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In this paper, we describe methods for analysis of edge-emitted amplified spontaneous emission spectra measured as a function of the pumped stripe length. We show that both the modal gain and the unamplified spontaneous emission spectra can be extracted from the data, and we describe a means of calibrating the spontaneous emission in real units, without requiring the carrier populations to be described by Fermi functions. The gain and emission spectra can be determined for transverse electric and transverse magnetic polarizations and by summing the recombination currents for each polarization the total radiative current can be measured. This enables the overall internal radiative quantum efficiency to be calculated. Once the calibration factor is known the internal stimulated recombination rate at the facet can also be estimated. The experiment can be configured to give a measurement of the passive modal absorption of the gain medium. The internal optical mode loss can be determined from the long-wavelength region of the gain spectrum or the modal absorption spectrum. In summary, we show that measurements of amplified spontaneous emission spectra provide a full characterization of the gain medium.