# Experimental Study on the Damage of Optical Materials by out of Band Composite Laser
## 1 Introduction  
Laser-induced damage to optical components is a key research issue in high-energy laser emission systems, and it is also one of the key technologies that need to be resolved for the development of high-power optoelectronic countermeasure systems. Starting from the basic principle of the interaction between lasers and matter, a laser can interact with optical systems and optical elements through the laser thermal effect and laser-electron interactions. This provides a theoretical basis for a single-band laser to achieve full-band photoelectric loading. Based on this principle, researchers have proposed the concepts of \"in-band damage\" and \"out-band damage.\" \"In-band damage\" refers to the damage of an optoelectronic system by a laser in its operating band. Researchers generally believe that the damage mechanism of \"in-band damage\" comprises the semiconductor band structure theory, thermoelectric effect, etc., and \"in-band damage\" has been widely used in contemporary optoelectronic countermeasures. \"Out-of-band damage\" refers to the damage of photoelectric systems by lasers outside the operating band. Earlier studies have suggested that optoelectronic components do not respond or respond weakly to the lasers outside the operating band. However, with the advancement of laser technology, more and more high-power and high-energy lasers have begun to be applied to high-power laser emission systems. This has caused the risk of damage to optical systems by lasers outside the high-energy band. Therefore, it is necessary to carry out systematic experimental research on the interaction between high-energy lasers outside the band and optical elements. At present, the research reports on \"out-band damage\" have mainly focused on photodetectors. The related literature has conducted experimental studies on interference and damage of HgCdTe, InSb,and Si-CCD (charge coupled device) detectors [1; 2; 3; 4; 5; 6; 7]. The mainstream view believes that the mechanism of \"out-band damage\" is mainly the semiconductor band structure theory (CCD) and thermoelectric effects (Mid-wavelength infrared and Long-wavelength infrared). Some researchers have found that out-of-band lasers can also damage window mirrors in experiments of laser interference effects on optical systems. [PERSON] [8] used a Deuterium fluoride laser to perform cumulative damage experiments on a visible light plane array CCD and found that multiple irradiations of the laser at different positions on the CCD surface and multiple irradiations at the same position damaged the K9 (A glass model) optical window. [PERSON] [9] experimented with a 3.8 \(\upmu\)m continuous-wave laser to destroy the ternary Photoconductive type HgCdTe detector system and found that the film and substrate damage occurred in the Ge window at the laser irradiation spot; they also found the internal filter had a melting phenomenon. Existing research believes that the key to \"out-band damage\" lies in whether the laser source has a sufficient damage ability, and the multi-mode composite laser has this characteristic. A multi-mode composite laser consists of lasers with different wavelengths, different systems, and different frequency changes that act on the target at the same time or alternately to obtain a better damage effect than a single continuous-wave or Pulsed-laser. Related studies have been conducted on composite lasers: [PERSON] [10] carried out an experimental study on the combined damage of a 1030 nm continuous laser and a 1064 nm pulsed laser and found that the \"non-linear avalanche ablation\" effect occurred under the combined or alternating effects of two lasers, which made the combined laser have a stronger ablation effect than the pulse laser. The experimental results showed that the average single-pulse ablation amount of the composite laser was 13 times that of the pulse laser. [PERSON] [11] found, in a study of pulse-pulse composite lasers, that the increase in target damage was a result of an increased power density.