A systematic study of sub-10 femtosecond pulse laser induced damage threshold (LIDT) determination was performed for the metal mirrors, (i.e. silver mirrors, aluminum mirrors, and aurum mirrors, etc.) with different thickness of protective layers. The damage morphology of metal mirrors with different thickness protective films at fluences below the single-pulse LIDT was studied to investigate the mechanisms leading to the onset of damage. The study found that the increase in the thickness of a single protective layer has little effect on the initial location of damage, and the use of a protective layer does not necessarily increase the damage threshold of a metal mirror. The damage threshold of the metal mirror is affected by the competition between the electric field in the protective layer and the material band gap and the degree of integration of the electric field with the metal-dielectric interface. However, the metal film without additional treatment has a lower degree of integration at the metal-dielectric interface and is more susceptible to damage. Therefore, even if the thickness change of the protective layer affects the peak intensity of the electric field in the protective layer, the degree of bonding at the metal-dielectric interface is still the decisive factor in the damage threshold of the metaldielectric film. This work is helpful to find new technologies to improve the damage threshold of metal mirrors used in ultrafast high-power laser systems.
The Dispersive Mirrors (DMs) offer high reflectivity and precise control of dispersion compensation, making them essential elements in ultrashort pulse systems. With the development of ultrashort pulse technology, the dispersion oscillations of the DMs become more stronger with the increase of the dispersion compensation bandwidth and target group delay dispersion (GDD) value. To reduce the GDD oscillation of DMs, two pairs of chirped mirrors with a central wavelength of 800 nm and a bandwidth of about 200 nm were designed and fabricated, which provide about -100fs² and -200fs² GDD respectively in the wavelength range of 700-900 nm. The GDD oscillation is reduced from ±100 fs² for a single chirped mirror to nearly 0 fs² using chirped mirror pairs. The chirped mirror pairs were fabricated by dual-ion beam sputtering deposition, and their GDD was tested with a white light interferometer. To verify the compression performance, we simulated the propagation of a Gaussian pulse through our chirped mirrors. We added +1200 fs² of positive dispersion to a Gaussian pulse at the Fourier transform limit and reflected it 12 times on a pair of fabricated chirped mirrors with a GDD of -100 fs². The simulated results showed that the fabricated chirped mirror pairs closely match the design specifications and effectively reduce the oscillation of GDD.
In response to the significant demand for high-damage threshold, broadband high-reflection films for high-power laser systems, composite high-reflection films have been developed. The composite high-reflection films combine the advantages of high-damage threshold materials with the advantages of high-refractive index materials by adding several cycles of high-damage threshold material Al₂O₃/SiO₂ on the surface of traditional Ta₂O₅/SiO₂ high-reflection films. The impact of the number of the Al₂O₃/SiO₂ protective layer cycles on the damage resistance of the composite high-reflection films in the 532nm band is examined. The 1-on-1 laser damage test demonstrated that there was no significant distinction in the laser damage threshold of the composite films with three or six cycles of protective layer. This was primarily due to the presence of various impurities and defects in the films. Six cycles of protective layer protection were found to be more effective. All laser damage of the composite films with six cycles protective layer was observed to occur within the protective layer. Moreover, the interface of Al₂O₃/SiO₂ and Ta₂O₅/SiO₂ was identified as a potential weak region in the composite films. This study provides a valuable reference for the subsequent application of composite dielectric films in high-power laser systems.
Notch filter serves as a vital optical filter, selectively blocking specific wavelength bands while transmitting both shorter and longer wavelengths. Traditional notch filter, designed by alternating layers of high and low refractive index materials, often suffer from undesired higher-order reflection bands. To address this problem, rugate filters with a sinusoidal variation in refractive index eliminate this issue. The refractive index distribution will affect the performance of the notch film. Therefore, the impact of refractive index distribution on the sidelobes of rugate filters was investigated using a developed software. The factors influencing the bandwidth and reflectance of rugate filters were also investigated. It was found that the sinusoidal index distribution modulation function could be more effective in sidelobe suppression. Furthermore, we found that index distribution modulation function decreases the reflectance of the reflection band, and the bandwidth was determined by maximum refractive index contrast. Additionally, we proposed a method for designing arbitrary multi-band rugate filters.
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