The Study On Thermal Effect Of HgCdTe Detectors Irradiated By Repetitive Frequency CO₂ Laser

The thermal effect of a HgCdTe detector irradiated by laser makes the temperature of the detection unit rise and the voltage signals of the detector distorted and even the detector can not work. In order to study the relation between thermal effect and laser parameters, both photoconductive and photovoltaic detectors were irradiated by a Q switched and tunable CO₂ laser to measure the saturation threshold and interference threshold of thermal effect. Investigation of the effects of the repetition frequency and average laser power density on the voltage signals was made and observation on the procedure of waveform distortion by thermal effect was taken. The increment of saturation threshold and thermal effective interference threshold and the reduction of the difference between them were found with the rise of the repetition frequency.To interpret the experimental result, first of all, the photoconductive model was established to calculate the distribution of temperature. The distribution of temperature about the detection unit irradiated by laser of different repetition frequency and different average laser power density under two kinds of boundary condition was calculated by using the finite difference method to solve the equation of heat conduction with variable coefficients. The calculation results indicated that the distribution of temperature about the detection unit irradiated by laser would get to a dynamic equilibrium state when the temperature of bottom of the detection unit was a constant. The setup time was independent on the average laser power density, the repetition frequency and the temperature of the bottom of the detection unit when the average laser power density was lower than 5000W/cm2. Then the relation between the responsivity of the detector and the temperature was calculated and found that the responsivity would reduce with rise of temperature under a certain doping content. Finally the interference phenomenon of thermal effect was interpreted qualitatively by the calculation results of the distribution of temperature and the relation between the responsivity of the detector and the temperature.