Aninsitulaser reflectometer is used to determine the surface roughness of III–V laser heterostructure as it etches in an electron cyclotron resonance etcher. A stochastic model links the reflectometer signals with the fluctuations of the surface height and slope, and with the size of the illuminated surface area. This technique is valuable for improving the quality of optical waveguides etched in III–V heterostructure, and the electrical contacts for optoelectronic devices. ©1996 American Institute of Physics.