As an integral component of laser systems, infrared glasses directly influence output characteristics of the laser, beam quality, and overall system stability. The demand and specifications for optical glass are expected to escalate continuously with the broad application prospects of laser technology in laser medicine, environmental monitoring, precision machining, laser communication, and laser guidance. On the contrary, femtosecond lasers play a crucial role in contemporary optical manufacturing processes because of their high instantaneous power, cold processing capabilities, and proficiency in 3D engraving. Various photoinduced phenomena in optical glasses have been extensively studied, including laser-induced damage, ion migration, expansion, darkening, flow dynamics, microstructural alterations  and changes in refractive index. Most are related to the breakdown induced by electron avalanche. This phenomenon occurs when seed electrons accelerate within the laser field, thereby creating other free electrons through collisions. The breakdown occurs when the plasma generated by the avalanche electrons reaches a critical density and transfers energy to the lattice ions, disrupting their bonds with the structure. Subsequently, the lattice ions expand away from the surface after the pulse passes. Given the nonselective characteristic of ultrashort pulse ablation, the LIDT is a crucial material parameter that must be understood when processing specific materials.

    In this work, the effects of different laser pulse numbers on the damage morphology of three types of infrared glasses (As₂S₃, 70TeO₂-15ZnO-5La₂O₃-10WO₃ (TZLW), and 53ZrF₄-20BaF₂-4LaF₃-3AlF₃-20NaF (ZBLAN)) with different bandgaps were investigated as shown in Fig. 1. The effects of damage region selection and bandgap of the glasses on laser damage were also investigated. Figure 2 show a serious edge collapse is found at the edge of the damage crater for the wide-bandgap ZBLAN glass under femtosecond laser irradiation, because the high laser energy density affects the accurate calculation of the laser-induced damage threshold (LIDT). The laser direct writing strip damage and using the linear regression method to accurately calculate the damage threshold was proposed as shown in Fig. 3. The LIDT of ZBLAN glass calculated were 1.014 J/cm² by laser direct writing strip damage method. After a while, the same laser parameters were set and the calculated value of LIDT of 1.097 J/cm² as shown in Fig. 4, indicating that the LIDT of the material calculated by laser direct writing damage is highly accurate and the calculation results have high repeatability. This process involves measuring the width of the strip damage instead of the diameter of the circular damage to deter­ mine accurately the damage crater size and provide reliable parameters for obtaining the LIDT of the wide-bandgap material.

Fig. 1 Optical microscope images of different infrared glasses under varying pulse numbers: (a) As₂S₃ and (b) TZLW observed at 500x magnification, and (c) ZBLAN observed at 1000x magnification. (d) The relationship between the diameter of the damage crater and the number of pulses.

Fig. 2 (a) The image of the inner and outer circle diameters of the damage craters after increasing power irradiation on the ZBLAN glassthe (The largest circle tangent to the inside of the damage craters as theinner circle of the damage craters and the smallest circle tangent to theoutside of the damage craters as the outer circle of the damage craters). (b) The relationship between the square of the diameters of the inner and outer circles of the damage craters and the logarithm of the laser energy

Fig. 3 Diagram of the femtosecond laser direct writing damage.

Fig. 4 (a) ZBLAN glass strip damage microscope. (b) Damage threshold calculation in different environments.