SEMICONDUCTOR LASER DIODE MODELING AND LINEARIZATION BY PREDISTORTION
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Fiber optic transmission systems are expected to become viable options for delivery of telephone, data and cable television to subscribers. Semiconductor lasers have been used as transmitters in digital applications for many years, however, new analog applications are now being proposed. The inhibiting factor for analog applications such as cable television or common antennae television (CATV) distribution is the inherent nonlinearity exhibited by semiconductor lasers. These nonlinear characteristics result in harmonic distortion and intermodulation distortion which degrade the system performance and limit the number of CATV channels that can be multiplexed on a fiber. Understanding the nature of these nonlinearities in semiconductor lasers is essential to designing compensation circuits to linearize the laser and improve its performance. This thesis presents predistortion as an appropriate linearization technique for semiconductor lasers. The predistortion block is incorporated prior to the laser and its ideal characteristics are the inverse of the laser characteristics. Two models of a single-mode Fabry-Perot semiconductor laser have been developed in order to predict the laser's analog transmission performance. The first model is based on an analysis of the single-mode semiconductor laser rate equations in the frequency domain. The laser and predistorter blocks have been modeled with Volterra kernels. This thesis also presents a second laser model based on a time domain simulation of the rate equations using a circuit simulation software package. The time domain model for the predistorter was developed from its Volterra kernels. The performance of the Volterra based predistorter was then evaluated by simulating the time domain predistorter model together with the time domain model of the Fabry-Perot semiconductor laser. Finally, performance of this compensation design was tested by building a hardware prototype. All results show that nonlinear distortion levels, as measured by harmonic distortion and intermodulation distortion, decrease when predistortion is employed prior to the laser.