Self-injection locking of the DFB laser through ring fiber optic resonator

We study self-injection locking of DFB laser through ring fiber optic resonator in the different regimes. Experimentally measured transmitted and reflected from fiber cavity powers at undercoupled, critically coupled, and overcoupled regimes are properly coincide with the calculated values. Significant reduction of locked laser linewidth was observed. For the free running laser, the full-width at half-maximum linewidth was equal to 2.55 MHz assuming that line shape is Lorentzian. Meanwhile the self-injection locked DFB laser demonstrates linewidth equal to 2.5 KHz for undercoupled and critically coupled regimes and about two times more for overcoupled one.