Owing to the continuous advancement in laser technology, lithium triborate (LBO) has emerged as a widely used nonlinear frequency-doubling crystal in high-power laser systems. Optical thin films are typically applied to the surfaces of LBO crystals to mitigate energy losses caused by Fresnel reflections. However, the laser-induced damage thresholds of both LBO crystals and films deposited on their surfaces typically limit the maximum output power of laser systems. This paper comprehensively reviews several decades of studies on LBO crystals and films deposited on their surfaces. In particular, it includes an examination of the properties of LBO crystals, the laser-damage thresholds of LBO crystals, and the laser-damage thresholds of thin films deposited on LBO crystals, with a summary of research achievements and recommendations on issues to be further investigated. Additionally, this paper discusses the promising prospects for the widespread application of LBO crystals.
Multilayer Dielectric Gratings (MLDGs) are the core optics of the picosecond-petawatt laser systems based on Chirped Pulse Amplification (CPA). The MLDGs encounter nanosecond- and picosecond-laser irradiation during the broadening and recompression of the laser pulse in CPA. Therefore, a comprehensive evaluation of the laser-induced damage performance of MLDG in the nanosecond (ns) and picosecond (ps) regimes is required. Herein, we examined the laser damage characteristics of MLDGs induced by the 8-ns and 8.6-ps laser pulses. In the two tested laser pulse widths, the damage of the MLDG was dominated by the nano absorbing defects and nodular defects, but the damage mechanism has changed. When the laser pulse width is shortened from ns to ps, the damage sites caused by the absorbing defects transfer from the interface to the grating pillars, where there is the maximum electric field. And the nodular ejection pit changes from a complete eruption to a local damage corresponding to the electric field enhancement region. For the nodular defects, ns-laser conditioning was introduced for removing them and achieved a maximum enhancement of 40% in the ps laser-induced damage threshold.
Time-resolved pump-probe technology is an effective method to study the dynamic damage process of optics. In this paper, the dynamic damage process of HfO2/SiO2 anti-reflection coating, for the cases that the coating located on the laser incidence (forward process) and exit (reverse process) surface, irradiated by a 1064nm nanosecond laser was studied based on the time-resolved pump-probe technology of intensified charge-coupled device (ICCD). Under the irradiation of the same fluence (52J/cm2), pits without and with the layer peeling existed in both forward and reverse processes. However, the lateral size and depth of the small pits generated by the reverse process are larger than those of the forward process. The finite element analysis shows that the electric field intensity (EFI) on the substrate-coating interface for the forward and reverse processes is similar, which is not enough to form the difference in damage morphology. These results indicate the develop process of the plasma after its formation under the subsequent laser pulse irradiation determines the damage difference in the above two cases. The time-resolved study of anti-reflective coating damage is of great significance for its damage mechanism analysis and practical application.
Multilayer dielectric gratings(MLDGs)have been widely used in chirped pulse amplification due to their high laser induced damage thresholds(LIDTs). The quest for MLDG LIDT improvement is endless. For MLDGs applied in picosecond(ps) lasers, damage shows the characteristics of both thermal effect and nonlinear effect. The thermal damage of multilayer dielectric films (MLDFs) and MLDGs were investigated using a 1064 nm laser with a duration of 8 ns in our study. Differently from previous 1-on-1 studies, Raster Scan method is adopted to investigate the effect of low-density defects on the laser damage resistance of MLDFs with different top layers and MLDGs. The results show that the LIDTs of MLDGs are half of those MLDFs. For MLDFs with the top layer of HfO2, the damage behaves the ablation of the top layer material due to the surface strong electrical field. For MLDFs with top layer Ta2O5 and SiO2, the typical morphologies are nodule ejections. The initial damage of MLDGs fabricated by etching these three kinds of grating films are similar, and all behave nodule ejections. This indicates that reducing nodule defects can help the MLDGs LIDT improvement in ps pules. These results provide guidance for process optimizations of MLDG fabrication.
Multilayer dielectric gratings (MLDGs) have been widely used as pulse compression grating (PCG) in chirped pulse amplification (CPA) technology due to their high laser induced damage thresholds (LIDTs). The quest for MLDGs LIDTs improvement is endless. As one of the core components of CPA process, MLDGs will encounter laser irradiation of nanosecond, picosecond and femtosecond. Therefore, the damage characteristics of MLDGs should be studied at various pulse widths. We performed the LIDTs test on a Nd:YAG laser system with a wavelength of 1064 nm and a pulse width of 8 ns. Damage characteristics of both MLDFs and MLDGs were investigated. MLDFs were deposited on the substrates cleaned by hand wipe or ultrasonic cleaning. The results show that the LIDTs of MLDGs are approximately 60% of MLDFs. Besides, LIDTs of MLDFs with HfO2 top layer will not be affected by the methods of substrates cleaning due to its surface damage characteristic related to the non-zero EFI on the surface material. However, for the MLDFs with top layer of Ta2O5, LIDTs of MLDFs deposited on substrates cleaned by hand wipe are higher than those deposited on the ultrasonically cleaned substrates.
The dichroic mirror is broadly used in the broad-field multi-object spectrometer, which is the key component to separate incident light into several wavelength channels. The design and fabrication of the broad angular spectrum dichroic mirror is investigated in this paper. The global optimization is applied to obtain the low passband ripple and the sharp transition between the transmissive and reflective wavelength range. The dichroic mirror was prepared by ion beam sputtering deposition. The results showed the average reflectance was larger than 99% between 310nm and 550nm and the transmittance (single side, mean-polarization) was larger than 98% between 570nm and 1000nm with the angle of incidence 28°± 5°. The film thickness distribution and film sensitivity of the dichroic film were also analyzed. This research relieves the feasibility of the design and fabrication of the broad angular spectrum dichroic mirror by ion beam sputtering deposition process.
This work is dedicated to the study of fatigue effects upon femtosecond laser-induced damage of Ta2O5/HfO2/SiO2 highreflective coatings irradiated by pulse train at 1Hz (65 fs, 800 nm). Upon on comparative measurements of different pulse numbers involving between 10 and 300 pulses, laser-induced damage threshold (LIDT) decreases and the multipulse LIDT decreased to the level of 70~75% of the single pulse LIDT. In addition, we found that the probability of damage performs an increasing trend with the number of pulse increases when the coating is irradiated with the same fluence. The evolution of LIDT and 100% damage probability threshold under multipulse irradiations revealed that fatigue effects were affected by both laser fluence and shot numbers. The deep defects play an important role in the multi-shot mode. A correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena.
In this work, 532-nm high-reflection (HR) coatings have been deposited at different deposition temperatures by electron-beam evaporation technology. The spectral performance, e-field distribution, surface roughness, stoichiometry, as well as the laser resistance of the prepared 532-nm HR coatings are investigated. Experimental results indicate that the LIDT of the 532-nm HR coatings can be greatly influenced by deposition temperature. A relatively high deposition temperature benefits the crystallization and oxidation, and improves the LIDT of the 532-nm HR coatings. In addition, the SiO2 overcoat layer is also demonstrated to be effective in suppressing the delamination damage morphology and improving the LIDT of the 532-nm HR coatings.
Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.
Two kinds of hafnia-silica polarizer coatings were prepared by electron beam evaporation, using hafnia-silica mixture (MPOL) and hafnia (POL) as the high refractive index materials, respectively. The spectral performance, surface and interfacial properties, as well as the laser induced damage performance were studied and compared. The M-POL coating shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property, while maintaining high laser induced damage threshold.
BK7 glass substrates were precleaned by different cleaning procedures before being loaded into a vacuum chamber, and then a series of plasma ion cleaning procedures were conducted at different bias voltages in the vacuum chamber, prior to the deposition of 532-nm antireflection (AR) coatings. The plasma ion cleaning process was implemented by the plasma ion bombardment from an advanced plasma source. The surface morphology of the plasma ion-cleaned substrate, as well as the laser-induced damage threshold (LIDT) of the 532-nm AR coating was investigated. The results indicated that the LIDT of 532-nm AR coating can be greatly influenced by the plasma ion cleaning energy. The plasma ion cleaning with lower energy is an attractive method to improve the LIDT of the 532-nm AR coating, due to the removal of the adsorbed contaminations on the substrate surface, as well as the removal of part of the chemical impurities hidden in the surface layer.
The lifetime of optics in high power laser system is typically limited by both laser-initiated damage and the subsequent
growth of laser-initiated damage sites. The single- and multiple-shot irradiations for HfO2 /SiO2 high-reflective coatings,
deposited from hafnia and silica at 1064nm in nanosecond were investigated. It was found that when shot number
increased, the possibility of damage growth increased as well. The relationship between field distribution and damage
morphologies and inner structures was discussed to reveal mechanism of damage initiation. Additionally, the damage
morphologies under different laser fluence and shot numbers were characterized to discuss the damage growth
mechanism upon subsequent pulses. The tested results illustrated the absorbers which induced damages were random
distribution, and the second highest peak of field intensity at the fourth interface was high enough to induce the
micron-sized damage pits. It was found that defect density had a significant impact on the damage site whether growing
or not upon subsequent laser pulses. Additionally, the growth resulted in delamination, and in turn delamination
accelerated damage growth, finally the catastrophic damage happened.
AlF3 thin films were prepared by thermal evaporation at different substrate temperatures and deposition rates. The
relationships between optical properties, mechanical properties and laser-induced damage threshold (LIDT) at 355nm of
AlF3 films were discussed. Both absorption and stress increased with increasing substrate temperatures and deposition
rates, which was a disadvantage to laser-induced damage resistance. Meanwhile, interfacial adhesion and hardness
increased with substrate temperatures and deposition rates, which was an advantage to enhance the LIDT. The LIDT
increased from room temperature to 200°C duo to increasing interfacial adhesion and hardness, and then decreased to
300°C duo to increasing absorption and stress. The LIDT decreased with deposition rates due to increasing absorption
and stress.
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