The effect of cladding layer thickness on large optical cavity 650-nm lasers

The reduction in penetration of the optical mode into the cladding layers in large optical cavity (LOC) laser structures offers the possibility of reducing the cladding-layer thickness. This could be particularly beneficial in GaInP-AlGaInP high-power devices by reducing the thermal impedance and the electrical series resistance. We have designed and characterized 650-nm LOC lasers by modeling the optical loss due to incomplete confinement of the optical mode by the cladding layers and calculating the thermally activated leakage current. This indicated that the cladding thickness could be reduced to 0.5 /spl mu/m without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1-/spl mu/m-wide cladding layers. The measured optical mode loss of the 0.3-/spl mu/m-wide cladding device was 36.2 cm/sup -1/ compared with 12.4 and 11.3 cm/sup -1/ for the 0.5- and 1-/spl mu/m-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0-/spl mu/m devices were similar over the temperature range investigated (120-320 K), whereas the 0.3-/spl mu/m devices had significantly higher threshold current density. We show that this can be attributed to the higher optical loss and increased leakage current through the thin cladding layer. The intrinsic gain characteristics were the same in all the devices, irrespective of the cladding-layer thickness. The measured thermal impedance of 2-mm-long devices was reduced from 30.7 to 22.3 K/W by reducing the cladding thickness from 1 to 0.5 /spl mu/m. Our results show that this can be achieved without detriment to the threshold characteristics.

I. Introduction

GaInP–AlGaInP high-power lasers operating in the wavelength range of 630–690 nm are required for a number of applications, including photodynamic therapy and read/write optical storage systems. Unfortunately p-doped AlGaInP has a large electrical resistivity due to incomplete activation of the dopant [1], low hole mobility [2], and a relatively poor thermal conductivity [3]. These properties exacerbate the thermal management problems associated with high power lasers. However, the reduction in penetration of the optical mode into the cladding layers which occurs in large optical cavity (LOC) structures [4] opens up the possibility of reducing the cladding-layer thickness, thereby reducing the electrical and thermal resistance. In addition to the need to maintain coupling of the optical mode to the quantum wells, there are two further factors which limit the extent to which the cladding thickness can be reduced. The first is the leakage of the mode into the absorbing GaAs contact layer and substrate, increasing the gain requirement and hence the intrinsic threshold current. The second is the thermally activated leakage current, which in GaInP-based lasers is due to drift and diffusion of electrons through the p-cladding layer. The leakage current increases as the cladding thickness is reduced, increasing the threshold current, which could cause an increase in power dissipation and increased internal temperature rise, negating the benefits of the reduced thermal impedance. In this paper, we investigate the interplay of these effects by analyzing the performance of LOC structures with three different values of cladding-layer thickness 0.3, 0.5, and 1 m.