Kerr E L
Appl Opt. 1973 Oct 1;12(10):2520-7. doi: 10.1364/AO.12.002520.
A sensitive infrasonic method for measuring the absorption of thin films at laser wavelengths has been demonstrated. A fraction of the heat absorbed from the beam by the film passes through a thin layer of gas to the back wall of the Alphaphone, while the remaining absorbed energy heats the supporting window. The gas temperature and pressure rise is then measured with a capacitance microphone. Sensitivity as great as 1.5 x 10(-7) absorption per surface could be achieved with 10 W input by allowing irradiation to continue until the window is saturated with heat. However, the waiting time for saturation even with thin windows would be about 5 min, and with lock-on amplifier demodulation the total measurement time would be several hours. If the chopping speed is increased to 1.5 sec for irradiation and 1.5 sec for cooling, adequate sensitivity of 10(-4) absorption with a signal-to-noise ratio of 9 is achieved with a total measurement time of 1 min. The samples we tested were KRS-5 substrates coated with layers one quarter-wave thick at 10.6 microm. One sample was a single layer of calcium fluoride. Other samples had a layer of zinc sulfide over a binder layer of thorium fluoride. The instrument may be calibrated absolutely with a thin-film heater described large as several percent.
已证明一种用于测量激光波长下薄膜吸收的灵敏次声方法。薄膜从光束吸收的一部分热量通过一薄层气体传递到阿尔发风声器的后壁，而其余吸收的能量则加热支撑窗口。然后用电容式传声器测量气体的温度和压力升高。通过让辐照持续进行直到窗口热饱和，输入10瓦功率时可实现高达每表面1.5×10⁻⁷吸收的灵敏度。然而，即使是薄窗口，达到饱和的等待时间约为5分钟，采用锁定放大器解调时总测量时间将为几小时。如果将斩波速度提高到辐照1.5秒、冷却1.5秒，总测量时间为1分钟时可实现10⁻⁴吸收的足够灵敏度，信噪比为9。我们测试的样品是在10.6微米处涂覆有四分之一波长厚层的KRS - 5衬底。一个样品是单层氟化钙。其他样品在氟化钍粘结层上有一层硫化锌。该仪器可用一个大型薄膜加热器进行绝对校准，校准精度可达百分之几。
