We describe a single-pass technique for the independent measurement of optical modal gain and internal mode loss in semiconductor lasers structures using a single, multi-section device which gives the loss and the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized gain spectra also identifies the transparency point, provides the quasi-Fermi level energy separation and a second means for determination of the mode loss. Measurements are described for AlGaInP quantum well laser structures with emission wavelengths close to 670 nm, yielding an internal loss of 10 cm-1 and peak gain values up to 4000 cm-1 for current densities up to 4 kAcm-2. We have also made an independent measurement of the spontaneous emission spectrum through a top-contact window on the same device structure and have converted this to local gain using the usual thermodynamical relationship. By this means we have been able to confirm the validity of this relation between gain and emission for excited semiconductor structures of this type.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.