Surface thermal lensing technique was developed into a high-sensitive apparatus for weak absorption test and defect analysis of optical coatings. A continuous-wave 1 064 nm Nd:YAG laser and a He-Ne laser were employed as pump source and probe source

respectively. Low noise photoelectrical components and a lock-in amplifier were used for photo-thermal deformation signal detection. In order to improve sensitivity

the apparatus configuration was optimized by choosing appropriate parameters

including pump beam spot size

chopper frequency

detection distance

waist radius and position of probe beam. Coating samples were mounted on a

x-y

stage. Different procedures

such as single spot

linear scan and 2-dimension area scan

could be performed manually or automatically. Various optical coatings were prepared by both electron beam evaporation and ion beam sputtering deposition. High sensitivity was obtained and low to 1×10

-7

weak absorption was tested in low-loss coating samples. For the sensitivity extreme of the system

1×10

-8

absorption was reason out to be measured by surface thermal lensing technique. Very small standard deviation was achieved for the reproducibility evaluation. Moreover

a spatial resolution of 25 micron was proved according to the area scan which traced out the profile of photo-thermal defects inside optical coatings. The system was employed in the analyses of optical absorption

absorption uniformity and defect characterization

and revealed the relationship between laser-induced damage and absorption of optical coatings.