Locking of the DFB laser through fiber optic resonator on different coupling regimes

We study self-injection locking of DFB laser obtained with a feedback loop that comprises a ring optical fiber cavity. The cavity is tuned to operate in the under-coupled, critically coupled, and over-coupled regimes. The laser operation is found to depend on the cavity coupling regimes. For the locked laser the experimental dependences of the transmitted and reflected from the ring cavity powers are well described by the analytical expressions for the ring cavity only. The best laser stability is observed with critical coupling. In this case the laser power is accumulated inside the cavity providing strong feedback for laser locking and leading to significant narrowing of the laser emitting spectrum. Reduction of the laser linewidth more than 1000 times is demonstrated with the ring cavity operating in under-coupled and critically coupled regimes.