Directly modulated laser transmitter driven by DC-balanced line-coded signals

Directly modulated laser (DML) based optical transmitters provide cost-effectiveness, small footprint, high output power, and high reliability. However, the signal quality of the DML output is poorer than that of external modulator-based counterpart, mainly because of the frequency chirp accompanied by current modulation of DMLs. Especially, the large current modulation to achieve high extinction ratio of the output signals induces severe chirp and waveform distortion that the transmission distance is significantly limited in high data-rate systems. This can be overcome with frequency modulation of DMLs. By making good use of the frequency modulation of DMLs, not only can we reduce the driving voltage but we also substantially reduce the deleterious effects of chirped modulation. However, frequency modulation (FM)-based DML transmitters suffer from the non-uniform FM response of the laser at around 100 kHz to 10 MHz. It gives rise to severe pattern-dependent performance degradation to directly modulated baseband signals. In this paper, we investigate through experiment the use of line coding to deplete the low-frequency spectral contents of the signals and thus to alleviate the degradation. We measure the performance improvement of various line codes, including 8B/10B, 5B/6B, 7B/8B, 9B/10B, and 64B/66B, for continuous-phase frequency-shift keying (CPFSK)/amplitude-shift keying (ASK) signals generated by a DML and a delay interferometer. Experimental demonstrations are performed with a long pseudorandom bit sequence length of 2²⁰ -1 and the bandwidth expansion by the line coding is taken into consideration. We also propose and demonstrate DML-based transmitters driven by highly band-limited electrical signals to alleviate the modulation bandwidth limitation. By using duobinary coding together with DC-balanced line coding, we generate 10-Gbps signals using a DML driven by 3.5-GHz-bandwdith signals and transmit them over 20-km of standard single-mode fiber w- - ithout any dispersion compensation.