Calcium fluoride (CaF₂), as an important material in the ultraviolet-infrared (UV-IR) broad-spectrum optical material system, exhibits ultra-high transmittance (average transmittance of >94%) in the spectral range of 0.13?8 μm. Combined with its extremely low intrinsic absorption coefficient and low refractive index, these properties make it the core material of high-power laser windows and micro- and nano-photonics devices, and its damage characteristics are the key basis for the processing of high-precision micro- and nano-structures. However, the laser damage threshold of CaF₂ crystals under high-frequency femtosecond lasers is still insufficient. On the one hand, the UV lasers commonly used in conventional tests exhibit significant spectral response differences compared to the current mainstream near-infrared(NIR) lasers, resulting in limited applicability of the data. On the other hand, even in the near-infrared band, the asymmetric nature of the actual damage morphology generates measurement errors, leading to large errors in the threshold calculation. These two shortcomings limit the accurate calibration of laser processing parameters.

In this study, the laser damage characteristics of CaF₂ crystals are evaluated using a dual-mode system of traditional standard irradiation and dynamic direct writing. Based on the traditional test standard, a five-level energy step of 0.9?2.2 μJ and 10?10⁴ pulse numbers are selected for the experiments, and the dynamic direct writing experiments simulate the actual processing scenarios by the parameter combinations of 0.52?2.04 μJ energy step and 5?200 μm/s scanning speed. The samples are placed in a scanning electron microscope for observation after the damage test. The surface morphology of CaF₂ crystal samples after laser damage is investigated by scanning electron microscope, the damage size after laser damage is accurately measured, and the laser damage threshold of CaF₂ crystals is calculated by the linear regression method.

In this study, the key damage mechanism of CaF₂ crystals under the action of a high repetition rate femtosecond laser is revealed by comparing the damage characteristics under conventional single-point irradiation with those under the dynamic direct writing mode. In the conventional single-point irradiation experiments, the damage point morphology shows significantly asymmetric chipping characteristics (Fig. 2). Due to the fuzzy boundary of the cracked area, the artificial selection of the damage diameter (inner or outer circle) leads to the deviation of the threshold calculation results as high as 136% (Fig. 4), which highlights the limitation of the traditional method. To overcome this problem, this study innovatively proposes a dynamic direct writing linewidth method: continuous damage lines are inscribed by controlling the scanning speed (5?200 μm/s), and the linewidth of the steady-state scanning region is accurately measured by using a scanning electron microscope (Fig. 5). The experiments show that the linewidth in the low-speed (<40 μm/s) region is stable and the edges are smooth without chipping, while the edges in the high-speed (>40 μm/s) region show a jagged chipping morphology (Fig. 6). Based on the linewidth data in the low-speed region, the damage threshold of CaF₂ is obtained by fitting a linear regression model to be (4.52±0.08) J/cm2, which reduces the error to 8% compared with the traditional method (Fig. 8). The results provide an important parameter basis for the ultra-precision processing of similar optical materials.

In this paper, the multipulse and direct-writing damage behavior of CaF₂ crystals is investigated under a high-repetition-rate femtosecond laser irradiation with a central wavelength of 1030 nm and a pulse duration of 290 fs. In the traditional damage test, the central damage radius of CaF₂ crystals does not change significantly with the increase of pulse number, but the edge of the damage point is irregular, and the inaccuracy of measuring the diameter can easily lead to the error of damage threshold calculation, and the error can be up to 3.4 J/cm². In the case of the damage under direct writing, the width of the damage line basically does not change when the direct writing speed is lower than 40 μm/s and the edge is regular. When the writing speed is greater than 40 μm/s, the edge of the damage line appears to be cracked, and the cracking phenomenon is intensified as the writing speed increases. The damage threshold of CaF₂ crystals is calculated to be (4.52±0.08) J/cm² by substituting the width of the damage line at low speeds (direct writing speeds of 5?40 μm/s) for the diameter of the damage under the conventional damage, which reduces the error of the threshold calculation from 136% in the conventional methods to 8%, and the critical speed threshold of 40 μm/s for suppressing crack propagation is determined. This study provides a high-precision parametric benchmark for the ultra-precision machining of CaF₂ optical components and the reliable design of high-power laser systems, and offers a new method for the threshold evaluation of similar brittle crystals.