Line coding to enhance the performance of 10-Gb/s CPFSK-ASK directly modulated signals

The major drawback of frequency modulation (FM)-based directly modulated laser (DML) is its non-uniform FM response at low frequency range which gives rise to a severe pattern-dependent performance degradation. In this paper, we investigate the use of line coding to deplete the low-frequency spectral contents of the signal and thus to alleviate the degradation. We examine various line codes (8B/10B, 5B/6B, 7B/8B, 9B/10B, and 64B/66B) with continuous-phase frequency-shift keying/ amplitude-shift keying (CPFSK/ASK) signals generated using a DML and a delay interferometer. Experimental demonstrations are performed with a long pseudorandom bit sequence length of 2(20)-1 and the bandwidth expansion by each code is taken into consideration. The results show that among the five codes we tested, 9B/10B code outperforms the other codes in terms of receiver sensitivity an dispersion tolerance. We demonstrate successful transmission of 10-Gb/s CPFSK-ASK signals over 65-km standard single-mode fiber with a bandwidth expansion of only 11.1%.