Recently, a research team from the Department of High Power Laser Element Technology and Engineering, Shanghai Institute of Optics and Precision Machinery (SIPM), Chinese Academy of Sciences (CAS), has made new progress in evaluating the anti-laser damage performance and the damage mechanism of 532 nm thin film polarizers using different laser damage test protocols. The results were published in Optical Materials under the title of "Nanosecond laser damage of 532?nm thin film polarizers evaluated by different testing protocols". Optical Materials.
Thin-film polarizers play an important role in high-power laser systems because they transmit P-polarized light and reflect S-polarized light. 1064 nm thin-film polarizers are commonly used as optical switches and optical isolators in large laser systems, such as the U.S. National Ignition Facility (NIF), the OMEGA EP laser system, the Laser Megajoule, and the SG II-UP device. UP devices. However, with the development of high-power short-wave lasers, polarized beam combining technology has been introduced to solve the problem of limited laser damage resistance of short-wave thin-film optical elements, but laser damage assessment of second- and third-harmonic polarizers is also crucial.
Currently, the main laser damage test protocols are 1-on-1, S-on-1, Raster scan, R-on-1 and N -1-on-1 laser damage testing involves applying a single laser pulse to each test point on the sample to study the initial damage morphology of the optical element. S-on-1 laser damage testing involves applying multiple laser pulses to the same test point to evaluate the cumulative effect and lifetime of the optics over a long period of time. raster scan laser damage testing scans a 1 cm2 area of the sample at 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, the R-on-1 laser damage test can be selected to determine the damage threshold, which uses increasing steps of laser energy density to irradiate the same test point. Reducing the number of laser energy density steps simplifies the R-on-1 test to an N-on-1 test. The use of different laser damage testing protocols can help uncover sources of damage to thin-film optical components, identify potential mechanisms of film failure, and inform improvements in thin-film optical component preparation processes.
The team evaluated the laser damage resistance of 532 nm thin-film polarizers in different polarization states using 1-on-1, S-on-1, and Raster scan laser damage test protocols. The damage threshold of thin film polarizers prepared using electron beam evaporation was significantly lower in P-polarized light than in S-light. The 1-on-1 and S-on-1 zero-chance damage thresholds of the 532 nm polarizers are very close to each other in P-polarized light. The damage morphology characterization shows that the damage of the samples under P polarization is mainly flat-bottomed craters caused by structural defects at the interface between the substrate and the film layer and shell-like damage caused by fused silica sub-surface damage, and both kinds of damage are very stable. Under S-polarized light, the damage threshold of S-on-1 is lower than that of 1-on-1, and the influence of cumulative effect appears. The main damage morphology is incompletely ejected nodule damage craters, and the damage caused by absorptive defects is also exhibited under multi-pulse laser irradiation. The Raster scan zero damage threshold is the lowest for both polarized lights, indicating that for thin-film polarizers, the defect density and film layer quality are the key limiting factors affecting their laser damage resistance.
Figure 1. Comparison of laser damage thresholds and typical damage morphology of 532 nm thin-film polarizers
