Estimation of parameters controlling direct modulation of semiconductor lasers

This paper presents a theoretical estimation of the parameters controlling direct modulation of semiconductor lasers and their dependence on modulation conditions. The study is based on the rate equation model of the emitted photon number and the number of injected charge carriers within the active region. The rate equations are solved numerically in the time domain to determine the transient parameters, and analytically in the frequency domain to determine the small-signal modulation parameters. The transient properties include, turn-on delay, setting time of the relaxation oscillations, damping rate and frequency of the relaxation oscillations. The small-signal modulation parameters include the small-signal response and the associated modulation bandwidth. The numerical calculations are applied to InGaAsP lasers emitting at 1.55 mum. The obtained results for the considered laser showed that the maximum modulation bandwidth is 11.1 GHz when the current is four times its threshold level. This value is nearly twice the maximum bit rate estimated from the laser transients. The maximum bit rate that results in optical signal modulated by pseudorandom returned to zero (RZ) or nonreturned to zero (NRZ) format and free from the bit-pattern effects exceeds 1 Gb/s when the modulation current is four times the threshold current.