Research Of Thin Film Damage Testing Based On Photothermal Deflection

Optical film is one of the important components of the laser system, and its ability of anti-laser induced damage not only is the main index evaluation of the laser system, but an important area of research. Several main domestic LIDT testing methods are studied in this article, including phase contrast microscopy, image method, and photothermal deflection method. Phase contrast microscopy test method is specified in the national standard.Different methord has different result. In order to match with other methods of test results, reserach on laser film damage identification which based on the photothermal deflection method is proposed in this paper.First of all, the theoretical of the photothermal deflection method was studied in this paper.Then, formula derivation and data simulation of pulsed photothermal deflection offset theoretical was conducted, and the model of film damage identification which based on reflective photothermal deflection method was established. Two kinds of photothermal deflection damage offset detection system were set up based on the model. One is based on quadrant detector(QD), the other is based on CCD. Laser film damage system in the above systems is the same. System based QD includes damages optical signal conversion module, signal amplification module and operation module. System based CCD includes damages optical signal receiving module and the detection of light intensity extrema location program.Photothermal deflection experiments of SiO2 thin film was conducted by using the two systems. Curves between offset and pump beam energy were obtained by using the two systems. Experiment show that:There is a sudden increase (inflection point) on the curve between offset and pump beam energy which is obtained by using experiment system based on QD and CCD detector when the pump beam energy is 85mJ and 75mJ respectively. In view of the inflection point, damage morphology and optical properties of SiO2 films on inflection point were studied. Got the damage criteria:SiO2 thin film is damaged when the offset is greater than 0.1mm using QD system; SiO2 thin film is damaged when the offset is greater than 1.4 Pixel using CCD system. Combined with the damage criterion, LIDT of SiO2 film was obtained by methord of zero probability damage thresholds:LIDT of SiO2 thin film is 12.7 J/cm2 by using system based on QD; LIDT of SiO2 thin film is 9.3 J/cm2 by using system based on CCD. In order to comparative analysis, LIDT of SiO2 thin films were tested by using the microscopy and image method. Test result of microscopy methord is 11.9 J/cm2.Test results of image method is 12.5 J/cm2. The experimental results show that:There are 0.8 J/cm2 and 0.2 J/cm2 difference on LIDT of SiO2 thin film which from system based on QD compare with the national standard and image methord respetly. There are 2.6 J/cm2 and 3.2 J/cm2 difference on LIDT of SiO2 thin film which from system based on CCD compare with the national standard and image methord respetly. Gathered from these:Laser film damage can be recognized by photothermal reflective photothermal deflection method. Compared with test results by using CCD, test results of stytem based on QD are more close the test results of the national standard and image methord.In addition to the SiO2 film, the reflective thermal deflection method is also applied to other reflective film damage identification. The damage criterion obtained in this paper is only for the SiO2 film, for other film materials, we need to have more experiments.