Abstract

We investigated theoretically the turn-on delay (t_on) of a SCH-QW-laser biased above the threshold current density. The dependence of this delay time on the geometrical laser structure is important for high-frequency characteristics. In our model the single QW is shifted from the center of the waveguide layer (10¹⁷ cm⁻³ p-doped) towards the n- or p-doped cladding regions with a doping concentration of 10¹⁸cm⁻³ (see inset in Fig. 1). We performed a one-dimensional simulation of the carrier dynamics with respect to carrier transport based on a drift diffusion model, including the influence of Fermi–Dirac statistics^1,2 and Poissons equation for the electrostatic potential. For the photon number we use a rate equation.³ Changing the boundary conditions for the electrostatic potential, we increase the injected current on a timescale of 1 ps. To obtain comparable results the power ratio P_on/P_off is kept constant, as well as the difference of the injection currents (j_on–j_off). Differential gain, transparency carrier concentration, and confinement factor are assumed to be equal for the three different structures. Figure 1 shows the light output power versus current density characteristics, revealing a change even in the stationary properties of the laser. This is a first hint that carrier transport becomes important. Looking at the response of the photon number we observe a significant dependence on the location of the QW. t_on increases more than 20% if the QW is shifted towards the n-doped cladding layer and it decreases more than 20% due to the shift versus the p-doped cladding layer (see Fig. 2). The pulse shape was kept identical in all three cases. This allows us to compare the results.

© 1994 IEEE