Time-Domain Simulation of Ultra-Short Pulse Process in Semiconductor Lasers

Abstract

The propagation of optical signals in a semiconductor laser is governed by the interactions between the photon and carrier densities. These interactions are commonly described by the rate equations in which the photon density is assumed to be evenly distributed in the cavity. This is valid when the signals are of low temporal frequency compared with the photon life time or low spatial frequency compared with the cavity length, which is the case in most applications where the bandwidth is below 20 GHz, due to the large carrier life time. However, semiconductor lasers do have gain spectrums that provide >30,000 GHz of bandwidth and in principle, ultra-short pulses of picosecond width can be generated, such as in the mode-locked semiconductor lasers[l], [2], At such short time domain, there are several factors must be taken into account: 1) the spatial distribution of the photon and carrier densities, 2) strong non-linear effects as the results of gain/loss saturation, which is primarily responsible for the generation of ultra-short pulses, and 3) modulations of the gain/loss and their bandwidth, or time response, which is very important in shaping the pulse profiles. In this paper we will present our approach by first deriving the rate equation incorporated with the wave equation of the photon density, then discussing the transitional process of the gain/loss, and finally our initial results from GaAs-AlGaAs passive mode-locked laser simulations.