We describe a technique for the measurement of optical gain and loss in sem
iconductor lasers using a single, multisection device. The method provides
a complete description of the gain spectrum in absolute units and over a wi
de current range. Comparison of the transverse electric and transverse magn
etic polarized spectra also provides the quasi-Fermi-level energy separatio
n. Measurements on AlGaInP quantum well laser structures with emission wave
lengths close to 670 nm show an internal loss of 10 cm(-1) and peak gain va
lues up to 4000 cm(-1) for current densities up to 4 kA cm(-2). (C) 1999 Am
erican Institute of Physics. [S0003-6951(99)04243-6].