DIRECTLY MODULATED LASERS FOR OPTICAL ACCESS NETWORKS

Abstract

Directly-modulated lasers (DMLs) have been the transmitter of choice for cost-sensitive access networks thanks to their cost-effectiveness, compactness, high output power, and low driving voltage. These merits notwithstanding, the current modulation of DMLs gives rise to a large frequency chirp which severely limits the transmission distance over standard single-mode fiber at 1550 nm. The limited transmission distance can be remarkably improved by making use of the frequency chirp through the frequency modulation (FM) of DMLs. The main problem in this scheme, however, is that DMLs exhibit highly non-uniform FM response at low frequencies which degrades the system performance.

In this thesis, we mitigate this issue by employing DC-balanced line coding in order to deplete the low-frequency contents of the signal, such that only the uniform region of the FM response is utilized. Various DC-balanced line codes, including 5B/6B, 7B/8B, 8B/10B, 9B/10B, 64B/66B, and scramble-and-select, are first investigated in a 10-Gb/s DML-based transmission system. DC-balanced line coding is then utilized to improve the performance of dually modulated electro-absorption modulated lasers where a laser diode and an electro-absorption modulator are integrated on a single chip as well as some other DML-based optical transmitters. Finally, we apply DC-balanced line coding to the downlink modulation in a single-fiber, wavelength-shared wavelength-division-multiplexed passive optical network which employs a 10-Gb/s DML and 2.5-Gb/s reflective semiconductor optical amplifier for downlink and uplink, respectively. The use of line coding not only improve the performance of the downstream signals, but it also greatly suppresses the interference between the downstream and upstream signals.