Highly stabilized optical frequency comb interferometer with a long fiber-based reference path towards arbitrary distance measurement

An optical frequency comb interferometer with a 342-m-long fiber-based optical reference path was developed. The long fiber-based reference path was stabilized to 10−12-order stability by using a fiber noise cancellation technique, and small temperature changes on the millikelvin order were detected by measuring an interferometric phase signal. Pulse number differences of 30 and 61 between the measurement and reference paths were determined precisely, with slight tuning of the 53.4 MHz repetition frequency. Moreover, with pulse number difference of 61, a 6.4-m-wide scanning for the relative pulse position is possible only by 1 MHz repetition frequency tuning, which makes pulses overlapped for arbitrary distance. Such wide-range high-precision delay length scanning can be used to measure arbitrary distances by using a highly stabilized long fiber-based reference path

Mode-filtering technique based on all-fiber-based external cavity for fiber-based optical frequency comb

We developed a mode-filtering technique based on the all-fiber-based external cavity for a fiber-based optical frequency comb for high repetition rate (frep) frequency comb, and the carrier envelope offset frequency (fceo) can be detected and stabilized and is robust to environmental fluctuations. To achieve multiplication of the frep with a high multiplication factor using the fiber ring cavity, a long fiber was developed to mitigate the physical limitation inhibiting the shortening of the cavity length. In this study, the length of the fiber cavity was set to 6.7 m (free spectral range = 44.7 MHz) as the fiber-based comb length was 6.1 m. We were able to demonstrate a multiplication factor of 11, i.e., frep increased from 48.7 MHz to 536.0 MHz with a side mode suppression ratio of about 25 dB using the double-pass configuration

High-coherence ultra-broadband bidirectional dual-comb fiber laser

Dual-comb spectroscopy has emerged as an attractive spectroscopic tool for high-speed, high-resolution, and high-sensitivity broadband spectroscopy. It exhibits certain advantages when compared to the conventional Fourier-transform spectroscopy. However, the high cost of the conventional system, which is based on two mode-locked lasers and a complex servo system with a common single-frequency laser, limits the applicability of the dual-comb spectroscopy system. In this study, we overcame this problem with a bidirectional dual-comb fiber laser that generates two high-coherence ultra-broadband frequency combs with slightly different repetition rates (frep). The two direct outputs from the single-laser cavity displayed broad spectra of > 50 nm; moreover, an excessively small difference in the repetition rate (< 1.5 Hz) was achieved with high relative stability, owing to passive common-mode noise cancellation. With this slight difference in the repetition rate, the applicable optical spectral bandwidth in dual-comb spectroscopy could attain ~479 THz (~3,888 nm). In addition, we successfully generated high-coherence ultra-broadband frequency combs via nonlinear spectral broadening and detected high signal-to-noise-ratio carrier–envelope offset frequency (fCEO) beat signals using the self-referencing technique. We also demonstrated the high relative stability between the two fCEO beat signals and tunability. To our knowledge, this is the first demonstration of fCEO detection and frequency measurement using a self-referencing technique for a dual-comb fiber laser. The developed high-coherence ultra-broadband dual-comb fiber laser with capability of fCEO detection is likely to be a highly effective tool in practical, high-sensitivity, ultra-broadband applications