4-ary FSK modulation of a semiconductor diode laser by injection current modulation has been demonstrated at a rate of 100 Mbit/s. The injection current modulator has a modular design which allows expansion to M-ary FSK with independent control of each tone frequency. The design allows considerable flexibility in the logical relationship between the source data and the selected frequency tones.
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Welford et al. (1985) studied this question.