Abstract
Characterization of the weak absorption loss of optical materials requires highly sensitive photothermal detection. To address this requirement, the temperature field induced by a spatially modulated Gaussian heat source imposed on an optical glass is analyzed and compared with that induced by the same heat source modulated temporally. The theoretical analysis reveals that the temperature variation due to the spatial modulation (SM) of the heat source is much larger than that due to the temporal modulation (TM) because the irradiated site not only has a high temperature peak in the quasi-steady state of heating but also has a temperature valley as low as the ambient temperature. This distinct advantage allows for a stronger thermoelastic response of the material and accordingly highly sensitive photothermal detection. The results of the experiment on a fused silica glass verified that the photothermal image due to the SM shows up the weak absorptive defects that have not appeared in that due to the TM under the same experimental conditions. The improvement in the sensitivity is calculated to be 1.8, and the degradation of the sensitivity due to the inevitable experimental imperfections is finally discussed.
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