Passive equalization of semiconductor diode laser frequency modulation

Optical heterodyne communication systems using direct-frequency-modulated (FM) semiconductor diode lasers often exhibit system degradation because of the nonuniform injection-current-to-FM transfer function present in most semiconductor diode lasers. In a FSK system, a nonuniform FM transfer function causes tone drift which results in increased crosstalk between time slots and a corresponding degradation in system bit-error rate (BER). Using simple passive electrical networks it is possible to equalize the nonuniform FM response and substantially reduce this transfer-function-induced BER degradation. The theory, computer optimation, construction, and test of various equalization networks and their use in realizing an FM-equalized transmitter for a 100-Mb/s binary FSK communication system is described.>