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摘要:
光学薄膜在制备和使用过程中会因缺陷和污染等产生吸收中心，当薄膜受激光辐照后，吸收中心吸收光能会产生热信号，根据热信号可以测量光学薄膜的光学吸收损耗。本文提出基于红外热像仪测量薄膜光学吸收损耗的方法，在测试中加入参考样品可以减少环境温度变化和热像仪噪声对于温度测试结果的影响，对测量过程温度场取一定面积进行平均减少了激光指向波动和光斑分布不理想导致的有限元仿真计算误差。使用本方法测试了小尺寸45°的高反膜吸收损耗，测试得到吸收损耗为7.60 ppm，且测试了同批次大尺寸光学薄膜样品吸收损耗的空间分布情况。使用本方法测量的光学薄膜吸收率和激光量热测试结果一致，不需要长时间的恒温和严格环境温度控制，且测试样品尺寸不受限制。
Abstract:During the preparation and use of the optical thin film, the absorption center will be generated due to defects and pollution. When the optical thin film is irradiated by a laser, the absorption center absorbs light energy and generates thermal signals, according to which the optical absorption loss of an optical film can be measured. The method proposed in this paper for measuring the optical absorption loss of a thin film based on a thermal imager. The addition of a reference sample in the test can reduce the impact of the changes of environmental temperature and the thermal imager noise on the temperature test results. Taking a certain area of the temperature field recorded by the thermal imager during the entire laser irradiation process can reduce the errors of the finite element simulation calculation caused by the laser pointing fluctuations and the unsatisfactory spot distribution. Using this method, the absorption loss of a small-size 45° high-reflection film was tested to be 7.60 ppm, and the spatial distribution of the absorption loss of the same batch of large-size optical film samples were tested. The absorption of the optical film measured by this method is consistent with the result of the laser calorimetry test. This method does not require long-term constant temperature and strict environmental temperature control, and the tested sample size is not limited.
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Key words:
- optical thin film /
- absorption loss /
- infrared thermography /
- finite element /
- absorption measurement
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Overview: During the preparation and use of the optical thin film, the absorption center will be generated due to defects and pollution. When illuminating optical film by laser light, the absorption center will absorb light energy and effectively generate the thermal signals. There are many methods for measuring the optical absorption loss of optical films, such as the laser calorimeter method, photothermal deflection method, surface thermal lens method, and so on. These methods have some problems, such as long measurement time, the influence of scattering on optical absorption, and the high requirement of spatial light position detector. The accuracy of the infrared thermal imager has been able to measure relatively small temperature changes in recent years, which provides the possibility for the measurement of the optical thin film absorption based on the infrared thermal imager. Infrared thermal imaging has unique advantages in measuring the absorption loss of optical films due to their simple measurement of the optical path, rapid measurement, and non-contact accurate temperature measurement.
The method proposed in this paper is based on thermal imager to measure the optical absorption loss of thin films. The method proposed in this paper for measuring the optical absorption loss of a thin film based on a thermal imager is simpler than the existing method, and it can measure the absorption of thin films of optical elements in the laser. The addition of a reference sample in the test can reduce the impact of the changes of environmental temperature and the thermal imager noise on the temperature test results. Taking a certain area of the temperature field recorded by the thermal imager during the entire laser irradiation process can reduce the errors of the finite element simulation calculation caused by laser pointing fluctuations and the unsatisfactory spot distribution. The temperature change curve of the film during laser irradiation is obtained by a thermal imager, and the accurate absorption value of the film sample can be calculated through finite element simulation.
In this paper, a thermal imager was used to test the absorption loss of a small 45° high-reflection film. The absorption loss was 7.6 ppm, and the spatial distributions of the absorption loss of the same batch of large-size optical film samples were tested, the optical absorption was consistent. The absorption of the optical film measured by this method is consistent with the result of the laser calorimetry test. This method does not require long-term constant temperature and strict environmental temperature control, and the tested sample size is not limited.
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图 3 热像仪测量吸收装置示意图
Figure 3. Schematic diagram of the thermal imager measuring absorption device
图 4 激光辐照时待测和参考样品温度变化
Figure 4. Temperature changes of measured and reference during laser irradiation
图 5 激光辐照点中心为圆心不同面积区域平均温度变化曲线
Figure 5. Average temperature changes curves of different areas
图 6 有限元方法温度变化拟合
Figure 6. The temperature variation fitting of the finite element method
图 7 激光量热法Ta2O5/SiO2多层45°高反镜测试结果
Figure 7. The measured results of the laser calorimetry Ta2O5/SiO2 multilayer 45° high mirror
图 8 大尺寸光学薄膜吸收损耗径向分布
Figure 8. The distribution of the radius and the absorption loss in large size optical thin film
表 1 熔融石英材料热物理参数
Table 1. Thermophysical parameters of fused silica
Material Thermal conductivity/(W·m-1·K-1) Density/(kg·m-3) Heat capacity/(J·kg-1·K-1) Fused silica 1.4 2200 746 
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表 2 不同面积区域平均温度计算吸收值
Table 2. The absorption values of average temperature in different areas were calculated
Area/(mm2) Max 0.95 2.76 10.02 38.30 149.79 Absorptivity/ppm 7.45 7.45 7.45 7.48 7.60 7.80
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