An optical communication channel is analyzed, in which a light beam is amplitude modulated at the source by a filter of continuously variable transmittance, and the detector counts the received photons. Such a communication channel has intrinsic noise limitations because there is not a one-to-one correspondence between the modulated beam power and the number of counts registered. The information rates achievable with single-mode and multimode lasers are evaluated as functions of the mean number N of detected photons per symbol, for several different input statistics. For large N the information rate increases logarithmically with N. It is shown that, when the symbol length is short, there is a minimum number of independent modes for which the multimode laser gives a greater information rate than the single-mode laser, if the laser power is equally divided among all the modes, and the power per mode is regarded as constant. However, for even moderate numbers of detected photons per symbol, this minimum number of modes is so great that the single-mode laser is to be preferred. When the light beam is derived from a thermal source, the information rate in the channel is, in effect, governed by the same equations as those for the single-mode laser, so long as the detector area is limited to a coherence area.
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