Abstract

Calculation of harmonic and intermodulation distortion produced by direct intensity modulation of a semiconductor laser diode is of great importance in subcarrier multiplexed (SCM) lightwave communication systems. Sources of such distortion are (i) overmodulation,^1,2 (ii) device imperfections,³ and (iii) nonlinearity of the rate equations governing lasing action.⁴ Overmodulation has attracted considerable attention from researchers worldwide and research so far has been restricted to low rf subcarrier frequencies. Device imperfections, such as leakage current, appear to be the dominant source of distortion at low rf frequencies compared to the relaxation frequency of the laser diode. At high rf (microwave) frequencies comparable with the relaxation frequency of the laser diode, it is generally agreed that the nonlinearity of the rate equation is the dominant source of harmonic and intermodulation distortion. It should be noted that the laser should be modulated with the highest possible subcarrier frequencies in order to transmit the highest number of channels. In this paper, we consider the calculation of harmonic and intermodulation distortion for subcarrier frequencies close to the relaxation frequency of the laser diode.

© 1994 IEEE