Therefore, the individual pulses of the PPM signal are shifted from their non-modulated positions of ??t0=tp/4 within the time slot of ???tp/4<??t<tp/4. [0039] The linearity of an optical PPM receiver using top hat generators, whether using the coherent detection technique described above and shown in FIG. 2 or other techniques or apparatus, depends on the quality of the rectangular pulses generat