Linewidth reduction of cleaved-coupled-cavity lasers by optical feedback from a single-mode polarisation-preserving fibre external cavity

Linewidth characteristics of a cleaved-coupled-cavity laser with a single-mode polarisation-preserving fibre external cavity have been investigated as a function of feedback power level and side-mode suppression ratio. The optimum power feedback ratio is near 2×10¿4 for a minimum linewidth of about 30 kHz. A weak dependence of linewidth on side-mode suppression ratio is observed as long as the suppression ratio is greater than 500:1.