Frequency-shift-keying (FSK) is an attractive modulation format for coherent optical fiber transmission systems, since the transmitter laser frequency can be directly modulated by current modulation. However, most semiconductor lasers have a nonuniform frequency modulation (FM) characteristic1 due to temperature changes in the laser for modulation frequencies below ~10 MHz. For nonreturn-to-zero (NR2) signals, modulation of the laser temperature can lead to drifts in the optical frequency during long strings of zeros or ones, causing errors for long pseudorandom bit patterns, even for data rates as high as 2 Gbit/s.2 Various techniques have been previously employed to eliminate unwanted thermal frequency modulation, including Manchester coding,3 equalization of the laser drive signal,4 and direct modulation of phase tunable DFB lasers.5 This paper demonstrates an alternate approach which overcomes the problem of nonuniform FM response of diode lasers by using a bipolar signal format. A bipolar signal, which is similar to the time derivative of an NRZ signal, has the property that the power in the low-frequency portion of its baseband spectrum is suppressed. With the suppression of the low-frequency components of the baseband signal, the unwanted thermal frequency modulation of semiconductor lasers can be avoided, independent of the frequency modulation response of the particular laser. Furthermore, the baseband bandwidth required for the bipolar signal is less than that for an NRZ signal. Manchester coding, which also suppresses the low-frequency components, requires twice the bandband bandwidth of a bipolar signal and requires a coder and decoder.
© 1988 Optical Society of AmericaPDF Article