Yaowei Wei

Tantalum pentoxide (Ta₂O₅) films deposited on fused silica substrates are critical components of high-power laser systems. Ion-assisted electron beam evaporation (IAD-EBE) is the mainstream technique for fabricating Ta₂O₅ films. However, it commonly requires extensive experimental efforts for deposition quality optimization, while each coating cycle is extremely time-consuming. To solve this issue, this work establishes a dataset targeting the surface roughness (Rq) and refractive index (n) of Ta₂O₅ films using atomic force microscopy, as well as ellipsometer and deposition experiments. Influence of assisting ion source beam voltage (V)/current (I) and Ar (Q₁)/O₂ (Q₂) flow rate on the n and Rq of Ta₂O₅ films are analyzed. Combining energy-field mechanism analysis with a Bayesian optimization approach (PI-BO), both deposition quality prediction and feature analysis of process parameters are achieved. The determination coefficient/mean absolute error for the prediction models of n and Rq reach 0.927/0.013 nm and 0.821/0.049 nm, respectively. Based on sensitivity analysis, the weight factors of V, I, Q₁, and Q₂ affecting n/Rq of Ta₂O₅ films are determined to be 0.616/0.274, 0.199/0.144, 0.113/0.582, and 0.072/0.000. V and Q₂ are identified as the core factors for regulating deposition quality. The optimal ranges for V and Q₂ are 600~700 V and 70~80 sccm, respectively. This study proposes a PI-BO method for predicting Rq and n of Ta₂O₅ films under small-data conditions, while determining the preferred parameter ranges and their sensitivity weight factors. These findings provide effective theoretical support and technical guidance for IAD-EBE strategy design and optimization of optical films in high-power laser systems.

In a wide range of laser applications, the optical losses of optical materials used in the laser systems are closely linked to the laser-induced damage and laser beam quality deterioration. It is demonstrated in this paper that when the pulsed cavity ring-down (CRD) technique is employed to measure the optical loss of uncoated substrates inserted in the ring-down cavity with normal incidence, the surface reflection of the uncoated substrate causes a significant overestimation of the optical loss. The degree of overestimation increases rapidly with the increasing surface reflectance. By taking into consideration the influence of the surface reflection on the measured CRD signal and developing an approximately linear dependence of the measured loss on the actual loss, the actual loss of the solid substrate is determined accurately from the measured loss. A theoretical description is developed to establish a simple relationship between the CRD measured loss and the actual loss, so to eliminate the influence of the surface reflection on the optical loss measurement. Experimentally the optical losses of fused silica and KDP substrates with 10 mm thickness at 355 nm are determined to be 116 parts per million (ppm) and 567 ppm, respectively. The results demonstrate the usefulness of pulsed CRD for accurate determination of optical loss below the measurement limit (∼3000 ppm) of spectrophotometry.

Previous research on the laser damage resistance of thin films deposited by atomic layer deposition (ALD) is rare. In this work, the ALD process for thin film generation was investigated using different process parameters such as various precursor types and pulse duration. The laser-induced damage threshold (LIDT) was measured as a key property for thin films used as laser system components. Reasons for film damaged were also investigated. The LIDTs for thin films deposited by improved process parameters reached a higher level than previously measured. Specifically, the LIDT of the Al2O3 thin film reached 40 J/cm2. The LIDT of the HfO2/Al2O3 anti-reflector film reached 18 J/cm2, the highest value reported for ALD single and anti-reflect films. In addition, it was shown that the LIDT could be improved by further altering the process parameters. All results show that ALD is an effective film deposition technique for fabrication of thin film components for high-power laser systems.