Published papers

We performed a transmission experiment to compare microcomb states using a silicon nitride microresonator. A modulation instability comb with the correct amount of detuning can provide a light source almost comparable to a soliton comb.

We conducted controlled experiments on nighttime thermoelectric generation using cost-effective, commercially available materials under various weather conditions, both with and without a blackbody filter for radiative cooling, achieving efficiencies of up to 53.3 mW/m2,.

Ultrahigh-repetition-rate lasers will become vital light sources for many future technologies; however, their realization is challenging because the cavity size must be minimized. Whispering-gallery-mode (WGM) microresonators are attractive for this purpose since they allow the strong light–matter interaction usually needed to enable mode locking. However, the optimum parameter ranges are entirely unknown since no experiments have yet been conducted. Here, we numerically investigate pulsed operation in a toroidal WGM microresonator with gain and saturable absorption (SA) to study the experimental feasibility. We show that dispersion is the key parameter for achieving passive mode locking in this system. Moreover, the design guideline provided in this work can apply to any small resonators with gain and SA and is not limited to a specific cavity system.

We detail a dispersion engineering strategy for microresonators allowing us to generate microcombs in the near-infrared and mid-infrared regimes, and identifying optimal shapes for achieving anomalous dispersion suited to different wavelength regimes.

The demand for high-speed and highly efficient optical communication techniques has been rapidly growing due to the ever-increasing volume of data traffic. As well as the digital coherent communication used for core and metro networks, intensity modulation and direct detection (IM-DD) are still promising schemes in intra/inter data centers thanks to their low latency, high reliability, and good cost performance. In this work, we study a microresonator-based frequency comb as a potential light source for future IM-DD optical systems where applications may include replacing individual stabilized lasers with a continuous laser driven microresonator. Regarding comb line powers and spectral intervals, we compare a modulation instability comb and a soliton microcomb and provide a quantitative analysis with regard to telecom applications. Our experimental demonstration achieved a forward error correction (FEC) free operation of bit-error rate (BER) <10−9 with a 1.45 Tbps capacity using a total of 145 lines over the entire C-band and revealed the possibility of soliton microcomb-based ultra-dense wavelength division multiplexing (WDM) with a simple, cost-effective IM-DD scheme, with a view to future practical use in data centers.

The presence of fabrication errors often degrades the performance of nanophotonic devices. Fabrication errors in the photonic crystal waveguide often appear as the Anderson localization of light. We show that the random localization of light can enhance the resolution of a photonic crystal spectrometer by utilizing this localization information in advance of the measurement.

We investigated the stability mechanism of a Raman comb by measuring RF beat noise and dispersion. Our measurements suggest that the Raman comb is stable even when only the stimulated Raman process is present.

We demonstrate the all-precision-machining fabrication of ultrahigh-quality-factor (Q) crystalline optical microresonators. The obtained Q exceeds 100 million for both magnesium fluoride (MgF2) and calcium fluoride (CaF2) materials. This constitutes the highest Q factor obtained for whispering gallery mode crystalline microresonators fabricated solely by ultra-precision machining. We also achieve octave-wide optical parametric oscillation using an MgF2 resonator fabricated with a computer-controlled machining process. This method readily enables advanced dispersion engineering for optical parametric oscillation and makes it possible to explore microcavity nonlinear optics and quantum optics without the need for skilled manual polishing techniques.

Fabrication of silicon nitride (SiN) based high-Q microring resonators prepared by the hot-wire chemical vapor deposition (HWCVD) method is presented. By the virtue of low-stress HWCVD films, no special precautions against crack propagation were required for high confinement waveguide device fabrication. By using an additional annealing process, the intrinsic Q factor in excess of 5 × 105 was obtained in the telecommunication C band, and which allowed us to observe frequency comb generation. We also investigated into the anneal temperature dependence of the residual hydrogen concentration in the film as well as the optical properties of the microring resonators.

Dissipative Kerr soliton microcombs have inspired various intriguing applications such as spectroscopy, ranging, telecommunication, and high purity microwave generation. Mechanically actuated soliton microcombs provide enhanced controllability and flexibility for Kerr solitons, thus enabling technological progress to be made on such practical applications. Here, we present architectures for coherent dual-comb techniques and ultralow-noise microwave generation by exploiting the mechanical actuation of ultrahigh-Q crystalline microresonators. By unifying a pump laser, we demonstrate highly coherent dual-soliton combs using distinct resonators with slightly different repetition rates. We also report significant phase-noise reduction achieved by directly generating Kerr solitons from a sub-Hz linewidth ultrastable laser. This study paves the way for further advancements in a wide variety of applications based on Kerr soliton microcombs.

SUMMARY We study how to optimize the coupling efficiency between silicon nitride and silicon waveguides using an edge coupler with an inverse-tapered structure, specifically targeting broad bandwidth applications. Initially, we hypothesized that mode matching alone would suffice for effective coupling. However, our results indicate that maximizing the coupling efficiency requires the simultaneous optimization of mode coupling and effective refractive-index matching. Theoretical simulations predicted a maximum coupling efficiency of 0.963, while experimental measurements yielded an efficiency of 0.588. We analyzed the wavelength dependence of our coupler, and found that the edge coupler exhibited broader bandwidth properties than a standard multilayer coupler, making it more suitable for applications across a wide wavelength range. These results indicate the potential for integrating such couplers with microresonator frequency comb systems, offering the possibility of a wide range of photonic applications.

Dissipative Kerr soliton (DKS) microcombs have various intriguing applications thanks to their features including compactness, high-repetition rate, low-power operation, and a broadband spectrum covering up to one-octave. Such unique features of DKSs have been utilized in dual-comb technologies to achieve accuracy, a high acquisition rate, and a small footprint simultaneously. However, conventional dual-comb systems using microcombs face challenges in controlling the difference in repetition rates (Δfrep) between two soliton combs while maintaining mutual coherence. When independent excitation sources and distinct microresonators are used for dual comb generation [1], it is usually difficult to maintain mutual coherence. On the other hand, the dual-combs with a single excitation source in a counter-propagating method [2] can achieve mutual coherence, but the controllability of Δfrep is limited. In this presentation, we report on a coherent dual-comb system with a shared excitation source using mechanically actuated soliton microcombs [3]. Soliton generation enabled by rapid resonance control allows us to obtain mutually coherent dual-combs with a fixed-frequency excitation source in two individual microresonators, balancing the flexibility of Δfrep and mutual coherence [4]. Figure 1 shows the experimental setup. Magnesium fluoride microresonators with FSRs of ~9.4 GHz are used as platforms. Figure 2(a) shows the combined optical spectrum of the dual soliton combs. The electrical spectrum of the dual-comb is shown in Figs. 2(b, c), where peaks are observed at intervals of Δfrep. Furthermore, we confirmed that the RF comb lines have very narrow linewidth compared to the linewidth when distinct pump lasers and resonators are used to generate dual soliton combs [Fig. 2(d,e)]. This study envisions practical dual comb applications and deployment that fully exploit the features of Kerr microcombs.

We show theoretically and numerically that mode-locking is feasible with a coupled-cavity system with gain and loss, notably, without any natural saturable absorber. We highlight that in the vicinity of the exceptional point, system Q exhibits substantial modulation even with minor refractive index changes and a minimal Kerr effect contribution. Leveraging this unique behavior, we propose an unprecedented approach wherein the lossy auxiliary cavity functions as an efficient artificial saturable absorber, thus facilitating mode-locking. This approach is not only novel, but also presents considerable advantages over conventional systems where both gain and saturable absorption are contained within a single microcavity. These benefits include reduced operational power and ease of post-adjustment, achievable through the manipulation of the coupling strength between the two microcavities.

We investigated the feasibility of IM-DD communication in a microcomb. Turing pattern and soliton combs are ideal, though MI comb can transmit signals when the pump-detuning is well controlled and excessive noise is suppressed.

We investigated the feasibility of IM-DD communication in a microcomb. Turing pattern and soliton combs are ideal, though MI comb can transmit signals when the pump-detuning is well controlled and excessive noise is suppressed.

We demonstrated low-latency optical transmission with a microresonator frequency comb using a 9-km optical fiber in a metropolitan area. The delay time of the entire system, excluding the transmission fiber, was only 3.1 us.

We demonstrated low-latency optical transmission with a microresonator frequency comb using a 9-km optical fiber in a metropolitan area. The delay time of the entire system, excluding the transmission fiber, was only 3.1 μs.

We report frequency comb generation in a silicon nitride (SiN) based microring resonator fabricated by the hot-wire chemical vapor deposition (HW-CVD) method with an intrinsic quality factor in excess of 5×105

We report frequency comb generation in a silicon nitride (SiN) based microring resonator fabricated by the hot-wire chemical vapor deposition (HW-CVD) method with an intrinsic quality factor in excess of 5×105,.

We used up-conversion to capture the localized light of a photonic crystal spectrometer with an inexpensive CMOS camera. We employed this data to detect a single wavelength using deep learning.