This issue highlights the latest research and advancements in integrated frequency combs, with a focus on microresonator frequency combs (microcombs) and device combs. In this article, I provide an overview of the development roadmap for microcombs.

View full abstract

Fundamentals and applications of soliton microcombs in high-Q microresonators are briefly reviewed. Several methods of soliton generation and stabilization are introduced while focusing on sophisticated techniques for microresonator dispersion engineering. Recent demonstrations of practical applications using soliton combs are also discussed.

View full abstract

The application of microcombs has been growing owing to their mass producibility in compact form factors and their large comb mode spacing. Among various applications of microcombs, such as optical coherent communications, optical computing, and low noise micro/mm/THz waves generation, LiDAR is one of the most fascinating, aiming for the realization of autonomous vehicles. In this letter, several LiDAR techniques using microcombs are introduced.

View full abstract

This paper presents photonic generation of ultralow-phase noise sub-terahertz signals and its application to wireless communications systems. Optical sources based on stimulated Brillouin scattering lasers are studied, and applied to both the transmitter and receiver in 300-GHz-band wireless links exhibiting over- 200-Gbit/s data rate at transmission distance of over 200 m.

View full abstract

Optical frequency combs are expected to reduce the SWaP (Size, Weight, and Power) of optical transceivers in space. We review the progress of optical communication in space and summarize requirements for using the optical frequency comb as the seed laser of an optical transmitter.

View full abstract

Recent expanding microcomb research has been exploring various nonlinear waveguide materials combining with the on-insulator structure, which are opening pathways to highly functional integrated microcomb devices. For that, we developed nonlinear SiN, LiNbO3 and LiTaO3, and AlGaAs waveguides to utilize their unique characteristics. In this article, we provide our achievements for each and future forecasts on promising configurations for integrated microcomb devices.

View full abstract

Microcombs allow for the miniaturization of optical frequency comb sources and offers interesting opportunities for a variety of applications, ranging from astronomy to telecommunication. We review the recent progress on the fabrication of SiN-based microring resonator devices and focus on low-temperature silicon nitride film deposition methods. We also discuss the possibility of integrating a uni-directional travelling carrier photodiode with a microcomb source for achieving stable and compact signal modules in the sub-terahertz frequency range.

View full abstract

We introduce the principles of electro-optic-modulation (EO) comb generation and the recent research trends in this paper. EO combs, with the high number of degrees of freedom associated with electro-optic modulators, find applications in various fields such as dual-comb spectroscopy, optical imaging, distance measurement, microwave generation, and astrocombs. We demonstrated that an EO comb can work as a noise booster and a high-sensitivity detector for detecting the magnified noise from microwave and millimeter-wave signal generators. Our method can generate low-noise and continuously frequency- variable microwave and millimeter-wave signals. For achieving lower noise generation from EO combs, we succeeded in stabilizing the carrier-envelope-offset frequency of the EO comb. Furthermore, we present research on integrating frequency combs, achieving supercontinuum generation and f-3f self-referencing interference using SiN waveguides with an Erbium-doped fiber laser. We also demonstrated EO comb generation with 25-GHz mode spacing and over 500 GHz spectral bandwidth using 3-stage integrated silicon modulator.

View full abstract

Hybrid laser ablation processing using a combination of femtosecond and nanosecond laser pulses is a method that can efficiently couple laser pulses to materials. We measured shockwave and plume dynamics that are unaffected by the timing jitter of a Q-switched nanosecond laser pulse using a single-shot ultrafast burst imaging method (sequentially timed all-optical mapping photography with lens-array: LASTAMP) with an observation time window of 2.4 ns, a frame interval of 300 ps, and the nine frames. The nanosecond laser pulse irradiated with a time delay relative to the femtosecond laser pulse is efficiently absorbed by an ablation plume-front generated by the femtosecond laser, improving the ablation efficiency by backward propagating secondary shockwave.

View full abstract

To achieve single-shot ultrafast burst imaging with THz-wave pulses, we developed a THz-STAMP (THz-wave-sequentially timed all-optical mapping photography) optical system using a heterodyne detection method that enables high-sensitivity EO sampling. Such a heterodyne EO sampling method, which is particularly effective for measurements using low-intensity THz-wave pulses, improves the SNR and dynamic range. In the single-shot THz-STAMP, five 2D burst images with a frame interval of 374 fs are acquired, and we successfully captured the spatiotemporal distribution of photocarriers generated in a GaAs thin film by a femtosecond laser pulse.

View full abstract