Significant Performance Advantage of Electroabsorption Modulator Integrated Distributed Feedback Laser (EML) Transmitter in Transporting Multi-carriers QAM Signals

We find that for transporting quadrature amplitude modulated (QAM) RF sub-carriers in suboctave frequency range, electroabsorption modulator integrated distributed feedback lasers (EMLs) can be modulated with significantly higher (2.5 times) modulation index without any in- band signal distortion as compared to the directly modulated and commercially available externally modulated DFB laser transmitters in 1.55micron band. This occurs when the selection of frequency range and biasing the modulator section of the EML is such that the third-order inter- modulation distortion is suppressed and the second order distortion is outside the in-band frequency range. We have used an equivalent of forty 64- and 256-QAM carriers in 550-800 MHz with the test carrier modulated with 5.063 M symbols/s using DAVIC forward error correction scheme.