate wafer-scale periodic poling exceeding chiplet-size without any loss in the periodic poling quality. Our work presents a key step towards highly-efficient, narrow-bandwidth and low-pump power nonlinear optical devices.

Thin-film lithium niobate (TFLN) has emerged as an attractive platform for integrated tunable photonic filters owing to its strong electro-optic response and low optical loss. However, conventional resonant filters, such as micro-rings, are intrinsically constrained by a limited free spectral range (FSR), which hinders their use in broadband and multi-channel operations. Here we present a TFLN-based add-drop filter that overcomes this limitation by employing a side-coupled travelling-wave Fabry–Pérot (FP) cavity formed with asymmetric multimode waveguide gratings (AMWGs). By engineering the inter-modal coupling through waveguide width tailoring and the reflection bandwidth of AMWGs, we realize a cavity with an intrinsic quality factor of 2.5 × 10 5 . The add-drop filter device also exhibits an FSR-free response across 1,500–1,630 nm wavelength band. A single resonance with a through-port extinction ratio of 20.23 dB and a drop-port insertion loss of 1.81 dB. Wavelength tuning by thermo-optic and electro-optic effect is demonstrated with efficiencies of 34.82 pm/K and 6.9 pm/V, respectively. Furthermore, a four-channel add-drop filter array with 3.2 nm channel spacing and 1.35 dB total through-port insertion loss validates the scalability of the present device. This work demonstrates an efficient approach to overcome the FSR constraint of sharp wavelength filters on TFLN. It can be potentially adopted in dense-wavelength-division-multiplexing communication systems, narrow-bandwidth microwave photonic filters, or high-resolution spectrometers.

Driven by the development of AI applications, optical communication systems experience an exponential surge in the demand for high data rates. Thin-film lithium niobate (TFLN) electro-optic (EO) modulators have been extensively studied and show potential for application in next-generation optical communication systems. In this paper, we present a dual-polarization (DP) TFLN EO modulator integrated based on an EO equalizer, fabricated on the lithium-niobate-on-insulator (LNOI) platform. The device consists of an 11-mm-long forward modulation section and a 4.5-mm-long reverse modulation section. It achieves a half-wave voltage (V π ) of 4 V for both Y-polarization (Y-pol) and X-polarization (X-pol), and exhibits an on-chip insertion loss of 2.5 dB for Y-pol and 2.8 dB for X-pol at a wavelength of 1,550 nm. A 3-dB EO bandwidth exceeding 110 GHz with low EO roll-off is achieved for both TE and TM modes. Furthermore, the modulator supports a data transmission rate of 512 Gbit/s in 4-level pulse amplitude modulation (PAM4) format, corresponding to 256 Gbit/s per polarization. This work demonstrates a beyond 400 G/λ solution for implementing a high-speed, and large-bandwidth modulator on a conventional LNOI platform.

Integrated optoelectronic oscillators (OEOs) have emerged as pivotal enablers of compact, energy-efficient solutions for generating high-frequency radio-frequency (RF) signals with exceptional spectral purity – an essential demand in the advancement of radar and communication technologies. Yet, the quest for ultra-low phase noise near the carrier remains hampered by laser frequency instability and environmental fluctuations. In this work, we unveil the first photonic-integrated, high-order subharmonic injection-locked OEO realized on a thin-film lithium niobate (TFLN) platform, seamlessly uniting a Mach–Zehnder modulator (MZM) and an add-drop microring resonator (MRR) in a monolithic architecture. By harnessing an external RF source operating at a fractional subharmonic (1/2 N , with N = 1, 3, 5…) of the OEO’s free-running frequency, our system achieves robust locking to the 2 N -th harmonic of the injected signal, made possible through the beating of ± N -th order modulation sidebands – precisely selected by the dual resonances of the MRR – at the photodetector. We experimentally demonstrate the generation of 28.7 GHz signals via second- and sixth-order subharmonic injection locking, employing external RF injections at 14.35 GHz and 4.78 GHz, respectively. This yields an outstanding side-mode suppression ratio (SMSR) exceeding 78 dB and remarkably low spurious emissions. Furthermore, the measured phase noise achieves values below −80 dBc/Hz at 100 Hz and below −115 dBc/Hz at 10 kHz offsets from the 28.7 GHz carrier, delineating a new standard for integrated OEO performance.

Thin-film lithium niobate (TFLN) has emerged as a powerful platform for integrated nonlinear optics owing to its large χ (2) nonlinearity, tight confinement and flexible tunability. To fully excavate such superior nonlinear optical properties, domain engineering is commonly adopted to fulfill the phase matching condition of χ (2) processes. During the past decade, various domain engineered TFLN nonlinear optical devices have been demonstrated, showing extremely high length-normalized nonlinear optical conversion efficiencies. However, application-driven scenarios demand absolute energy conversion in nonlinear frequency conversion rather than length-normalized efficiencies, but the progress has been limited by imperfect fabrication processes. In this work, we realize effective on-chip nonlinear energy conversion by developing low-loss and high-quality domain engineered TFLN waveguides with long interaction length. Ion beam trimming (IBT) technique and an etching-prior-poling workflow are adopted for such fabrication. Optical characterization yields an overall second-harmonic generation (SHG) efficiency of 2,590 %/W. A high pump depletion of 85.7 % is demonstrated under continuous-wave operation, which directly reflects strong nonlinear energy conversion. These results may lead to breakthroughs in applications like classical optical frequency conversion, quantum frequency conversion, and quantum light generation.

Thin-film lithium niobate (TFLN) has enabled efficient on-chip electro-optic modulation and frequency conversion for information processing and precision measurement. Extending these capabilities with optical frequency combs unlocks massively parallel operations and coherent optical-to-microwave transduction, which are achievable in TFLN microresonators via Kerr microcombs. However, fully integrated Kerr microcombs directly driven by semiconductor lasers remain elusive, which has delayed integration of these technologies. Here, we demonstrate electrically pumped TFLN Kerr microcombs without optical amplification. With optimized laser-to-chip coupling and optical quality factors, we generate soliton microcombs at a 200 GHz repetition frequency with an optical span of 180 nm using only 25 mW of pump power. Moreover, self-injection locking enables turnkey initiation and substantially narrows the laser linewidth. Our work provides integrated comb sources for TFLN-based communicational, computational, and metrological applications.