Measurements of internal optical loss inside an operating laser diode

Abstract

An experimental technique for measuring internal optical loss in high-power edge-emitting semiconductor lasers is demonstrated. The technique is based on coupling a probe beam into the waveguide of a pulse-pumped laser diode. It allows measuring free-carrier absorption (FCA) in a laser heterostructure at different temperatures and at pump current levels up to 30 kA/cm². Measurement results are presented for two laser heterostructure designs, which vary in the waveguide doping level and material. For both heterostructures, the pump current increase induces a significant rise in FCA and a corresponding increase in internal optical loss, from 0.4–0.7 cm⁻¹ at the threshold current to 2–2.5 cm⁻¹ at 15 kA/cm². At higher temperatures, the dependence is even stronger and the internal optical loss rises to 6 cm⁻¹ (65 °C, 27 kA/cm²). The gradient of the FCA current dependence is lower for the laser heterostructure with a doped GaAs waveguide, while the heterostructure with an undoped AlGaAs waveguide displays a larger increase in FCA but better internal quantum efficiency at high currents. These results show that the proposed experimental method has significant potential.