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.
Semiconductor lasers based on MOVPE-grown asymmetric separate-confinement heterostructures with a broadened waveguide and emitting in the wavelength range 1.0 -1.1 mm are studied. It is found that the intensity of spontaneous emission from the active region increases with increasing pump current above the lasing threshold and that this is caused by a growth in the concentration of charge carriers in the active region due to the modal gain enhancement needed to compensate for the growing internal optical loss at high pulsed pump currents. It is shown that the increase in the internal optical loss with increasing pulsed pump current is one of the main reasons for saturation of the light -current characteristics of high-power semiconductor lasers.
We continue the comparative studies of high-power AlGaAs/InGaAs/GaAs semiconductor lasers with different waveguide designs. In this work, measurements were carried out for a continuous-wave mode of operation: light-current and current-voltage characteristics, threshold characteristics and their temperature dependences and crystal temperature depending on the pump current. The data obtained made it possible to reconstruct the dependences of internal optical losses and internal quantum efficiency using calculations. It was shown that the main factor affecting the power characteristics in the continuous mode of operation is the temperature stability of the laser and its relationship with internal optical losses and internal quantum efficiency. Thus, lasers based on a heterostructure with a doped GaAs waveguide with characteristic temperatures T 0 and T 1 of 120 K and 170 K, respectively, demonstrated an increase in internal optical losses to 2 cm −1 and a drop in the quantum efficiency to 90%, which led to saturation of the output power by high pump currents. Lasers based on the heterostructure with an AlGaAs waveguide, which are characterized by higher temperature stability (T 0 = 161 K and T 1 = 280 K), had lower internal optical losses and a higher quantum efficiency. This made it possible to obtain optical power 1 W higher than that for lasers with GaAs waveguides.
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