Output Facet Temperature of High-Power Semiconductor Lasers Using Optical-Thermal Reflection Method
Objective Semiconductor lasers have been widely used in industrial,medical,and other fields owing to their high electro-optical conversion efficiency,wide spectrum,and high power-to-volume ratio characteristics.However,as the application field expanded,higher power and reliability requirements have been stated.When manufacturing a high-power semiconductor laser,catastrophic optical mirror damage(COMD)is a key factor limiting the output power and reliability characteristics.COMD occurs due to a local temperature rise at the facet,which exceeds the material damage threshold,and it denotes the irreversible physical damage inflicted on the facet.Note that the occurrence of COMD is closely related to the output facet temperature;thus,accurately measuring the temperature and plotting its distribution are crucial for assessing the failure characteristics of high-power semiconductor lasers.Methods This study is based on the optical thermal reflection method used to construct a semiconductor laser output surface temperature measurement system.Accordingly,the distribution characteristics of the output surface temperature are studied.First,the thermal reflection coefficient of the output facet material used in the semiconductor laser is measured,based on which the measurement system is calibrated.Second,the lock-in method is used to improve the signal-to-noise ratio of the measurement system by increasing the number of image acquisitions.Finally,the output facet temperatures are measured under different operating currents,and the temperature information along the fast and slow axes is extracted and analyzed.Results and Discussions The thermal reflection coefficient of the active region is 5.06 × 10-4[Fig.3(a)],and that of the substrate is 6.03 × 10-4[Fig.3(b)].After 1000 iterations,the amplitude fluctuation of the thermal reflection signal tends to a smooth curve,causing a temperature fluctuation of less than 0.4 ℃(Fig.6).The output facet temperature under the 1-10 A current is measured;the output facet temperature of the active region of the semiconductor laser increases with an increase in the injection current(Fig.8).The output facet temperature of the quantum well layer exhibits strong non-uniformity along the slow axis.At 10 A,the maximum temperature difference at the output facet is approximately 7.5 ℃.However,at 1 A,the maximum difference exceeds 3 ℃(Fig.9).The output facet temperatures of the quantum well region under currents of 2,4,6,8,and 10 A are 1.4,3.1,4.6,6.9,and 8.7 ℃ higher than the junction temperature,respectively.In the region with an approximate thickness of 1.3 μm at both sides of the quantum well,the output facet temperature is higher than the junction temperature.However,in other regions,the output facet temperature is lower than the junction temperature(Fig.11).Conclusions This article presents a study on the high-resolution measurement of the temperature distribution at the semiconductor laser output facet using the optical thermal reflection method.The temperature distribution information from the output facet of the semiconductor laser is collected under working currents of 1-10 A.The results indicate that the measurement method presented in this study can distinguish small temperature variations at the output facet of the semiconductor laser.Moreover,it is observed that the temperature distribution at the output facet of the semiconductor laser exhibits strong non-uniformity along the slow axis,primarily due to heat generation from light absorption and non-radiative recombination occurring at the facet defects.The highest temperature is observed near the quantum well layer at the output facet,which is consistent with the fact that COMD usually occurs in this region,indicating that abnormal temperatures exceeding the damage threshold are the direct cause of COMD failure in semiconductor lasers.The research method and results presented in this study contribute to obtaining a better understanding of the heat generation mechanism at the output facet of semiconductor lasers,which hold significant practical value for optimizing their design for improving their output performance and reliability.
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