Absorption is a critical performance parameter for optical thin-film components. Under high-power laser irradiation, the absorption of thin film elements becomes the primary cause of damage and failure. Thermal reactions between film layers and substrates can be attributed to the thermal energy generated by this absorption, which leads to a local temperature increase and subsequent thermal coupling effect. Ultimately, these processes result in macroscopic damage such as melting and tearing of either the film layer or substrate. Furthermore, absorption loss is associated with optical thermal distortion, which induces phase changes in the laser beam and deteriorates the beam quality and focusing ability, thereby significantly impacting laser systems and equipment. Therefore, the accurate measurement of absorption is essential for developing high-performance, low-loss, optical thin-film components.

Traditional measurement methods can be broadly categorized into spectrophotometry, extinction coefficient, and photothermal radiation methods. With the development of photothermal techniques, new methods have been developed for measuring the weak absorption of thin films. Photothermal technology is widely used because of its high measurement accuracy, simplicity, and scalability. Photothermal technologies can be classified into two types: photothermal deflection and thermal lens. Photothermal deflection calculates the absorption by measuring the beam deflection, whereas thermal lens technology primarily relies on the diffraction effect caused by light and heat. Surface thermal lens (STL) methods are commonly used to measure thin film absorption. For devices utilizing this technology, pump and probe beams can be delivered to a sample in a splitter or common path. Although the former offers high accuracy, it involves complex light paths and numerous components that are challenging to adjust and susceptible to environmental influences. In contrast, the latter is simpler to adjust but has fixed parameters and relatively low measurement accuracy. To address these challenges, an improved design based on a collinear device was proposed. In this design, pump and probe beams are delivered in parallel and focused on the surface of the sample by the same lens.

Based on this improvement, a single-lens measuring device (Fig. 2) was constructed and used to measure optical film samples. A single layer of HfO₂ was deposited on a fused quartz substrate via electron-beam evaporation. The film thickness was approximately 700 nm. The absorption of the six samples was measured using a splitter and single-lens device. To obtain the overall characteristics of the sample absorption, 12 points were measured for each sample, and the average value was calculated. The results for the same sample measured using the two methods were similar; the deviation was only approximately 10% (Fig. 3), which proves the effectiveness of this method. However, as per the results, the measured data of all samples in the single-lens device are slightly smaller than those in the splitter device, probably due to the difference between the aperture of the photodetector used in the two detection methods and size of the pump spot. In this experiment, owing to device limitations, the focal lengths of the convergent lens used by the splitter and single-lens devices were different, resulting in inconsistent focal spot sizes. However, the same detector and receiving aperture were used at the receiving end of the detection beam, which effectively increased the detection aperture for the single-lens structure, resulting in errors. To test this hypothesis, the splitter device parameters were adjusted. Specifically, by moving the probe beam-converging lens in the splitter path, the ratio of the pumping and probing spots on the sample surface was matched to that of the single-lens device. Finally, the parameters of the two devices were the same, and the results showed a deviation of approximately 1%, proving that the above diagnosis of the error source was accurate.

Building on the common light path structure of surface thermal lens technology, we propose an enhanced design featuring altered incident modes for pump and probe lights along with single-lens focusing. This simplifies the experimental equipment, reduces debugging complexity, and maintains the advantages of the splitter path. A measuring device was constructed according to an improved design to measure optical thin film samples prepared using electron beam evaporation technology. The measurement results from the two methods were compared with those from the splitter device, and the data from both the methods were found to be similar. Furthermore, the measured data of all the samples in the single-lens device were slightly smaller than those in the splitter device, probably due to the difference between the aperture of the photodetector used in the two detection methods and size of the pump spot. The splitter device was modified to test this hypothesis. The analysis focused on the variations in the aperture size of the photodetector and size of the pump spot used in the two detection methods. Consequently, following the unification of the measurement parameters of the two structures, the sample was measured again. The deviation between the two methods was less than 1%, demonstrating the accuracy of the error-cause analysis and potency of the single-lens construction.