Jungwon Kim

High-resolution non-contact non-destructive evaluation (NDE) utilizing MHz-range ultrasound in air-coupled environments faces two significant challenges: severe signal attenuation and reduced sensitivity due to transducer miniaturization. Optical ultrasound sensors have emerged as promising candidates to overcome these limitations; however, widespread adoption remains challenging. Conventional discrete optical sensors are limited by complex fabrication and rigid form factors. Furthermore, the efforts to implement these on photonic integrated circuits (PICs)—while addressing scalability—are typically constrained by limited sensitivity. To address these requirements, we report a chip-scale optomechanical ultrasound receiver implemented, for the first time, on a silicon photonics MEMS platform. This platform facilitates CMOS-compatible scalability and design freedom to independently optimize mechanical components and optical sensitivity. Consequently, the receiver demonstrates a compact active acoustic sensing area of 4×10 ⁻⁴ mm ² . Despite this size reduction, in the noise-equivalent pressure (NEP)–active-area comparison, the proposed receiver’s measured and estimated NEP values are positioned within a range comparable to or lower than those for previously reported air-coupled optical and CMUT sensors, when interpreted relative to the ideal area-scaling trend. Designed for a 2.8 MHz mechanical resonance, the receiver was benchmarked against a commercial 1 MHz air-coupled PZT transducer to provide a practical, controlled experimental baseline. In this controlled experimental comparison, the receiver showed a more than 25,000-fold higher received response per unit active acoustic area than the PZT baseline, serving as a complementary metric for unit-area received response and structural scaling potential. These findings suggest that the silicon photonic MEMS platform can provide a route toward compact, dense-array-compatible optical ultrasound receivers for localized air-coupled sensing.

Microcombs can generate optical pulse trains with high repetition rates and ultralow timing jitter. Owing to their ultrahigh-Q resonators, silica microcombs have demonstrated excellent jitter characteristics at microwave repetition rates. However, their jitter performance degrades when the cavity free-spectral range (FSR) is extended to the mm-wave regime. Perfect soliton crystals (PSCs), formed by multiple evenly spaced solitons, offer an effective route to multiply the repetition rate of a microcomb while potentially preserving its low-noise properties. However, their jitter properties have not yet been systematically explored. Here, we characterize the timing jitter of PSCs in a silica resonator using a heterodyne fiber interferometer. We find that the ultralow jitter of the single-soliton state is preserved in 2-FSR and 3-FSR PSCs, yielding an integrated root-mean-square jitter of 3.62 and 3.52 fs at 44 and 66 GHz repetition rates, respectively. These results demonstrate that PSCs enable the generation of low-noise, mm-wave-rate optical pulse trains, with potential applications in ultralow-noise mm-wave generation, high-speed data converters, precision timing distribution, and advanced metrology.

Microresonator‐based Kerr frequency combs (microcombs) provide compact femtosecond pulse sources with repetition rates exceeding 10 GHz, combining a small footprint with intrinsically low timing jitter. However, free‐running microcombs suffer from long‐term timing drift, and direct stabilization to optical references is hindered by the difficulty of carrier‐envelope offset frequency detection at such high repetition rates. Here we demonstrate optical‐to‐microcomb stability transfer via a mode‐locked laser (MLL), which enables femtosecond synchronization and directly imprints optical‐reference stability onto the microcomb repetition rate. Using an electro‐optic‐sampling‐based timing detector, we synchronize a 22‐GHz silica microcomb to a 250‐MHz fiber MLL, reaching 1.3 as/Hz ^0.5 timing resolution and fractional frequency stability of 10 ⁻¹⁸ level at 1000‐s averaging. When the MLL is stabilized to a compact and alignment‐free optical fiber‐delay reference, its stability is transferred to the microcomb, yielding a single‐sideband phase noise of −125 dBc/Hz at 100‐Hz offset from a 22‐GHz carrier—the lowest phase noise reported to date for a microcomb‐based microwave source at offset frequencies below ∼1 kHz. This approach enables direct transfer of optical‐reference stability to microcombs, providing a practical route toward high‐repetition‐rate and ultralow‐noise photonic oscillators for ultrastable timing, precision metrology, coherent communication, and next‐generation sensing systems.

Very long baseline interferometry (VLBI) enables high-angular-resolution observations in astronomy and geodesy by synthesizing a virtual telescope with baselines spanning hundreds to thousands of kilometres. Achieving high instrumental phase stability in VLBI relies on the generation of high-quality, atomic-referenced RF local oscillator (LO) and RF-comb signals for the effective downconversion of celestial RF signals and precise phase calibration, respectively. As observing frequencies move into higher ranges with wider bandwidths, conventional electronic methods face significant challenges in maintaining the quality of these signals. Here, we demonstrate that an optical frequency comb (OFC) can be used as a versatile tool to generate and distribute low-noise and atomic-referenced RF-comb and RF-LO signals in the VLBI telescope. Hydrogen maser-stabilized optical pulses are transmitted over a timing-stabilized fibre link from the observatory building to the VLBI receiver system at the telescope, where photodetection converts them into the required RF signals. In VLBI test observation, we successfully detected VLBI fringes and extracted the RF-combs characteristics in a format suitable for VLBI instrumental phase calibration. These results highlight the high potential of OFC-based technology for enhancing next-generation broadband VLBI measurements, advancing astrophysical research and facilitating intercontinental clock comparison.

The field of micro- and nano-mechanics has seen rapid advances driven by applications in sensing, microscopy, and precision instrumentation. Accurate, time-resolved characterization of mechanical dynamics is essential for understanding device behaviour and improving performance. However, conventional optical and electrical methods face trade-offs between sensitivity, linearity, and bandwidth, while frequency-domain approaches are limited in capturing transient dynamics. Here, we present a frequency comb-based time-domain tracking technique for directly observing the full-range dynamic motion of atomic force microscopy (AFM) micro-cantilevers. By leveraging electro-optic sampling between femtosecond optical pulses and ultra-precise photocurrent timing signals, our system enables real-time measurements across six orders of magnitude – from ~ 30 pm thermal fluctuations to ~ 20 µm nonlinear oscillations. The technique reveals complex behaviours including mode coupling, hysteresis, bifurcation, and transient modulation, while maintaining calibration fidelity through thermomechanical noise. This approach bridges the longstanding gap between ultra-sensitive and wide-range motion tracking, offering a powerful tool for studying nonlinear dynamics in micro- and nano-scale mechanical systems. Looking ahead, the method lays the groundwork for advances in high-resolution force sensing, AFM probe optimization, and the broader exploration of dynamic behaviour in precision microsystems.

We demonstrate a self-homodyne detection method to stabilize a continuous-wave 1550-nm laser to a 1-km optical fiber delay line, achieving a frequency instability of 6.3 × 10⁻¹⁵ at a 16-ms averaging time. This result, limited by fiber thermal noise, is achieved without the need for a vacuum system, highlighting the potential of our approach for ultra-stable laser systems in non-laboratory environments. The system utilizes only a few passive fiber optic components and a single balanced photodetector, significantly simplifying the laser stabilization process while maintaining high performance. The entire optical setup is compactly packaged in a portable metal air-tight case.

Very long baseline interferometry (VLBI) enables high-angular-resolution observations in astronomy and geodesy by synthesizing a virtual telescope with baselines spanning hundreds to thousands of kilometres. Achieving high instrumental phase stability in VLBI relies on the generation of high-quality, atomic-referenced RF local oscillator (LO) and RF-comb signals for the effective downconversion of celestial RF signals and precise phase calibration, respectively. As observing frequencies move into higher ranges with wider bandwidth, conventional electronic methods face significant challenges in maintaining the quality of these signals. Here, we demonstrate that an optical frequency comb (OFC) can be used as a versatile tool to generate and distribute low-noise and atomic-referenced RF-comb and RF-LO signals in the VLBI telescope. Hydrogen maser-stabilized optical pulses are transmitted over a timing-stabilized fibre link from the observatory building to the VLBI receiver system at the telescope, where photodetection converts them into the required RF-comb and RF-LO signals. In VLBI test observation, we successfully detected VLBI fringes and extracted the RF-combs characteristics in a format suitable for VLBI instrumental phase calibration. These results highlight the high potential of OFC-based technology for enhancing next-generation broadband VLBI measurements, advancing astrophysical research and facilitating intercontinental clock comparison.

We are excited to present this special issue on microwave photonics in the Journal of Lightwave Technology (JLT), coinciding with the 2023 International Topical Meeting on Microwave Photonics (MWP 2023). As an interdisciplinary field, microwave photonics mainly focuses on the utilization of photonic devices, systems, and techniques for applications in wireless communications and sensing systems, particularly in the microwave, millimeter-wave, and terahertz (THz) wave bands. It also encompasses the development of high-speed photonic components. In recent years, the field has experienced remarkable growth, propelled by innovations in integrated microwave photonics and their applications in next-generation wireless systems. This special issue provides an important platform for researchers to share their latest discoveries across a broad spectrum of topics, including photonic devices, chips, systems, and networks.