Gain measurements on semiconductor lasers by optical feedback from an external grating cavity

Two methods are developed to determine the curvature around the maximum of the mode gain curve of homogeneously line-broadened semiconductor lasers. In the first method, the gain curvature is indirectly determined from the wavelength dependence of the threshold current. This method includes the assumption of parabolic wavelength dependence and the linear current dependence of unsaturated mode gain. In the second method, the gain is measured directly from the value of the tuning range versus the amount of external feedback in a way that requires no assumptions concerning the functional dependence of the gain upon the wavelength and current. The mode gain width and the current-induced peak-gain wavelength shift for a GaAlAs CSP laser were determined to be 175 A and 2.1 A/mA, respectively.