Shanghai Optics and Machinery Institute has made new progress in evaluating the anti laser damage performance of thin film optical components using different laser damage testing protocols

Recently, the research team of the High Power Laser Element Technology and Engineering Department of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made new progress in evaluating the laser damage resistance and damage mechanism of 532nm thin film polarizers using different laser damage test protocols. The related achievements were published in Optical Materials under the title "Nanosecond laser damage of 532? Nm thin film polarizers evaluated by different testing protocols".

Thin film polarizers can transmit P-polarized light and reflect S-polarized light, playing an important role in high-power laser systems. 1064 nm thin film polarizers are commonly used as optical switches and isolators in large laser systems, such as the National Ignition Facility (NIF) in the United States, OMEGA EP laser systems, Laser Megajoule, and SG II-UP devices. But with the development of high-power shortwave lasers, in order to solve the problem of limited resistance to laser damage in shortwave thin film optical components, polarization beam combining technology has been introduced. However, laser damage assessment of second and third harmonic polarizers is also crucial.

At present, laser damage testing protocols mainly include 1-on-1, S-on-1, Raster scan, R-on-1, and N-on-1. The 1-on-1 laser damage test is to apply a single pulse laser to each test point on the sample to study the initial damage morphology of optical components. The S-on-1 laser damage test is the process of irradiating multiple laser pulses at the same testing point to evaluate the cumulative effects and lifespan of optical components under long-term use. The Raster scan laser damage test scans a 1 cm2 area of the sample with the same energy density and can be used to detect discrete low-density defects in the film layer. When the testable area of the sample is limited, R-on-1 laser damage testing can be chosen to determine the damage threshold. This testing method uses progressively increasing laser energy density to irradiate the same test point. Reducing the number of steps in laser energy density can simplify R-on-1 testing to N-on-1 testing. The use of different laser damage testing protocols helps to identify the sources of damage in thin film optical components, identify potential mechanisms of thin film failure, and provide reference for improving the preparation process of thin film optical components.

The research team evaluated the laser damage resistance of 532 nm thin film polarizers under different polarization states using 1-on-1, S-on-1, and Raster scan laser damage testing protocols. The damage threshold of thin film polarizers prepared by electron beam evaporation under P-polarized light is significantly lower than that under S-polarized light. Under P-polarized light, the 1-on-1 and S-on-1 zero probability damage thresholds of the 532 nm polarizer are very close. Through damage morphology characterization, the damage of the sample under P-polarization is mainly caused by flat bottomed pits caused by structural defects at the interface between the substrate and the film layer, and shell shaped damage caused by sub surface damage of fused silica, both of which are very stable. Under S-polarized light, the damage threshold of S-on-1 is lower than that of 1-on-1, resulting in cumulative effects. The main damage morphology is incomplete jet nodule damage pits, and the damage caused by absorption defects is also manifested under multi pulse laser irradiation. The Raster scan zero damage threshold under two types of polarized light is the lowest, indicating that for thin film polarizers, defect density and film layer quality are the key limiting factors affecting their resistance to laser damage performance.
The research was supported by the foreign cooperation project of the Bureau of International Cooperation of the Chinese Academy of Sciences and the Scientific and Technological Research Council of Türkiye.

Figure 1.5Comparison of laser damage thresholds and typical damage morphology of 32 nm thin film polarizers

Source: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences