Spectral Peaked Optical Frequency Comb for Highly Sensitive Spectroscopy

Optical frequency combs represent a breakthrough in the field of metrology. Because an optical frequency comb has a large number of comb modes, the optical energy is dispersed over the comb modes, and the optical energy per comb mode is very small. In terms of their application to highly sensitive spectroscopy, such as cavity ring‐down spectroscopy and saturated‐absorption cavity ring‐down spectroscopy, because a higher optical energy per mode is a mandatory factor, comb mode selection and power scaling are important issues. This article reviews the development of a spectrally peaked optical frequency comb using the novel phenomenon of spectral peaking for highly sensitive spectroscopy. First, the physical mechanisms and fundamental characteristics of spectral peaking are described. Next, the intense spectral peak generation in a passively mode‐locked ultrashort pulse fiber laser is explained. Background suppression techniques are important for applying generated spectral peaks to highly sensitive spectroscopy. Three techniques developed for spectral peak filtering are presented. Furthermore, the high‐repetition‐rate optical frequency comb developed for the seed comb source of the freely controllable comb and the recently developed coherence‐preserving amplification of an optical frequency comb using a fiber Raman amplifier are discussed. Finally, this study is summarized, and the future prospect is outlined.