Carrier Phase Recovery for Coherent Optical Transmission Systems

[摘要] This thesis presents studies concerned with the evaluation of the suitablility of an optical-phase-locked-loop (OPLL) as a component in a coherent optical transmission network. The performance of various binary optical transmission schemes, measured by the probability of error (Bit Error Rate), is presented. The linewidths of semiconductor diode lasers are, in general, not optimally suited to coherent transmission formats and therefore an additional power penalty must be paid if such sources are to be used. This power penalty is included in the analysis of a binary PSK transmission. The performance analysis of coherent transmission schemes is extended to encompass multi-level signalling formats. It is shown that, with the use of a sufficient number of signalling levels, these schemes can provide a receiver sensitivity which is better than a shot noise limited binary PSK transmission even when subject to the effects of high laser line width. A discussion on the properties of the laser diode, which are of interest in the design of a coherent optical transmission scheme (the source linewidth, modulation charateristics, wavelength tunablity and frequency and power stability), is provided. Measurements of these properties is given along with documentation of experimental attempts to reduce the linewidth of the laser emission. Linewidth reductions from between five and ten Megahertz down to tens of kilohertz were obtained. An analysis of the performance of an OPLL, taking into consideration the effect of phase instability of the laser diode source, is given. This analysis predicts that present day laser diodes can be used in the construction of the OPLL provided that the loop bandwidth is sufficiently large to account for the laser instability, while not being excessively large such that the performance is impaired by the influence of shot noise. This analysis is extended to describe the effect which the loop transport delay will have on such a loop, as the required loop bandwidth is extended. Similar calculations are performed to evaluate the effect of the loop receiver amplifier bandwidth. An evaluation of the OPLL response in the time domain, with the use of the Root Locus construction technique and a digital simulation, is presented. This analysis supports the findings of the analysis described above. The simulation of the loop dynamic response is found to be in reasonable agreement with the experimental findings of a previous worker. The components for the construction of an optical heterodyne-phase-locked-loop were assembled and attempts were made to lock both free running laser diodes and external cavity line-narrowed diodes. Reliable aquisition of lock was never achieved in any of the experiments performed despite the fact that beat notes of less than 50 kHz were achieved on a regular basis. The main factor which frustrated the succesful implenentation of the OPLL was deduced to have been mechanical disturbances, of the order of nanometers in magnitude, in the reflecting mirror of the external cavity sources. Hysteresis in the mechanical positioning equipment used to control the external reflector positioning was responsible for limiting the effectiveness of the frequency control loop. Experimentation was terminated by the deterioration of the laser sources, the suspected cause of which was the clustering of material defects within the active region of the device.

[发布机构] University:University of Glasgow

[关键词] Electrical engineering