Abstract

Frequency-shift-keying (FSK) is an attractive modulation format for both direct detection and coherent detection systems because the laser is directly modulated and the excessive wavelength-chirping of direct intensity modulation is avoided. For conventional non-return-to-zero (NRZ) signals, the FSK transmitter laser should have a frequency modulation (FM) response which is uniform over the bandwidth of the baseband signal. FSK modulation has been demonstrated up to 11 Gb/s using a conventional distributed-feedback (DFB) laser.¹ Conventional DFB lasers have a nonuniform FM response, however, with a dip in the magnitude and a 180 degree change in phase near 1 MHz. This nonuniform FM response, which stems from the competition of thermal and carrier density effects, can cause pattern-dependent systems degradations. Multi-section DFB lasers can have a uniform FM response without the so-called “thermal dip.” Recently, FSK modulation was achieved at 1.7 Gb/s using a two-section (2S)-DFB laser module² and at 8 Gb/s using a three-section-DFB laser module.³

© 1991 Optical Society of America