At low temperatures, indirect excitons formed at the in-plane electron-hole
interface in a coupled quantum well structure undergo a spontaneous transition
into a spatially modulated state. We report on the control of the instability
wavelength, measurement of the dynamics of the exciton emission pattern, and
observation of the fluctuation and commensurability effect of the exciton
density wave. We found that fluctuations are strongly suppressed when the
instability wavelength is commensurate with defect separation along the exciton
density wave. The commensurability effect is also found in numerical
simulations within the model describing the exciton density wave in terms of an
instability due to stimulated processes
