An Ultra‐Efficient Integrated Plasmonic Lithium Niobate Electro‐Optic Mach‐Zehnder Modulator

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.

Electro-optic modulators (EOMs) are pivotal in bridging electrical and optical domains, essential for diverse applications including optical communication, microwave signal processing, sensing, and quantum technologies. However, achieving the trifecta of high-density integration, cost-effectiveness, and superior performance remains challenging within established integrated photonics platforms. Enter thin-film lithium niobate (LN), a recent standout with its inherent electro-optic (EO) efficiency, proven industrial performance, durability, and rapid fabrication advancements. This platform inherits material advantages from traditional bulk LN devices while offering a reduced footprint, wider bandwidths, and lower power requirements. Despite its recent introduction, commercial thin-film LN wafers already rival or surpass established alternatives like silicon and indium phosphide, benefitting from decades of research. In this review, we delve into the foundational principles and technical innovations driving state-of-the-art LN modulator demonstrations, exploring various methodologies, their strengths, and challenges. Furthermore, we outline pathways for further enhancing LN modulators and anticipate exciting prospects for larger-scale LN EO circuits beyond singular components. By elucidating the current landscape and future directions, we highlight the transformative potential of thin-film LN technology in advancing electro-optic modulation and integrated photonics.

Electro-optic (EO) modulators play a critical role in converting electrical signals into optical signals. With the growing demand for higher data traffic and broader network bandwidth, there is an increasing need for high-performance, highly integrated EO modulators. Thin-film lithium niobate (TFLN) modulators have emerged as a promising solution for next-generation integrated photonic circuits due to their low power consumption, wide bandwidth, compact footprint, and well-established fabrication process. Our research proposes a Mach–Zehnder modulator (MZM) based on silicon-based TFLN, aiming to achieve a balance between low half-wave voltage product (V π ·L), low optical loss, and high bandwidth. Through systematic design of the waveguide geometry and cladding structure, we demonstrate a modulator with a V π ·L of 0.69 V·cm, a propagation loss of 0.578 dB cm −1 , and a 3-dB bandwidth exceeding 110 GHz at 1550 nm.

Soliton microcombs are a cornerstone of integrated frequency comb technologies, with applications spanning photonic computing, ranging, microwave synthesis, optical communications, and quantum light generation. In nearly all such applications, electro-optic (EO) components play a critical role in generating, monitoring, stabilizing, and modulating solitons. Toward building photonic integrated circuits for next-generation applications that will simultaneously maximize system performance and minimize size, weight, and power consumption metrics, achieving soliton microcombs and efficient EO modulation on a chip is essential. X-cut thin-film lithium niobate (TFLN) has emerged as a leading photonic platform for the realization of high-performance integrated EO devices and systems. However, despite extensive research, soliton microcombs have remained elusive to X-cut TFLN due to its multiple strong Raman-active modes, in-plane refractive index anisotropy, and photorefractive effects. Here, we address this long-standing challenge and demonstrate versatile soliton microcombs on X-cut TFLN, with repetition rates spanning from the gigahertz (∼26GHz, single soliton) up to the millimeter-wave (∼0.156THz, soliton crystal) regime. The combs feature exceptional long-term stability, maintaining a direct injection-locked state for over 90 min (manually terminated), with repetition-rate phase noise closely tracking that of a high-quality electronic microwave synthesizer. Our finding broadly advances both the fundamental science and practical applications of integrated comb sources by enabling efficient EO modulation and broadband coherent solitons to be monolithically combined on the same chip.

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

The thin-film lithium niobate (TFLN)-based electro-optic (EO) modulator is one of the most important devices for optical communications in terms of the advantages of low voltages and large bandwidth. However, the large size of devices limits their applicability in large-scale integrated optical systems, posing a key challenge in maintaining performance advantages under restricted design space. In this paper, we propose a novel TFLN modulator on a quartz substrate incorporating barium titanate (BaTiO3, BTO) as the cladding material. The device is designed with silicon–lithium niobate (Si-LN) hybrid waveguides for operation at a wavelength of 1.55 µm. After theoretical analysis and parameter optimization, the proposed 10 mm long modulator demonstrates high-efficiency modulation, featuring a low half-wave voltage-length product of 1.39 V·cm, a broad 3 dB EO bandwidth of 152 GHz, and low optical loss. This theoretical model provides a novel design solution for TFLN modulators on quartz substrates. Moreover, it is a promising solution for enhancing the integration of photonic devices on the TFLN platform.

Thin‐film lithium niobate (TFLN) based electro‐optic (EO) Mach–Zehnder modulators (MZMs) have shown their potential for next generation high‐speed optical transmission modules and systems. Unlike modulators on other platforms, most of TFLN based EO MZMs adopt single‐drive push‐pull configuration. It is more attractive to design a differential‐drive and push‐pull compatible MZM, which would not only improve the modulation bandwidth, but also facilitate interfacing with electronic drivers. Here, we demonstrate, to the best of our knowledge, the first differential‐drive push‐pull EO modulator on TFLN using a two‐segment MZM structure. By compensating the delay mismatch between optical and microwave signals, the fabricated differential‐drive modulator features an ultra‐low Vπ of 670 mV and a large extrapolated EO bandwidth of ≈109 GHz. This corresponds to, so far, the best voltage‐bandwidth‐ratio performance of ≈243 GHz/V2 for TFLN modulators. Both intensity and coherent modulators are built using the proposed structure, and high‐speed data transmissions are also demonstrated. This differential‐drive MZM structure could promote co‐packaging of TFLN based modulators with electronic chips.

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.

In this work, we design, model, and characterize the Bragg grating modulator (BGM) on silicon-rich nitride and thin-film lithium niobate (SRN-TFLN) platform. In particular, the slow-light effect on the electro-optical modulation response is modeled and experimentally validated. The presented modulator shows a 3-dB electro-optical (E-O) bandwidth beyond 40 GHz, which agrees well with the simulation results. High-speed modulation up to 60 Gbps and 70 Gbps are experimentally obtained with extinction ratio (ER) of 4.7 dB and 2.7 dB, respectively. A measured modulation efficiency of 2.47 pm/V is also achieved. With optimized slow-light effect and proper design, the proposed BGM has great potential in high-speed data transmission.

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.

The escalating capacity limitations of conventional near-infrared telecommunication bands have spurred urgent investigations into wide-band optical communication systems spanning from the near-infrared to mid-infrared regimes. This has motivated the development of optical components combining broadband bandwidth with high-speed operation. The state-of-the-art modulators face challenges in achieving broad operational bandwidth due to waveguide dispersion and velocity mismatch. Here we demonstrate a thin-film lithium niobate (TFLN) electro-optic (EO) modulator with an unprecedented 800-nm operational bandwidth, covering the full O-U telecom bands and extending into the 2-μm regime. The TFLN modulator exhibits >67 GHz EO bandwidth across O-U bands (~100 GHz at O-/S-/C-/L-bands) and >50 GHz at 2-μm band. It enables single-lane exceeding 240 Gbps PAM-4 transmission across O-U bands and a record 170 Gbps PAM-4 transmission at 2-μm band. This breakthrough establishes TFLN as a compelling platform for multispectral photonics, bridging conventional telecom infrastructure with emerging 2-μm technologies for next-generation optical communications.

The interleaver was one of the key devices in dense wavelength division multiplexing (DWDM) applications. In this study, an interleaver with an asymmetrical Mach-Zehnder interferometer structure was designed, fabricated, and characterized in hybrid silicon and lithium niobate thin films (Si-LNOI). The interleaver based on Si-LNOI could be fabricated by mature processing technology of Si photonic, and it was capable of the electro-optical (E-O) tuning function by using the E-O effect of LN. In the range of 1530-1620 nm, the interleaver achieved a channel spacing of 55 GHz and an extinction ratio of 12-28 dB. Due to the large refractive index of Si, the Si loading strip waveguide based on Si-LNOI had a compact optical mode area, which allowed a small electrode gap to improve the E-O modulation efficiency of the interleaver. For an E-O interaction length of 1 mm, the E-O modulation efficiency was 26 pm/V. The interleaver will have potential applications in DWDM systems, optical switches, and filters.

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.

We demonstrate a Mach-Zehnder Interferometer (MZI) modulator characterized by a low voltage-length product, high 3-dB electro-optic (EO) bandwidth, and low light absorption loss. The sunken electrode structure enhances modulation efficiency, reduces optical loss, and allows the electrode to be positioned closer to the waveguide, thereby reducing it by orders of magnitude. This results in a highly efficient modulator. With an electrode gap of 5 um, the simulated voltage-length product is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.94\ \mathrm{V}\cdot \text{cm}$</tex>, and the 3-dB bandwidth exceeds 60 GHz.

We present low-loss (<1.5%) and power-efficient Mach-Zehnder interferometers (MZIs) on thin-film lithium niobate. To accurately measure small MZI losses, we develop a self-calibrated method using tunable Sagnac loop reflectors (SLRs) to build cavities. Fabry-Pérot cavities constructed from these SLRs achieve an intrinsic quality factor of 2×106. By implementing thermal isolation trenches, we also demonstrate a >10× reduction in power consumption for thermo-optic phase shifters, achieving a Pπ of 2.5 mW. These components are crucial for scaling up complex photonic integrated circuits.