Kenji Nishimoto

Self-injection-locked (SIL) narrow-linewidth lasers based on high-Q microresonators are promising sources for frequency-modulated continuous-wave (FMCW) LiDAR, but the SIL mechanism, as well as its key characteristics, such as the frequency sweep range and the noise performance, are often determined by uncontrolled backscattering in the resonator. Here, we investigate a tunable SIL laser based on a corrugated photonic-crystal (PhC) microresonator in which the feedback strength is set by design. Numerical and experimental results show that stronger SIL feedback expands the sweep range accessible through resonator modulation while also impacting the phase-noise and linewidth during sweeping, revealing a trade-off between frequency tunability and noise performance. Using CMOS-compatible microheater tuning (sub-1 V driving voltage), we demonstrate linearized up- and down-chirps with 224 THz/s over approximately 3 GHz and, in a proof-of-concept ranging experiment, measure a 10 m fiber length with a standard deviation below 3 mm. These results establish PhC microresonators with engineered SIL feedback as robust, compact, CMOS-compatible LiDAR engines.

Dissipative Kerr solitons (DKSs) generated in high-Q microresonators driven by continuous-wave (CW) lasers provide chip-scale optical frequency combs composed of mutually coherent CW lines. However, their small mode volume makes them highly susceptible to thermal fluctuations, and the resulting thermo-refractive noise perturbs the repetition rate, frep. Here, we experimentally demonstrate a “blue-detuned” DKS in a coupled-ring microresonator. By employing avoided-mode-crossing-induced dispersion engineering at the pump mode, DKSs are generated even when the pump laser is tuned to the higher-frequency (blue) side of the resonance. In this regime, the pump laser not only seeds DKS formation but also serves as a cooling laser for the thermally sensitive pumped mode. We observe a self-cooling effect that reduces the phase noise of frep by up to 14.5 dB, while achieving a pump-to-comb conversion efficiency as high as 37%. These results establish blue-detuned DKSs as a thermally robust and power-efficient solution for integrated microcomb systems, eliminating the need for auxiliary lasers.

We propose a relatively simple and robust system architecture for soliton microcomb generation, based on two key techniques: direct fiber coupling using high numerical-aperture polarization-maintaining single-mode fibers (high-NA PMFs) and active temperature control of the microresonator. These complementary strategies address two major challenges in microcomb implementation: environmental sensitivity and resonance instability. Building on prior work using single-mode fiber (SMF)-based direct coupling, which demonstrated device miniaturization and partial suppression of thermal drift in total fiber-to-fiber insertion loss, our PMF-based approach offers enhanced thermal stability and significantly greater robustness to environmental disturbances such as temperature fluctuations and vibration. In our system, the need for precision alignment using microscopes or multi-axis stages is alleviated, enabling a less complex optical setup and stable long-term operation. The direct coupling scheme achieved a total fiber-to-fiber insertion loss of 2.39 dB and maintained stable soliton operation for over 24 hours under external perturbations. In parallel, active temperature control of the microresonator was quantitatively evaluated, reducing the wavelength variation of the pump and auxiliary lasers by 79% and 97%, respectively. This stability enables reproducible soliton generation even in thermally dynamic environments. Comparative experiments with SMF-based direct coupling and lensed-SMF-based free-space coupling systems confirmed the superior performance of the PMF-based design in terms of coupling stability, soliton lifetime, and immunity to environmental noise. The architecture developed in this study lays a strong foundation for future integration into compact modules, with ongoing efforts aimed at reducing the reliance on laboratory-scale laser systems and optical components.

The increasing demand for higher data rates in 6G mobile wireless systems has sparked a keen interest in terahertz (THz) waves as a high-frequency, high-bandwidth carrier. This study presents a novel approach to wireless data transmission at 560 GHz, leveraging the use of THz waves generated through the injection-locking of a pair of distributed feedback lasers into a Kerr micro-resonator soliton comb. Experimental results demonstrate a Q-factor of 6.23 in 1-Gbit/s on-off-keying data transmission, which closely approaches the error-free limit represented by a Q-factor of 6.36. Additionally, the study achieves low error vector magnitudes for various modulation formats: 23.9% for 1-GBaud binary-phase-shift-keying, 23.6% for 1-GBaud quadrature-phase-shift-keying, and 8.07% for 0.1-GBaud 16-quadrature-amplitude modulation. This innovative approach holds promise for achieving high-quality, high-speed wireless data transmission, thereby advancing THz communication technology for integration into 6G systems.

Terahertz (THz) waves have attracted attention as carrier waves for next-generation wireless communications (6 G). Electronic THz emitters are widely used in current mobile communications; however, they may face technical limitations in 6 G with upper-frequency limits. We demonstrate wireless communication in a 560-GHz band by using a photonic THz emitter based on photomixing of a 560-GHz-spacing soliton microcomb in a uni-travelling carrier photodiode together with a THz receiver of Schottky barrier diode. The on-off keying data transfer with 2-Gbit/s achieves a Q-factor of 3.4, thus, satisfying the limit of forward error correction.

We report the thermal control of a dissipative Kerr microresonator soliton comb via an optical sideband generated from an electro-optic modulator. Same as the previous reports using an independent auxiliary laser, our sideband-based (S-B) auxiliary light also enables access to a stable soliton comb and reduces the phase noise of the soliton comb, greatly simplifying the set-up with an auxiliary laser. More importantly, because of the intrinsically high frequency/phase correlation between the pump and S-B auxiliary light, the detuning between the pump and resonance frequency is automatically almost fixed, which allows an 18 times larger “effective" soliton existence range than the conventional method using an independent auxiliary laser, as well as a scanning of the soliton comb of more than 10 GHz without using microheaters.