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A technique based on the integration of the product of amplified spontaneous emission spectrum and a phase function over one mode interval is proposed for measuring the gain spectrum for Fabry-Perot semiconductor lasers, and a gain correction factor related to the response function of the optical spectrum analyzer (OSA) is obtained for improving the accuracy of the measured spectrum. Gain spectra with a difference less than 1.3 cm/sup -1/ from 1500 to 1600 nm are obtained for a 250-μm-long semiconductor laser at an OSA resolution of 0.06, 0.1, 0.2, and 0.5 nm. The corresponding gain correction factor is about 9 cm/sup -1/ at a resolution of 0.5 nm. The gain spectrum measured at a resolution of 0.5 nm has the same accuracy as that obtained by the Hakki-Paoli method at a resolution of 0.06 nm for a laser with a mode interval of 1.3 nm.