The bit-error-rate impairment using a directly modulated DFB laser transmitter and an externally modulated YAG laser transmitter are compared in a hybrid multichannel AM-VSB/16-QAM video lightwave transmission system. A 16-QAM bit-error-rate floor at 10/sup -7/ is measured using the DFB laser transmitter (AM modulation index of 4.2%), which is much higher than the 10/sup -9/ BER floor measured for the EM-YAG laser transmitter (AM modulation index of 3.6%) with the same analog carrier-to-noise ratio and received optical power. A similar 16-QAM bit-error-rate floor at 10/sup -9/ is observed using both laser transmitters with the same AM modulation index of 3.7%. However, in this case, the analog CNR using the DFB laser transmitter is about 1.5 dB lower than that of the YAG laser transmitter.>
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Ovadia et al. (1994) studied this question.
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