ON THE PERFORMANCE OF A COHERENT HETERODYNE OPTICAL FIBER COMMUNICATION SYSTEM

Abstract

One of the most impressive engineering achievements of the past decade has been the remarkable progress in lightwave transmission technology. This is largely because of excellent characteristics of optical fibers, low transmi ssion loss, wide bandwidth and immunity to electromagnetic interference. Application of this technology may include high sensitivity point-to-point links, dense multichannel broadcasting networks, complex optical switching/routing architectures incorporating frequency-agile sources, and other more basic instrumentation uses as well. In recent years, coherent optical fiber communication systems have received considerable attention both theoretically and experi mentally,, as it offers excellent performance both in sensitivity and frequency selectivity. The aim of the thesis is to study the performance of different types of coherent heterodyne optical fiber communication systems. The performance has been evaluated in terms of bounds on error probability, power spectral density (PSD) and signal-to-interference ratio (SIR). A major problem in coherent optical communication systems is that of broad laser linewidths caused by phase noise in laser. Laser phase noise occurs due to temperature fluctua tions, spontaneous emission, refractive index fluctuations and carrier density fluctuations. It is generally agreed

that the maximum permissible laser linewidth depends on the system bit rate and on the modulation/demodulation techniques used. The effect of laser phase noise in terms of laser linewidth has also been studied. A simple and elegant approach has been developed to evaluate the bounds on error probability using moment generating function (MGF) and approximation techniques. The Chernoff bound (CB) and modified Chernoff bound (MCB) have been numerically computed for an optical ASK heterodyne system with envelope detection. The effect of chromatic dispersion on the system performance has also been investi gated. The performance of a subcarrier modulated heterodyne system using AMI, CMI , 5B6B and mBlC coded input data and the performance of an analog AM-DSB receiver have been assessed in terms of power spectral density (PSD). One of the attraction of coherent optical communication is the possibility of simultaneous transmission of large number of closely spaced channels using frequency division multiplexing (FDM) techniques. An important design parameter in such a multichannel coherent system is the laser linewidth, which significantly increases the crosstalk and hence the required channel spacing. In this thesis a simple frequency domain analysis approach has been used to study the effect of adjacent channel interference and laser linewidth on

system performance. A balanced receiver using ASK and FSK modulation formats has been analysed. The SIR has been computed at the system output. Typical system parameters have been used to compute the results numerically from the expressions derived.