Data are presented that show that, for transporting quadrature amplitude modulated (QAM) radiofrequency (RF) subcarriers 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 or externally modulated (using LiNbO3 MZ modulator) distributed feedback (DFB) laser transmitters in the 1.55-m band. This occurs when the selection of frequency range and biasing the modulator section of an EML is such that the third-order intermodulation distortion is suppressed and the second-order distortion is outside the in-band frequency range. We have used an equivalent of 64-and 256-QAM 40 carriers in 550-800 MHz with the test carrier modulated with 5.063 M symbols per second. For a given bit error rate (BER), the receiver sensitivity was as much as 4 dB (optical) higher with an EML-based transmitter compared to all other transmitters. The results are presented for QAM signals but they are equally applicable for other formats of digital modulation of RF carriers, such as quadrature phase shift keying (QPSK) in suboctave frequency range.
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