We observe modification of the spontaneous emission spectrum emitted through the substrate of a quantum well semiconductor laser. The effect is driven by the proximity of the quantum well active region to the p-side electrical contact of the device, representing an excellent example of cavity quantum electrodynamics. Modification of the spontaneous emission rate and spectrum can be substantial; it must be accounted for in order to infer modal gain or carrier heating phenomena in the device correctly. ©1997 American Institute of Physics.