Phase-locking of two coupled EDF ring lasers

The phase synchronization (phase-locking) of two lasers occurs in generating the same frequency (they can differ only in phase). The phase-locking effect can by forced by mutual radiation coupling between lasers. We investigated experimentally that phenomenon in two erbium doped ring fibre lasers. They were strongly coupled by using a 50/50 fibre coupler. Both lasers had the same resonator lengths in order to keep their free spectral ranges equal. The phase-locking state was diagnosed by observing the spectrum of the beat signals. Using a tuneable fibre Bragg grating, the locking range was investigated. One laser operated at 1551.91 nm, the second one was tuned around. Due to frequency pulling effect, the common frequency of both synchronized lasers changed from 1551.26 to 1552.22 nm. That locking range was about 225 GHz (1.8 nm). Although the lasers are not single-frequency ones, we showed, that proper designing, careful alignment and phase control, can give effective phase-locking effect.