This paper describes the overview of special issue on exploration of the new world by optical frequency comb technology. As an introduction of the special issue, I describe the history of technology development and new trends on optical frequency comb applications. Optical frequency comb is no longer a simple optical frequency ruler, but a platform of broad science and technology fields, which enables innovative applications. This special issue will visualize such new trends of optical frequency comb applications.

View full abstract

Optical measurements, which play a pivotal role in scientific, industrial, and medical fields, utilize diverse optical properties. The development of optical-frequency-comb is a breakthrough in such optical measurements. Optical-frequency-combs exhibit highly time-controlled pulse trains or meticulously structured multispectral arrays of narrow optical frequency modes and offer remarkable accuracy and precision. The controlled nature of optical-frequency-combs allows for highly accurate and precise optical measurements, contributing to a broad spectrum of precision measurements. This review describes the potential applications of optical-frequency-combs in optical microscopy. Integrating them into optical microscopy introduces a novel approach that leverages the temporal and spectral properties, including methods for extracting various light parameters through the time characteristics of optical-frequency- combs and superimposing spatial information on each comb mode by utilizing their spectral characteristics. Optical microscopy with optical-frequency-combs is a novel means of optical measurement with broad applicability across various fields.

View full abstract

We present a study that quantitatively measures the amplitude of propagating surface acoustic waves using an optical frequency comb. Because the vibration amplitude of these waves is on the order of several tens of picometers, it is crucial to minimize measurement errors caused by the optical equipment’s vibration effects. This paper outlines an experimental setup designed to reduce such errors, enabling precise measurement of surface vibrations in the tens of picometers range through optical frequency comb spectroscopy. Additionally, we will share preliminary findings indicating that the energy from surface acoustic waves can be converted into the energy of ferromagnetic resonance in a thin nickel film sample.

View full abstract

Dual-comb spectroscopy has recently attracted attention as an excellent spectroscopic application of optical frequency combs (optical combs). This article introduces an application of dual-comb spectroscopy to in-plane angle measurement. We extended dual-comb technology to the spatiotemporal dimension by combining it with an optical vortex, developed a dual-optical-vortex comb spectroscopy method, and demonstrated angle measurement for the first time. The result shows the great potential of optical combs for new technological development based on the spatiotemporal coherent controllability of light waves.

View full abstract

We describe the recent developments in optical-to-microwave frequency comparison, which is the most fundamental application of an optical frequency comb. We focus on a comparison between 10 MHz and an optical frequency, which is needed to compare Coordinated Universal Time (UTC) with an optical clock. We report on the worldwide research situation, the precision limit conditions, and our achievement of a 10 MHz-to-optical frequency comparison with a 10− 18 precision level at NMIJ/AIST.

View full abstract

Sound plays a vital role in various aspects of everyday life, including human communication, industrial monitoring, and medical diagnosis. Although sound is generally measured using microphones, research on optical sound measurements have recently progressed. This paper reviews the overview of optical sound measurement to date and describes a method for the absolute measurement of sound pressure in air using an optical frequency comb. Measuring the resonance frequency shift of an optical resonator caused by sound using the optical comb makes it possible to quantitatively determine the sound pressure.

View full abstract

In order to prove the advantages of a directly-irradiated laser peening with double spots (DSLP) over a conventional directly-irradiated laser peening with a single spot (SSLP), we investigated the changes in residual stress onto the surface of a metal treated with two varying parameters, the spot distance (dspot = 0 - 1.5 mm) and the shifting distance (dshift = 50 - 500 μm). The following were found from experimental results. (1) Respective compressive regions of residual stress were formed by double spots and overlapped each other when dspot ＞ ~1.0 mm. (2) The enhanced compressive residual stress, which was almost the same as the results of a SSLP treatment, was observed at dspot = 1.48 mm and dshift = 250 - 350 μm. We believe that the formation of the enhanced compressive residual stress observed after a DSLP treatment could be attributed to the overlapping effect of the compressive residual stress regions.

View full abstract