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, equalization of the laser drive signal, using bipolar signals, and direct modulation of phase-tunable DFB lasers. In this paper an alternate approach is reported which uses adaptive quantized feedback equalization (QFE) 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 AmericaPDF Article