## 1 Introduction  
Optical thin film is a critical component in the laser system, and its optical performance determines the quality of laser output to a large extent. However, the laser-induced damage threshold (LIDT) of thin films is one of the crucial bottlenecks that limit the development of laser technology to high power and high energy. It is also one of the critical factors affecting laser systems' stability and service life. As the output power of laser systems increases, the requirements for the ability of optical films to resist laser damage are getting higher and higher. Before a new type of optical film is used, evaluating its ability to resist laser damage is a necessary procedure. Among them, laser parameters include laser wavelength, pulse width, energy density, spot area, polarization state, etc. ; film properties include film material, thickness, processing technology, preparation methods, etc. From the above discussion, it can be seen that the laser damage of optical films is a very complicated and accidental problem. Many researchers have studied the laser-induced damage threshold (LIDT) of optical thin films. [PERSON] et al. investigated the LIDT of Ta\({}_{2}\)O\({}_{5}\) and Ta\({}_{2}\)O\({}_{5}\)/SiO\({}_{2}\) films at 532 and 1064 nm [1]; their results showed that the values of LIDTs of all samples, single layer, all quarter-wave multilayer and multilayer having some upper layers non-quarter wave thickness for 1064 nm wavelength were higher than LIDTs at 532 nm wavelength. [PERSON] et al. studied the damage characteristic of the GaAs PHEMT [2]. The results show that the pulse width, pulse period, and duty ratio of the injected signal have a significant influence on the damaging effect of the device. [PERSON] et al. investigated the electric field distribution, the LIDT, and the damage morphology of the prepared four mirror coatings [3]. Their results showed that the SiO\({}_{2}\) coating and electric field distribution greatly influenced the LIDT and damage morphology of the mirror coating. [PERSON] et al. studied the effect of optical parameters (such as extinction coefficient, thickness, and refractive index) and laser parameters (such as laser irradiation number and pulse width) on the LIDT of the SiO\({}_{2}\) thin film [4], finding that optical parameters had little effect on the LIDT, laser parameters had a great effect on the LIDT. [PERSON] et al. investigated the effect of pulse duration on the LIDT in thin triazenepolymer films on glass substrates [5]. They found a significant dependence of the LIDT on the pulse duration in the sub-picosecond regime. [PERSON] et al. used 1-on-1 and 10-on-1 modes to measure the LIDT of Mo/Si multilayers with an extreme ultraviolet pulse at a wavelength of 13.5 nm [6]. According to their results, the differences between these two measurement methods were compared and analyzed, finding that the damage threshold at the 10-on-1 mode was 60% lower than that at the 1-on-1 mode. [PERSON] et al. studied the melting and thermal-stress damage thresholds of silicon induced by long pulsed laser at 0.532, 1.064, and 10.6 \(\upmu\)m [7] and analyzed the damage mechanism of silicon. According to their results, the LIDT of silicon with 0.532 \(\upmu\)m was lower than that with 1.064 and 10.6 \(\upmu\)m, and the damage mechanism of silicon was mainly the melting damage. It can be seen from the above discussion that LIDT is not only limited by laser parameters, including laser wavelength, pulse width, energy density, spot area, polarization state, etc. It is also limited by the characteristics of the film, including the film material, thickness, processing technology, preparation method, etc. [5; 8; 9; 10; 11; 12; 13]. At present, there are few systematic studies on the damage mechanism of thin films under different polarization femtosecond lasers. This experiment studied the damage of thin films irradiated by laser with different polarization based on LIDT and microscopic morphology.