Carrier and gain spatial profiles in GaAs stripe geometry lasers

Abstract

The spontaneous emission generated in the active region of stripe geometry GaAs double-heterostructure (DH) laser diodes is observed through the n-GaAs substrate. The spatial variation of the spontaneous intensity along the junction plane is measured in a direction normal to the stripe axis at current levels below and above lasing threshold. It is observed that the carrier concentration profile is consistent with a model in which carriers out diffuse along the junction plane away from the active region. No evidence has been found of a local depletion of carrier concentration due to regenerative optical oscillations. Analytical expressions for the concentration profiles are obtained from a diffusion model. A comparison of the analytical expressions with experiment yields the carrier diffusion lengths inside and outside of the active region. The computed profiles are in excellent agreement with measurement. From the computed carrier concentration profiles and measurements of mode gain dependence on current it is possible to obtain the spatial variation of local gain in the active region. Coupling of the fundamental optical mode to this nonuniform gain distribution reduces the electronic gain. The lasing threshold of the fundamental mode in the stripe geometry laser is shown analytically to increase as the stripe width is decreased relative to a carrier diffusion length. Finally, the gain dependence on current density has been evaluated and shown to be in agreement with analytical results based on band-to-band recombination in lightly doped active regions.