Threshold variations in diode lasers induced by external resonator feedback

The effective reflectivity of an external resonator mirror coupled to a laser diode is typically inferred from the measured total reduction in threshold current when the laser diode is incorporated into the external resonator. The present paper demonstrates that this procedure overestimates the effective resonator feedback because the decrease in overall resonator losses causes a shift in laser output wavelength, which in turn leads to a reduction in wavelength-dependent losses. For the commercially available devices studied, the reduction in wavelength-dependent loss accounts for roughly half of the observed threshold current reduction. Experimental techniques are described in this paper for separating the two contributions to threshold reduction and thereby determining the actual value of the external resonator effective reflectivity, An additional complication arises because of the observed strong bias current dependence of the external resonator coupling in the region below lasing threshold where such lasers are typically biased in various mode-locking schemes. The magnitude of this bias current dependence, attributed to variations in laser mode guiding, has also been determined.