Research On Defect Detection Of Nd:YAG Slab Based On Photothermal Deflection Technique

Nd-doped yttrium aluminum garnet(Nd:Y₃Al₅O₁₂,Nd:YAG)has the advantages of high hardness,good optical uniformity,good thermal conductivity,high physical and chemical stability,good laser performance and mature production technology,etc.The laser with neodymium-doped yttrium aluminum garnet as the working material has been widely used in industrial production detection,medical equipment and military weapons.Due to the limitation of optical material properties and growth technology,many defects introduced in growth engineering of neodymium-doped yttrium aluminum garnet affect the optical properties of crystal and high power laser performance.To solve this problem,a defect detection system based on photothermal deflection technology was established in this paper to characterize the defects of Nd:YAG slab.The main research contents are as follows:（1）A three-dimensional one-layer temperature field model under continuous square wave modulation is established,and the alternating temperature field of the one-layer medium is solved.The relationship between the temperature change or refractive index gradient change of the sample and the thermal parameters and internal uniformity of the sample is obtained.（2）Based on the detection principle of four-quadrant limit position sensitive detector,a defect characterization system based on simultaneous measurement of photothermal deflection and scattering is established.The defects of transparent samples of Nd:YAG Slab were detected by the detection system.Three types of defects were found:absorption defect with photothermal deflection signal and scattering signal,absorption defect with photothermal deflection signal only and non absorption defect with scattering signal only.The experimental results were compared with the results under the Normansky microscope,and the defects of Nd:YAG Slab were qualitatively analyzed.（3）A three-dimensional,three-layer temperature field model with defects was established,and the depth information of the absorbent defects on the sub-surface of Nd:YAG slab was obtained by using a defect characterization system based on the simultaneous measurement of photothermal deflection and scattering.The absorption defects with only photothermal deflection signals are quantitatively analyzed by using a theoretical model.The defect depth of phase fitting is consistent with the actual situation,and the fitting residual is 0.13.The results show that the absorption defects without scattering can be detected by the photothermal deflection technique,and the defect depth can be determined by the corresponding theoretical model for this type of defects.