SIMULATION OF DISTORTION IN DIRECTLY MODULATED SEMICONDUCTOR LASERS

Abstract

Distortions can arise from: (i) device imperfections, such as leakage currents, (ii) the intrinsic nonlinearity manifest in the rate equations governing the lasing action of the laser diode, (iii) overmodulation resulting from modulation by multiple subcarriers, (iv) non.linear gain effects and, (v) spatial hole burning. Only the first three are considered in this thesis. The first source of distortion can be avoided or at least reduced by careful design of the laser. The second source of distortion due to relaxation resonance is important only for subcarrier frequencies comparable to the relaxation frequency of the laser diode. The third source of distortion, overmodulation, can result from a large number of subcarriers modulating a solitary laser diode. This type of distortion occurs whether the subcarrier frequencies are close to the relaxation frequency or not. To date, overmodulation distortion has been calculated by static methods. However, a directly modulated laser diode is a dynamic system governed by rate equations. The static calculations are therefore of restricted applicability. Our purpose is to analyse overmodulation through a numerical analysis of the laser diode rate equations. We will show that even for subcarrier frequencies much smaller than the relaxation frequency, dynamically calculated results can be significantly different from the static value. Moreover, as we use rate equations, our method can give distortion due to overmodulation as well as distortion due to intrinsic nonlinearity when the subcarrier frequencies are comparable to the relaxation frequency. By a minor modification to the rate equations, our technique can be successfully applied to predict distortion in the presence of optical feedback. Our simulation shows that the electron lifetime of the laser is an important parameter. It affects both static and dynamic effects of overmodulation. A higher value of electron lifetime gives a sharper cutoff in the light current characteristic near threshold, and hence lower static distortion. However, when the electron lifetime is increased, so does the dynamic effect of overmodulation. Therefore, there appears to be a trade-off in the value of the electron lifetime. Although our focus has been on a dynamic simulation technique, we have also derived a model that can be applied to modify hard-clipping models to account for soft-clipping effects. New formulae are derived that can be applied to predict distortion even when the light current characteristic is non-abrupt near the threshold of the laser. This is clearly not the case with current formulae, which are derived based on the assumption that the light current characteristic of the laser has a sharp cutoff near threshold.