The reliability of high-power single-mode GaAlAs lasers with nonabsorbing mirrors (NAMs) was investigated in detail. The lasers were tested at various output powers and temperatures. The aging test was performed under constant power in order to examine the output power dependence. In order to carry out the measurement with accuracy, the heat sink temperature was controlled with an error of +or-0.1 degrees C, and the operating current was measured every hour or every 10 h. As a result, very smooth aging test curves were obtained. Output powers of the coated lasers in the aging tests were 30, 50, and 100 mW, and the temperatures of the heat sink were 50, 70, and 90 degrees C. conventional lasers without a NAM were tested for comparison. The results indicate that the degradation of the lasers is dependent not on the output power, but on the operating current and temperature. The thermal activation energy and the degradation rate is estimated to be 0.85 eV. >