Damage effect of pulsed laser on Ta₂O₅/SiO₂ filter film on quartz substrate

  Objective   Studying the interaction process and damage mechanism between optical films and laser is of great significance for clarifying the effect of laser on imaging devices when films is damaged or not, improving the design and preparation technology of imaging devices and anti-laser hardening technology, and laying the application foundation for related industries and national defense. Over the past few decades, optical films have been widely used in high-energy laser systems, and their ability to resist laser damage is critical to the operation of the whole laser system. The first step to improve the film damage threshold is to accurately measure the damage threshold of the films. At present, the main factors affecting the damage threshold of optical films include the physical properties of the film material, the processing technology of optical films and the laser output parameters. Among these factors, the preparation method, processing technology and physical properties of the optical films have certain effects on the damage threshold of the films, but the output parameters of the pulsed laser are decisive. As a key optical component in multispectral cameras, multilayer filter is often designed according to the actual needs of the suitable medium multilayer film. The study of the interaction process and damage mechanism between the multilayer film and pulsed laser is of great significance for the improvement of the design and preparation of the multilayer film and the anti-laser hardening technology of multispectral camera.

  Methods   In the experiment, a Ti: sapphire pulse amplification system, EKSPLA picosecond pulse system and Nimma-900 nanosecond pulse system were used to output laser. The laser damage threshold of Ta₂O₅/SiO₂ multilayer films plated on quartz substrate by electron beam evaporation was measured by 1-on-1 test method. The experimental setup diagram for femtosecond, picosecond and nanosecond laser damage to the multilayer films is shown (Fig.2). The metallographic microscopy is used to observe the damage morphology of the film. We aim to analyze the "pulse width effect" of the damage threshold through the morphological method, and to lay a foundation for the subsequent damage mechanism analysis.

  Results and Discussions   The results show that laser-induced damage threshold of the multilayer film by the 800 nm femtosecond laser (1.67 J/cm²), 532 nm/1 064 nm picosecond laser (1.08 J·cm⁻²/1.98 J·cm⁻²) and 532 nm/1 064 nm nanosecond laser (9.39 J·cm⁻²/21.57 J·cm⁻²). The laser-induced damage threshold of the multilayer film by the femtosecond laser is equivalent to that by the picosecond laser, and the laser-induced damage threshold of the nanosecond laser is one order of magnitude higher. The laser-induced damage threshold outside the transmission passband is about twice that of the laser-induced damage threshold inside the passband. It is verified that the relationship between the material surface damage threshold and the laser pulse width obeys the law of under the thermal damage mechanism. Observing the damage morphology, it is found that with the increase of energy density under femtosecond laser, the film material is ionized, which leads to the obvious delamination spalling phenomenon, and the damage region outline is complete and clear. There are significant differences in the size and density of initial damage points and the thermal damage traces in severe damage although films are damaged by defects under nanosecond and picosecond laser.

  Conclusions   The damage characteristics of the multilayer film induced by femtosecond, picosecond and nanosecond pulsed lasers are studied, and the damage morphology and mechanism of the filters under different pulse widths are mainly discussed. Different pulse width lasers have different mechanisms of damage to the multilayer film, that is, the damage mechanism of femtosecond laser is mainly the ionization effect. In contrast, the damage mechanism of picosecond and nanosecond laser is mainly thermal effect. It is concluded that the difference of laser damage between picosecond and nanosecond laser is caused by the difference of laser sensitivity to different pulse widths. This study has certain reference value for the application of the multilayer film in laser application system and high power laser system.