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

All-polarization-maintaining, polarization-multiplexed, dual-comb fiber laser with a nonlinear amplifying loop mirror

We developed an all-polarization-maintaining, polarization-multiplexed, dual-comb fiber laser with a nonlinear amplifying loop mirror (NALM) mode-locking mechanism. Owing to the use of the slow and fast axes of a polarization-maintaining fiber (PMF), the dual-frequency combs with slightly different repetition rates from the single-laser cavity are generated at the same center wavelength without extra-cavity nonlinear spectral broadening. The narrow relative beat note between the two frequency combs is obtained with a full-width-at-half-maximum of ~1 kHz in the optical frequency domain. The two frequency combs have high relative stability and mutual coherence owing to passive common-mode noise cancellation

Precise and highly-sensitive Doppler-free two-photon absorption dual-comb spectroscopy using pulse shaping and coherent averaging for fluorescence signal detection

We demonstrated Doppler-free two-photon absorption dual-comb spectroscopy of 5S1/2 - 5D5/2 and 5D3/2 transitions of Rb. We employed simple pulse-shaping of the dual-comb source and eliminated Doppler-broadening backgrounds, which cause fitting errors of the Doppler-free signals. Moreover, to improve sensitivity, we investigated the coherence in dual-comb fluorescence signals and the coherent averaging method was applied to fluorescence dual-comb detection for the first time. The detection sensitivity was significantly improved by coherent averaging to reduce the noise floor. Observed Doppler-free spectra was fitted to Voigt profiles and we performed absolute frequency determination with a precision of about 100 kHz