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To characterize semiconductor lasers, it is often required to measure parameters such as the quasi Fermi-level separation, intrinsic optical loss, the position of the gain peak, and gain and absolute refractive index spectra. For these measurements, there are many different techniques available, but they neglect to take into account the dispersion of refractive index and cannot be used to extract the absolute refractive index spectrum. A novel technique is proposed to systematically and accurately measure all these parameters of semiconductor lasers. Compared with techniques often used, which will be briefly reviewed in the paper, this novel technique has the following advantages: (1) the determination process uses only the measured spontaneous emission spectra, without the requirement for knowledge of such parameters as intrinsic optical loss, facet reflectivity, and waveguide confinement factor, which presently are difficult to check experimentally; (2) results are obtained for each given current (for example, this technique measures intrinsic optical loss for each given current, rather than the average one over the whole current range); (3) the dispersion of refractive index is taken into account; (4) both the absolute refractive index spectrum for a given current and its change with current can be accurately measured; (5) the gain spectra and refractive index can be measured as wide as one wants; (6) the measurement accuracy is improved; and (7) no adjustable parameter or recalibration is needed.