One of the major problems in frequency-shift-keying (FSK) systems is the non-uniform frequency modulation response of semiconductor lasers which causes distortions of the optical FSK signal. For non-return-to-zero (NRZ) signals this effect leads to erroneous detection for long pseudo-random bit patterns, even for data rates as high as 2 Gbit/s. Various techniques have previously been employed at the transmitter to eliminate unwanted thermal frequency modulation, including Manchester coding,[1] equalization of the laser drive signal,[2] using bipolar signals,[3] and direct modulation of phase-tunable DFB lasers.[4] In this paper an alternate approach is reported which uses adaptive quantized feedback equalization (QFE)[5] at the receiver to overcome the problem of non-uniform FM response of diode lasers. The advantages of this technique are that only a few passive components need to be added to the receiver’s decision circuit to implement a quantized feedback equalization circuit and this technique docs not have to be tailored to the FM response of the transmitter laser.

© 1988 Optical Society of America

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