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A high efficiency optical frequency discriminator/photodetector exhibiting a conversion efficiency exceeding 1 mV/GHz is described. The operating principle consists in monitoring the intracavity optical power change induced by optical FM signals through the voltage change in the phase shifter region of an injection-locked 1.53 mu m GaInAs/GaInAsP/InP three-section two-contact lambda /4-shifted MQW DFB laser. This principle is demonstrated by demodulating FSK signals at 700 Mbit/s using neither an interferometer nor an additional photodetector. A receiver sensitivity of -21.9 dBm is reported with an NRZ (27-1) PRBS.