The wavelength of stimulated emission at 8175.25 A of GaAlAs broad-area, single-heterojunction laser diodes of the close-confinement type was experimentally found to increase as a function of emission angle. The shift was 0.42± 0.08 A/deg and approximately 0.01 A/deg in the lateral and transverse directions, respectively. The increase of wavelength is postulated to be due to momentum loss of the photons by inelastic reflections at the Fabry-Perot surfaces. An optical model was developed by which the changes in the following properties per photon per reflection were calculated for the optical cavity-air interface (no = 3.60, na = 1.00): wavelength, AX = 3.03 x 10-4 A; momentum, Ap = 3.00 x 10-30 erg.s/cm; momentum loss coefficient, n = 3.70 x 10-8; energy, DE = 2.50 x 10-20 erg; and increased angle of reflection, a = 2.04 x 10-4 deg. By determining the number of reflections per beam path in the cavity, the full width at half power for the lateral beam divergence was calculated to be 20.6°. The pressure and energy per pulse at the cavity surface of the diode were calculated from the momentum loss concept and are in reasonable agreement with the Knoop hardness and heat of fusion for the fracturing and melting of GaAs.