Integrated Microwave Photonics Multi‐Parameter Measurement System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Recently, optical phase modulation has been widely used in microwave photonics (MWP) systems, such as radio over fiber systems, photonic microwave filters, optical microwave and millimeter-wave signal generators, and optical subcarrier frequency up-converters. An optical phase-modulated signal can be converted to an intensity-modulated signal in a dispersive optical fiber. Due to the intrinsic nonlinearity of optical phase modulation, for linear applications such as microwave signal distribution and filtering, the modulation index should be kept small to minimize the unwanted modulation nonlinearity. However, for nonlinear applications such as microwave frequency multiplication and subcarrier frequency upconversion, the modulation index should be large to maximize the frequency multiplication and upconversion efficiency. In this paper, for the first time to our knowledge, we develop a thorough theoretical framework for the characterization of phase-modulation-based MWP systems, in which the phase modulation to intensity modulation conversion is realized using a dispersive fiber. Analytical models for the distributions of single-tone and two-tone microwave signals and for microwave frequency multiplication and subcarrier frequency upconversion are developed, which are verified by numerical simulations. The analytical models for single-tone and two-tone transmissions are further confirmed by experiments. The developed analytical models provide an accurate mathematical tool in designing phase-modulation-based MWP systems. </para>

Thin-film lithium niobate (TFLN) has rapidly emerged as a highly competitive integrated photonic platform in fields such as data communication, microwave photonics, and quantum information processing. However, as a critical interface for chip-fiber links, the TFLN edge coupler is limited by its mode field diameter, exhibiting extremely low assembly tolerance when coupling with a standard single-mode fiber (SSMF), which severely restricts its application. Here, we propose a TFLN edge coupler based on a silicon nitride trident structure. By converting the small mode field of the lithium niobate waveguide into the large mode field of the Si3N4 trident structure, a high-efficiency, high-tolerance, and process-compatible edge coupler is constructed. Three-dimensional simulation results show that the device exhibits a mode field diameter greater than 8 μm, with TE mode coupling loss less than 0.71 dB, TM mode coupling loss less than 0.91 dB in the C+L band, and polarization-dependent loss below 0.41 dB. This device provides a promising solution to the challenge of high-performance, high-tolerance interconnection between TFLN chips and SSMFs.

Optical filters based on integrated phase-shifted Bragg gratings (PSBGs) have attracted considerable interest because they feature a Lorentzian-shaped resonance, along with strong out-of-band rejection across a broad stopband region. The extent of the stopband can be conveniently adjusted by modifying the grating coupling coefficient [1]. Fabricating PSBGs on a thin-film lithium niobate (TFLN) platform enables the selection and modulation of specific wavelengths at ultra-fast speeds [2]–[3], offering substantial advantages in applications like datacom, sensing, and microwave-photonics systems [4]. In this abstract, a monolithic PSBG based on TFLN is proposed. The low waveguide propagation losses combined with the strong grating coupling coefficient of these devices, enable a narrow wavelength selection combined with a large stopband and effective electro-optic tunning capabilities.

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.

Coherent optical communication offers superior spectral efficiency and substantial transmission capacity, making it indispensable for accommodating the exponential growth of global data traffic. Next‐generation data‐centers demand higher baud rates and improved spectral efficiency, which in turn require optoelectronic devices for both ultra‐broadband operation and high modulation efficiency. However, conventional modulator design commonly faces a fundamental trade‐off between electro‐optic bandwidth and modulation depth. To address this limitation, we present a segmented modulation strategy and fabricate segmented I/Q modulator on the thin‐film lithium niobate (TFLN) platform. It features a 3‐dB bandwidth exceeding 110 GHz while maintaining a high signal‐to‐noise ratio for high‐order signal generation. For high baud rate signal generation beyond the limitation of digital‐to‐analog convertor sampling rate, we achieve 320‐Gbaud single‐polarization quadrature phase shift keying (QPSK) and 310‐Gbaud dual‐polarization (DP) QPSK signal based on polybinary coding scheme. For high spectral efficiency transmission, we demonstrate 2.02‐Tb/s line rate probabilistic shaped (PS) 144‐ary quadrature amplitude modulation (144‐QAM) signal over 125‐km single‐mode fiber, facilitated by the residual carrier‐based phase tracking. The results highlight the potential of segmented IQ modulator, in conjunction with advanced digital signal processing, can pave the way for next‐generation high‐speed coherent optical communications.

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

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.

A digital phase-locked receiver architecture is presented for microwave and millimeter wave amplitude and phase measurements. This architecture characterized by a built-in Nyquist rate digital phase-locked loop can reduce the accuracy degradation caused by frequency errors from microwave sources. The hardware logic implementation of core algorithm enhances its highest measurement speed up to 350,000 points per second, which is near an order of magnitude higher than that of the current popular amplitude and phase receivers, and potential increase is prospective. A complete microwave amplitude and phase measurement system has been developed and tested. The results of practical test show its performance of high speed and accuracy.

Integrated microwave photonics is an emerging field, where radio signals are generated, distributed, processed and analyzed using photonic integrated circuits. InP and Si3N4 are hybrid integrated to create complex microwave photonic functionalities.

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.

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.

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.

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.

Microwave photonics can be generally defined as the study of high-speed photonic devices operating at microwave or millimeterwave frequencies and their use in microwave or photonic systems. In this multidisciplinary field at the interface between microwave techniques, ultrafast electronics and photonic technologies, typical investigations include, for example, high-speed and microwave signal generation, processing and conversion as well as the distribution and transmission of microwave signals via broadband optical links. From pioneering experiments in the 70's, this field of microwave photonics has paved the way for an enabling novel technology with a number of commercially important applications. This paper is intended to give an overview on this multidisciplinary field of microwave and millimeterwave photonics.

On-chip polarization splitter–rotators (PSRs) are crucial components for coherent optical communication and polarization-division multiplexing systems. Here, we propose and experimentally demonstrate a polarization splitter–rotator based on the principles of adiabatic mode evolution and the stimulated Raman adiabatic passage on the thin-film lithium niobate (TFLN) platform. The experimental results indicate that the fabricated device exhibits an insertion loss of less than 0.5 dB and extinction ratios exceeding 20 dB over a 130-nm band, with the potential to extend up to 250 nm as predicted by simulations. Large fabrication tolerance is also demonstrated with extinction ratios of more than 20 dB in the wavelength range of 1500–1630 nm. The demonstrated PSR could become an indispensable component in the future TFLN photonic integrated circuits.