Performance issues in hybrid fiber radio communication systems due to nonlinear distortion effects in laser transmitters

With the increasing demand for broadband services, it is expected that hybrid fiber radio systems may be employed to provide high capacity access networks for both mobile and fixed users. In these systems, the radio frequency data signals are modulated onto an optical carrier at a mobile switching centre and then sent over fiber to a number of base stations, before being transmitted over air to the users. A possible method of generating the optical radio frequency data signals for distribution over fiber is to directly modulate the electrical signal onto an optical carrier using a laser diode. The major problem with this technique is that nonlinearities in electncal-to-optical conversion may seriously degrade the system performance. In this work we initially examined the distribution of a wideband code division multiple access signal (centered around 6 GHz) through an optically fed microwave system. Our results show that the adjacent channel leakage ratio is degraded from -52 to -32 dBc after passing through the optical system. We then examined the technique of externally injecting light into the directly modulated laser, to extend the bandwidth of the laser diode and hence, increase it’s linear region to beyond the frequency of interest With this technique an improvement of over 10 dB in the adjacent channel leakage ratio of the signal was achieved. We subsequently went on to examine the distribution of a 5-channel radio frequency signal (each channel carrying 10 Mbit/s) through a hybrid fiber system As in the previous work, we examined how external light injection into the directly modulated laser could be used to improve system performance, and our results show an improvement of up to 5 dB. Finally a model was designed using Matlab, which simulated the 5-channel system mentioned above. It used the laser rate equations to mimic the nonlinear effects of the laser diode Good correlation was observed between experimental and simulated results