Photothermal measurements of absorption in LBO with a "proxy pump" calibration technique

Photothermal technology provides sensitive detection of the optical absorption in bulk materials and coatings. To obtain the absolute absorption numbers it requires a proper calibration. In this work a new, “proxy pump” calibration approach is described. The proxy pump has a wavelength at which the material exhibits high enough optical absorption to be evaluated via direct loss measurements. The pump beam is shaped to have the same spot size as the main pump at which the optical absorption of the material is to be determined. Once the thermal field in the material has the same profile both with the proxy and main pump, the sample is self-referenced. Consecutive tests with proxy and main pumps provide absolute absorption numbers. LBO crystals are notorious objects for which the photothermal response is not easy to calibrate since the material has very low absorption in the UV, visible and near IR. In order to calibrate these materials using the above approach we used 2.3 nm laser as a proxy pump. At this wavelength LBO absorbs more than 15 % per cm length. For crystals oriented in XY and YZ planes the photothermal response was found to be 3 times weaker than for Schott glass NG12 with the same amount of absorbed power. With this correction, NG12 glass that has absorption more than 45 %/cm in the wide range of wavelengths can be used as a reference calibration material for LBO crystals.