Dual-Comb Build-Up Dynamics in Polarization-Multiplexing Fibre Laser

Dual optical frequency combs exploiting two pulse trains with slightly different repetition rates have recently found applications in the distance ranging (LIDARs), structural health monitoring, gas sensing, optical communications, and instrumentation calibration for astronomy. Unlike the dual-comb based on two synchronized ultrafast lasers, the single source dual-comb (dual-comb laser) has some advantages in common noise suppression and combs' coherence in a shared cavity. Though the individual repetition rate of a free-running dual-comb of the single mode-locked laser drifts over time, the offset between the two combs has superior long-term stability. So, the single-cavity dual-comb lasers do not need complex phase-locking systems and have simple control of the difference in repetition rates. This paper provides the results of an experimental study of the polarisation-multiplexed dual-comb laser, which was designed based on Er-doped fibre laser mode-locked by carbon nanotubes. The laser has a fundamental frequency of 12. 6 MHz and a tunable repetition rate difference of a few hundred Hz. The dual-comb regimes are relatively stable, having a slight drift of 6 Hz over 6 hours. Comb separation with the polarisation controller and polarisation beam splitter demonstrated the extinction ratio of 19 dB. The fast polarimeter (Novoptel PM1000-XL-FA-N20-D) has detected the polarisation dynamics with ten nanoseconds resolution time. Unlike the scalar interferograms, polarisation dynamics shows the beating of the signals for cross-polarised states of polarisation, the phase difference between them and the degree of polarisation. The analyses of polarization dynamics conclude that this polarization-multiplexed dual-comb source can be used to design the polarimetric LIDAR able to recognize the object's texture based on the polarimetric signatures. Also, the developed approach can find application in Structural Health Monitoring for detecting the shape distortions, such as stretch, twist, bend, and so to discriminate different structural defects by detecting the polarimetric signatures.