Published papers

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.

Mode-locked ultrashort pulse sources with a repetition rate of up to several tens of gigahertz greatly facilitate versatile photonic applications such as frequency synthesis, metrology, radar, and optical communications. Dissipative Kerr soliton microcombs provide an attractive solution as a broadband, high-repetition-rate compact laser system in this context. However, its operation usually requires sophisticated pump laser control to initiate and stabilize the soliton microcombs, particularly in millimeter-sized ultrahigh-Q whispering-gallery resonators. Here, a mechanically actuated soliton microcomb oscillator is realized with a microwave repetition rate of 15 GHz. This enables direct initiation, subsequent long-term stabilization, and fine tuning of soliton combs without pump laser tunability that is generally required for soliton microcomb operation. The prospects for using this method with a wide range of applications are revealed that will benefit from mechanical soliton actuation such as optical clocks, spectral extension, and dual-comb spectroscopy.

We numerically study the mode-locking behavior in a system in which an erbium-doped resonator (gain) is coupled to a carbon-nanotube functionalized resonator (nonlinear loss). We find that mode-locking is possible even with a small gain and relatively low Q by adjusting the coupling between two cavities.

We numerically study the mode-locking behavior in a system in which an erbium-doped resonator (gain) is coupled to a carbon-nanotube functionalized resonator (nonlinear loss). We find that mode-locking is possible even with a small gain and relatively low Q by adjusting the coupling between two cavities.

Optical frequency combs based on microresonators (microcombs) have been actively researched since the first experimental achievement of soliton mode-locking. Soliton pulses (dissipative Kerr solitons; DKSs) possess high repetition rates ranging from tens to hundreds of GHz. Thanks to this feature, DKSs are expected to benefit a wide range of applications, such as optical communications. In particular, DKSs are highly suitable for wavelength division multiplexing (WDM) transmission, which exploits an individual comb line as an optical carrier, owing to its broad bandwidth and low-noise properties [1] , [2]. Although coherent transmissions are currently the mainstream of optical communication, transmissions employing the intensity-modulation direct-detection (IM-DD) method remain in use, e.g., in data centers [3]. The advantage of an IM-DD transmission is that it does not require complex digital signal processors, resulting in a low power consumption and a small-signal delay, both of which are key characteristics required in the Beyond-5G technology. The IM-DD method can be used in conjunction with microcombs to achieve high-speed transmission while maintaining these advantages.

Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

Mode-locking usually requires saturable absorption or active modulation to trigger and build short pulses. We show through calculations that self-starting mode-locking may be achieved simply by coupling active and passive microresonators without a saturable absorber.

Spectra are reconstructed by capturing wavelength-specific patterns of localized light in a photonic-crystal-waveguide. We show that real-time reconstruction at a wavelength resolution of 0.07 nm is possible by using low-rank approximation with singular-value-decomposition and regularized-least-squares.

Spectra are reconstructed by capturing wavelength-specific patterns of localized light in a photonic-crystal-waveguide. We show that real-time reconstruction at a wavelength resolution of 0.07 nm is possible by using low-rank approximation with singular-value-decomposition and regularized-least-squares.

We investigated the stability, longitudinal spacing, and mutual coherence of a generated Raman microcomb and found that we can obtain a highly stable and coherent Raman microcomb even without a four-wave mixing process.

We investigated the stability, longitudinal spacing, and mutual coherence of a generated Raman microcomb and found that we can obtain a highly stable and coherent Raman microcomb even without a four-wave mixing process.

We investigated the stability and mutual coherence of a Raman microcomb in a silica microrod resonator by monitoring the output power and longitudinal mode spacings. The results indicate that we can obtain a stable Raman comb formation without the need for four-wave mixing processes. The use of a Raman comb will open the possibility of simplifying the setup because it will relax the phase matching condition usually required for microresonator frequency comb generation. Although there are some restrictions in regard to using a Raman comb for applications due to the coexistence of the comb components in different mode families, a proof-of-concept demonstration shows that it is sufficiently stable and robust for applications such as optical communications.

We numerically investigate soliton microcomb generation with a saturable absorption effect. We found that a cavity-integrated saturable absorber allows the formation of a much higher number of solitons than possible with a Kerr-only monolithic cavity.

We numerically investigate soliton microcomb generation with a saturable absorption effect. We found that a cavity-integrated saturable absorber allows the formation of a much higher number of solitons than possible with a Kerr-only monolithic cavity.

We explore the synchronization of chaotic microresonator frequency combs, emphasizing the modulation instability state, which is known for its inherent chaotic behaviors. Our study confirms that the synchronization of two such combs is feasible by injecting the output from the lead microresonator into the next microresonator’s input. We also identify the optimal parameters for this synchronization. Remarkably, even partial injection from the leader is sufficient for synchronization, paving the way for versatile future system configurations. Such systems could simultaneously utilize distinct spectral components for synchronization and transmission. This work advances our understanding of chaotic microresonator combs, showing them to be pivotal elements in next-generation optical communication systems.

Microresonator-based optical frequency combs emitted from high-quality-factor microresonators, also known as microcombs, have opened up new horizons to areas of optical frequency comb technology including frequency metrology, precision sensing, and optical communication. To extend the capability of microcombs for such applications, large and reliable tunability is of critical importance. Here, we show broad spectral tuning of Kerr soliton microcombs in a thermally controlled crystalline microresonator with pump-detuning stabilization. The fundamental elements composing frequency combs, namely the center frequency, repetition frequency, and carrier-envelope offset frequency, are spectrally tuned by up to −48.8 GHz, −5.85 MHz, and −386 MHz, respectively, leveraging thermal effects in ultrahigh-Q crystalline magnesium fluoride resonators. We further demonstrate a 3.4-fold enhancement of soliton comb power resulting from thermal expansion with a temperature change of only 28 K by employing quantitative analyses of the fiber-to-resonator coupling efficiency.

We report the thermal tuning of a Kerr soliton frequency comb in an ultrahigh-Q magnesium fluoride crystalline microresonator yielding broad pump frequency tuning of up to ~ 50 GHz and a repetition rate of ~ 6 MHz.

We report the thermal tuning of a Kerr soliton frequency comb in an ultrahigh-Q magnesium fluoride crystalline microresonator yielding broad pump frequency tuning of up to ~ 50 GHz and a repetition rate of ~ 6 MHz.