Bin Wang

We propose an approach for highly-stable and low-phase-noise microwave signal generation based on an actively stabilized multi-loop optoelectronic oscillator (ASML-OEO) by leveraging the distinctive spatial multiplexing capability of multi-core fiber (MCF). In the proposed ASML-OEO, a MCF is employed to create spatially parallel optical delay paths within a single cladding. This enables a compact multi-loop OEO design with intrinsically high side-mode suppression ratio (SMSR). To enhance the long-term frequency stability, an active time-delay compensation scheme is introduced by leveraging the inherent chromatic dispersion effect of the MCF. By precisely tuning the optical carrier wavelength, the environmentally induced time delay variations in the MCF can be dynamically compensated, thereby actively stabilizing the OEO loop. Notably, the compact multi-loop OEO design enabled by the MCF offers superior frequency stability compared to conventional designs. This enhancement stems from the fact that all fiber cores in the MCF experience nearly identical environmental disturbances, enabling precise and simultaneous time-delay compensation. The proposed system is experimentally validated, generating a 10.1-GHz signal with an SMSR of 69.6 dB and a phase noise of −124.5 dBc/Hz@10 kHz. Furthermore, active stabilization yields a remarkable improvement in long-term frequency stability, reducing the hourly peak-to-peak frequency drift from 40.2 kHz to 0.44 kHz and lowering the Allan deviation from 5.6 × 10⁻¹¹@1000s to 9.3 × 10⁻¹³@1000s. With its simultaneous achievement of high SMSR, low phase noise, and superior frequency stability, the proposed ASML-OEO architecture is uniquely suited for demanding applications including high-resolution radar imaging, modern broadband wireless communications and advanced electronic warfare systems.

Wideband electro-optic phase modulator (PM) serves as a pivotal component in modern photonic systems, enabling critical applications ranging from high-speed optical communication and broadband optical frequency comb (OFC) generation to high-resolution microwave photonic radar imaging. In this work, we propose a high-efficiency and ultra-compact thin-film lithium niobate (TFLN) PM based on a dual-noncentered waveguides recycling structure. In the proposed electro-optic PM, two specially-designed noncentered waveguides are placed within a shared modulation region of the capacitively-loaded travelling wave electrodes (CL-TWEs). By passing the optical signal through the modulation region twice via a loop-back waveguide and optical crossing, the interaction between the optical mode and electrical field is enhanced, thereby significantly boosting the modulation efficiency. The chip is designed, fabricated and evaluated. The experimental results demonstrate that the proposed TFLN PM achieves a low voltage-length product (V_π·L) of 3.5 V·cm at 11.6 GHz, a low-V_π bandwidth of 6.2 GHz, a high working frequency up to 46.6 GHz, and an ultra-compact footprint of 0.4×5.5 mm². By leveraging the fabricated TFLN-PM, tunable OFCs with comb spacings of 9 GHz, 11 GHz and 14 GHz are experimentally generated. The proposed TFLN PM delivers high modulation efficiency, wide operational bandwidth and an ultra-compact footprint, making it a promising candidate for next-generation optical communication and signal processing systems.

Wavelength-division multiplexing (WDM)-compatible mode-division multiplexing (MDM) approach is a promising optical communication technique that leverages multiple spatial modes on different wavelengths within an optical waveguide to boost the data capacity. In a WDM-compatible MDM transmission system, a wavelength-selective mode exchanger is a key component which can selectively fulfill the mode conversion on a specific wavelength of the light. Here we propose a compact and reconfigurable wavelength-selective mode exchanger on a silicon photonic chip. The device employs a symmetric Mach–Zehnder interferometer (MZI) structure, with optical mode couplers at both input and output for mode conversion. Each MZI arm features a single-mode rib waveguide, integrated with multiple ultra-compact microdisk resonators (MDRs) for wavelength selectivity, and a metallic micro-heater on each MDR for tuning. By adjusting the micro-heaters, a controlled phase-shift difference between the two arms enables wavelength-selective mode conversion. The mode conversion bandwidth is determined by the MDR resonance bandwidth, and can be enhanced by aligning the resonances of multiple MDRs. Experimental results demonstrate successful transverse-electric (TE0 and TE1) mode conversions across three wavelength channels. The device exhibits strong reconfigurability, allowing tunability of the central wavelength, channel spacing, and bandwidth. Using the chip in an optical transmission system, WDM-compatible MDM transmission is demonstrated. The proposed on-chip wavelength selective mode exchanger holds key advantage of scalability since the WDM channel can be hugely increased by adding more ultra-compact MDRs, which paves the way for future large-scale WDM-compatible MDM optical communication systems.

Microdisk resonator (MDR), as a common class of optical resonators, has emerged as a cornerstone of integrated photonics due to its ultracompact footprint and high light-confining capacity. In this work, a silicon photonic bend-coupled MDR with an ultrahigh Q-factor and a compact footprint is proposed, in which the bus waveguide is engineered to satisfy the strict phase-matching condition for the fundamental whispering gallery mode (WGM), while selectively suppressing the higher order WGMs. The proposed bend-coupled MDR features an ultrahigh Q-factor of $3.3\times 10$ ${}^{\mathbf {6}}$ and a large free spectral range of 2.67 nm. Leveraging the ultrahigh-Q bend-coupled MDR, widely tunable microwave photonic filtering and high-accuracy microwave frequency identification are experimentally implemented. The demonstrated microwave photonic filter achieves breakthrough performance with a narrow passband of 59 MHz and a wide frequency tuning range of 50 GHz. Concurrently, the photonics-assisted microwave frequency identification system exhibits a wide measurement range of 40 GHz, ranging from 6 to 46 GHz, a high measurement accuracy of 14.5 MHz, and a high-frequency resolution of 80 MHz. The proposed ultrahigh-Q bend-coupled MDR paves a way for revolutionizing large-scale and high-density photonic integrated circuits, enabling wideband microwave photonic signal processing, ultraprecision metrology and scalable quantum photonic networks.

We propose and experimentally demonstrate a programmable mode-locked optoelectronic oscillator (PML-OEO) for generating wideband arbitrary microwave waveforms. In the proposed system, a bandpass microwave photonic filter (MPF) is implemented based on phase-modulation to intensity-modulation (PM-IM) conversion, with center frequency controlled via optical carrier wavelength tuning. Upon closing the MPF loop, an optoelectronic oscillator (OEO) is created, where mode locking happens when the period of the wavelength-tuning driving signal matches the round-trip time of the OEO loop. By controlling the driving signal, the proposed PML-OEO can operate in three different mode-locking states, including Fourier-domain mode locking (FDML), active mode locking (AML), and dual mode locking (DML). An experiment is performed, in which wideband arbitrary microwave waveforms, including continuous-wave (CW) microwave signals with reconfigurable modulation patterns, frequency-tunable, and linearly chirped microwave pulses (LCMPs) with customizable duty cycles, are experimentally generated. Particularly noteworthy is the performance of the generated LCMP, exhibiting a bandwidth as broad as 6 GHz, a frequency tuning range of 9 GHz from 12 to 21 GHz, a chirped error as small as 1.95%, and a tunable duty cycle from 20% to 80%. With its unique waveform programmability, the proposed PML-OEO system provides an efficient solution for demanding RF applications spanning from multifunctional radar and high-speed wireless communications to high-resolution electromagnetic imaging.

We propose and experimentally demonstrate an approach for high-precision optical transfer delay measurement based on phase-derived ranging powered by a frequency-agile electro-optic comb. When the optical comb passes through a Mach-Zehnder interferometer, a reference frequency-shifted comb and a probe comb carrying distance information of targets are created. After filtering and heterodyne detection, two electrical signals are generated and sent to a narrowband phase detector for phase extraction, from which distance information or transfer delay can be recovered. Thanks to the broad comb spectrum, high-order comb lines exhibit high sensitivity to propagation distance and transfer the phase to the generated low-frequency electrical signals, significantly enhancing measurement precision. Experimental demonstrations show that a precision of ±10.5 fs is achieved, with a measurement range up to 50 km. Furthermore, a free-space distance measurement is performed, and a precision as small as $2.8~\mu $ m is realized. The proposed system holds great potential for widespread use in distributed network systems and 3D imaging systems.

Reconfigurable optical filters are pivotal components in modern optical systems, providing unparalleled flexibility to adapt to a wide range of application scenarios while significantly reducing the need for iterative design and fabrication processes. In this paper, we propose an on-chip reconfigurable high-order Fabry-Perot (FP) filter based on fully tunable Sagnac loop mirrors (SLMs), offering multi-parameter reconfigurability, particularly in enabling tunability of the FSR. Each SLM incorporates a tunable Mach-Zehnder interferometer (MZI) coupler realized using two variable-width multi-mode interferences (MMIs), which are optimized for a 3 dB power splitting ratio with minimal excess loss, enabling full reflectivity tuning. By thermally tuning the SLM reflectivity and the effective index of the connecting waveguide with the use of micro-heaters placed on top of the waveguides, the resonance wavelength, bandwidth, free spectral range (FSR), and filter order of the proposed high-order FP filter can be flexibly tuned. As a proof of concept, a reconfigurable three-order FP filter is designed, fabricated and experimentally demonstrated. The experimental results show that the proposed reconfigurable FP filter has a tunable order from first to third, a tunable FSR from 0.18 nm to 0.06 nm, a tunable bandwidth spanning from 2.5 GHz to 18.5 GHz, a resonance wavelength tuning range exceeding 0.19 nm, and a maximum extinction ratio of 42.1 dB. With its versatile reconfigurability and exceptional performance, the proposed high-order FP filter shows significant potential for applications in elastic optical networks and adaptive microwave photonic systems.

Due to its uniqueness of simultaneously processing multiband radio frequency (RF) signals, multiband microwave photonic filters (MPFs) have attracted plenty of interest in multifunction radar and multiband-communication applications. In this article, we propose a chip-based widely tunable and independently switchable multiple narrow bandpass MPFs based on a tapped-delay-line structure. This multiband MPF is realized using an incoherent broadband optical source (BOS), an optical spectral shaper (OSS), a chirped waveguide grating (CWG), and a high-speed photodetector (PD). The OSS splits the broadband optical signal into multiple channels: one of the channels incorporates a high-speed phase modulator (PM) for microwave signal modulation; the other channels provide tunable time delays with respect to the PM channel. Thanks to multiple channels incorporation in the OSS, the resulting MPF has multiple passbands. The center frequency of each passband can be independently tuned by tuning the corresponding channel time delay, and the passband switchability can be implemented by controlling the insertion loss of each channel. As a proof of concept, a four-channel MPF is experimentally demonstrated with the use of a four-channel tunable optical delay lines (ODLs) chip. The proposed four-channel MPF exhibits a narrow 3-dB bandwidth of 156 MHz, a wide frequency tuning range up to 24 GHz, a high extinction ratio (ER) over 30 dB, and an independent channel switchability, which holds great potential in multifunction radar and multiband-communication applications.