Linewidth reduction of 1.5-μm grating loaded external cavity semiconductor laser by geometric reconfiguration

Abstract

It is found experimentally that the linewidth of a grating loaded external cavity semiconductor laser is inversely proportional to the square of the linear dimension of the overall cavity. This observed behavior is consistent with the existing theories on the linewidth of a laser with an extended passive cavity. A slope of 6.5×105 Hz cm2 is obtained from least-squares fitting the linewidth vs 1/(cavity length)2 curve at 1 mW of power. However, the linewidth reaches a lower limit when the cavity length is extended beyond a certain limit. This result suggests that additional phase noise term should be included in the calculation of linewidths of these extended cavity semiconductor lasers. We also report a value as small as 18 kHz mW in the linewidth vs 1/(power) curve.