Measurement of frequency interval stability for dual-frequency phase-sensitive optical frequency-domain reflectometry

Dual-frequency phase-sensitive optical frequency-domain reflectometry shows an important potential with the enhanced sensing dynamic range. However, the sensing performance is limited by the stability of the frequency intervals and the sweep synchronization between two lasers. In this work, we report an approach for the frequency interval measurement based on the use of independently operating optical frequency comb. By taking advantage of the mutual coherence and stable mode-spacing of the comb modes, the relative frequency drift between two sweep lasers can be directly obtained, enabling accurate evaluation for the stability and the synchronization of the frequency interval between the two lasers. Experiments were conducted on optical phase-locked dual-frequency lasers with an interval of 1THz. An enhancement in the stability of the frequency interval from 1.98MHz to 7.03kHz before and after phase locking has been verified with the proposed method. The sensitivity reaches 9.23dB Hz/ √Hz@100Hz . This system does not require high-frequency detection, has no restriction on frequency interval, and is independent of highly stable references. In addition to the standalone operation of the comb, it thus exhibits versatility for further applications.