Abstract
In a long span and/or high speed optical fiber transmission system, frequency shift keying (FSK) modulation is favored over intensity modulation for the laser source because the FSK operation is not limited by fiber dispersion and requires less modulation current than the intensity modulation. A laser with uniform high sensitivity and wide bandwidth frequency modulation (FM) response is essential to the FSK optical fiber system. Although the quarterwave shifted distributed feedback (QWS-DFB) lasers are developed to have a larger sidemode suppression ratio and lower mode partition noise than those of conventional DFB lasers,1 they are also found to have lower thermal cutoff frequencies, which are desirable for the FSK operation. Since the thermal modulation and carrier modulation in a laser have the opposite phases, the combined effects usually result in an amplitude dip in a laser's FM response and a phase reversal at the thermal cutoff frequencies.2–4 Hence to reduce the performance degradation due to this FM deficiency in a FSK system, a laser source which has low thermal cutoff frequency is desirable. We report the fabrication and performance characteristics of QWS-DFB lasers and the system performance of a 1.7-Gbit/s and a 622-Mbit/s incoherent FSK system employing this type of laser.
© 1990 Optical Society of America
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