Wavelength-division-multiplexed(WDM) systems are expected to be deployed extensively for commercial services. This is mainly because these systems enable operating companies to meet the increasing demand for the bandwidth without replacing the embedded single-mode fiber. However, WDM systems would require each transmitter laser to operate at a specific wavelength to the end of the system's lifetime. At present, practical WDM systems might use DFB lasers. Thus, DFB lasers should be "wavelength-selected" for each channel, and then tuned with temperature to operate at the standardized wavelengths. Once the DFB laser is tuned at the factory, it must maintain the wavelength over the expected system's lifetime. However, because of aging, the wavelengths of some DFB lasers may drift up to a few tens of GHz even when their operating conditions remain constant. This drift would result in the reduction of system margin and cause crosstalk.
In this dissertation, a cold-start WDM system which could overcome the above problems is discussed. Firstly, the effect of laser wavelength drifts on a WDM system is analyzed. However, the system's performance could also be degraded by wavelength drifts of the demultiplexing optical filters. These effects are analyzed for three-types of optical filters, including a Fabry-Perot filter, a second-order Butterworth filter and a third-order Butterworth filter. The results show that the system using a Fabry-Perot filter requires the most stringent control of laser wavelength among these filters. The system using a third-order Butterworth filter shows the most relaxed requirements of laser wavelength control. Secondly, a cold-start WDM system which automatically adjusts the wavelengths and output powers of transmitter lasers is proposed. This system utilized the wavelength selected DFB lasers. The criteria for the wavelength selection would have the laser operating within one half of the channel spacing from the standardized wavelengths. However, these lasers would not necessarily be tuned to the standardized wavelengths at the factory. Thus, for the cold-start operation, each laser should be able to automatically find its operating wavelength without any prior knowledge of its operating conditions for the standardized wavelength. In addition, this system should be able to simultaneously adjust the output power of each laser to a desired value. This operation of wavelength control requires a device which could provide a comb of references at the standardized wavelengths. A simple technique to make an etalon filter is discussed, which provides a set of equally-spaced references at the standardized wavelengths. Using this filter, a 4×2.5 Gb/s cold-start WDM system is demonstrated. The resulting wavelengths and output powers of transmitter lasers were (1547.82+n×0.8)±0.01 nm and 0±0.1dBm, respectively. A cold-start procedure was completed within 40 seconds after turning on the system's power supply. This system is advantageous because there is no need to precisely adjust the wavelengths of WDM lasers in the factory or in the field, thus simplifying the installation and maintenance of WDM systems. In addition, the continuous adjustment of each laser to operate at its standardized wavelength could improve the reliability of WDM systems. Finally, use of cold-start laser control may reduce the cost of the DFB lasers usable in WDM systems by allowing a relaxed wavelength-selection procedure.