Study On Highly Sensitive Testing Methods For Weak Absorptance Of Optical Components

The development of laser technology puts forward higher requirements on the performance of optical components. Especially in high power laser system, the absorption loss of optical components is an important factor to restrict its further development. The accurate measurement of the optical absorption loss is of great importance for the optimization of coating process and reduces the absorption loss. Therefore, in this thesis, the highly sensitive testing methods for the weak absorption measurement of optical components are investigated.A method that using the laser calorimetry technique to measure the absorptance of the large aperture optical components is presented. A theoretical temperature model of the large aperture optical components excited by laser beam irradiation is developed to analyze the experimental data and calculate the absorptance of the large aperture optical components. Furthermore, the measurement sensitivity and the influence of the error of the temperature detection position on the measurement result are analyzed by using the temperature model.In laser induced deflection(LID) technique, resist heating approach is used to calibrate the LID signal to obtain the absolute value of the absorptance of the optical components. In order to analyze the calibration error of the resist heating approach, theoretical models describing the temperature distributions inside the test sample excited with flat-top beam irradiation. Based on the theoretical models and finite element analysis(FEA) method, the dependence of the LID signals and corresponding calibration error on the probe beam position are analyzed and some methods are presented to reduce the calibration error of the LID technique.According to the theory of surface thermal lens(STL) technique, we present a method that using a short wavelength laser to improve the measurement sensitivity of STL technique. Three probe lasers with different wavelength are also used to demonstrate the validity of this method. The theoretical and experimental results reveal that the optimal detection distance and the corresponding STL signal amplitude are both inversely proportional to the wavelength. The sensitivity of STL technique could therefore be improved by using a short-wavelength probe beam.Based on Fresnel diffraction theory, an accurate model is developed to describe the photothermal deflection(PTD) signal. A very simple expression of PTD signal and optimal parameters on the measurement configuration are derived. Furthermore, a PTD experiment is performed to investigate the dependence of the PTD signal on the experimental parameters, such as the radius, waist position, wavelength of the probe beam, and the detection distance. Both the experimental data and theoretical results reveal that the optimal detection distance highly depends on the waist position and wavelength of the probe beam. The sensitivity of the PTD technique can be enhanced by optimizing the probe beam radius as well as the detection distance and by using a short-wavelength probe beam.