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.
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.
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.
Polyimide film had been widely used on the spacecraft as thermal control material or structural materials. During in mission, polyimide film will be damaged by space radiation environments such as energetic charged particles and solar electromagnetic radiation, especially electron, proton and ultraviolet, which will lead to the degradation of its mechanical properties or structural rupture. In this paper, the mechanical properties of polyimide film under the synergistic effect of electron, proton and near ultraviolet was tested by the space integrated irradiation test facility in Beijing Institute of Spacecraft Environment Engineering, and the tensile strength and rupture elongation of it was studied. Under the synergistic effect of ultraviolet, electron and proton, the mechanical properties decrease in the early stage of irradiation larger than in the electron and proton irradiation, but less than in the degradation of mechanical properties caused by electrons and protons in the later stage of irradiation. During the initial stage of synergistic irradiation of electron, proton and near ultraviolet, the degradation law of mechanical properties of polyimide films is basically consistent with that under electron or proton irradiation. But its tensile strength and rupture elongation will exponentially increase with space combined radiation until tend to stable.
Optical coatings prepared by ion beam deposition are dense, with good mechanical properties and environmental stability, which have a wide range of applications in high-power lasers such as ultrafast lasers and space lasers. In the field of nanosecond-laser damage, the size, composition, density and distribution of defects in materials are closely related to laser damage. In this work, the types of defects in ion beam sputtered coatings and their effect on laser damage are investigated. Micron-scale defects and damage morphology on the surface of ion beam sputtered coatings are observed using focused ion beam scanning electron microscopy. The defects on the coatings surface are found to be mainly due to structural defects from the substrate surface and nodules formed during the coating process. Micron-scale defects have a greater impact on laser damage in coatings with stronger standing wave fields. Experimentally pits defects copied from substrates can reduce the damage threshold of high-reflective coatings by nearly a factor of two, and nodules can cause a greater laser damage threshold drop. This study helps to improve the laser damage threshold of ion beam sputtered coatings.
With the development of space lasers, research on the stability of laser films in space environments is becoming more and more important. The space laser film will be damaged by protons, gamma rays and other space radiation environments, and it will be affected by laser radiation, too. This puts forward higher requirements for space laser film elements, so it is essential to carry out space environment simulation tests on laser films. In this paper, the effect of 40keV proton on SiO2 film was studied and then the 355nm laser damage threshold test on it was performed. It was found that the ultraviolet absorption of the film irradiated by proton increased, resulting in the decrease of its transmittance and its ability to resist laser damage.
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.
Polyimide film can be used in spacecraft thermal control multilayer and large-scale deployment structure, and its mechanical properties will be degraded by the influence of space radiation environment such as electron and proton. In this paper, the mechanical properties of polyimide film under the synergistic effect of electron and proton was studied using the space integrated irradiation test facility in Beijing Institute of Spacecraft Environment Engineering. It is found that the combined irradiation of electron and proton damages the polyimide film more than the single environment of electron or proton. The synergistic effect of electron and proton causes the exponentially decrease of rupture elongation and the tensile strength of the polyimide film with. With the increase of the electron and proton irradiation fluence until its properties reaches a steady state.
Silicon rubber and gray cable commonly used on spacecraft was selected as contamination sources, quartz crystal microbalance (QCM) was used as a monitoring device for molecular contamination deposition, and quartz glass was used as a collection plate for molecular contamination. The influence of different temperatures such as 100°C, 125°C, 150°C on the outgas contamination of silicone rubber and gray cable research was carried out in a vacuum environment , and the influence of different deposition temperature such as 10°C, 25°C, 40°C, 55°C, 70°C on the amount of contamination deposition was studied too. The spectral properties and morphology changes of quartz glass at a heating temperature of 125°C on the contamination sources and a deposition temperature of 25°C were studied. The research results show that as the deposition time increases, the amount of contamination deposits approximately linearly increases. With the increase of space temperature, the amount of outgas contamination deposits increases significantly, and as the temperature of the deposition surface increases, the contamination deposits exponentially decreases. The contamination deposits on the surface of quartz glass are in the form of droplets, and the droplets are of different sizes and randomly distributed. After being contaminated by molecular contamination, the optical transmittance of quartz glass significantly decreases.
Laser technology and laser system has more and more important implication in spacecraft engineering. But different from ground environments, space laser system will encounter space environments include vacuum, extreme temperature and thermal cycling, vibration, space radiation environments such as electron, proton, heavy ions, ultraviolet, contamination, and so on. These space environments may have damage or threaten to the space laser system, especially to their optical devices. In this paper, space environments and effects are introduced firstly, and then the influence of space environments such as vacuum, temperature, energetic particles, contamination and vibration on space laser system are analyzed. At last, some advices are proposed to improve the reliability of space laser system are given.
Low dispersive with broad bandwidth mirrors serve as indispensable elements in ultrafast laser systems. We report on three coatings, quarter-wave high reflection mirror, metal-dielectric mirror and ternary composite mirror, which are suitable for broad bandwidth high reflection (BBHR) with 800nm center wavelength. Mirrors based on quarterwave Ta2O5/SiO2 layers are designed and fabricated. By depositing the quarter-wave HfO2/SiO2 layers on the metallic material, we have achieved the mirror with lager low-dispersive bandwidth, this design takes advantage of broad high reflection bandwidth of metal and high laser induced damage threshold (LIDT) of HfO2. Moreover, we have deposited HfO2/SiO2 layers on Ta2O5/SiO2 layers, since HfO2 demonstrates higher damage threshold than Ta2O5, such combination reach an ideal trade-off between the low-dispersive bandwidth and high LIDT comparing with traditional quarter-wave Ta2O5/SiO2 layers. Our BBHR mirrors are produced by means of dual-ion-beam sputtering technique. The designs afford low group delay dispersion (GDD) for reflected light over the broad bandwidth in order to minimize temporal broadening of the fs pulses. The design’s GDD behaves in a smooth way, and its electric field intensities show promise for high LIDTs. Reflectivity and GDD measurements indicate good performance of the BBHR design. LIDTs of the produced BBHR mirrors are also tested and compared.
Plasma-enhanced atomic layer deposition (PEALD) has been widely used in microelectronics due to its precise coating thickness control and high uniformity. Coating qualities are strongly affected by deposition parameters and can be tailored accordingly. In this work, SiO2 and HfO2 monolayers were deposited by PEALD on fused silica and BK7 substrates for different measurement. The influence of deposition temperature and precursor pulse time on both coatings were studied. Coating thickness was obtained by ellipsometer and coating roughness was extracted by atomic force microscope. Laser-induced damage threshold (LIDT) and damage morphology were also studied. By optimizing the process parameters, coatings with desired properties can be deposited.
Multiple-pulse laser-induced damage is an important topic for many applications of optical coatings. In this work, we study the performance of multiple and single pulse laser-induced damage of anti-reflection (AR) coatings with ns-pulsed laser irradiation at a wavelength of 355 nm. LIDT of AR coatings changes little under multiple and single pulse irradiation, around 15 J/cm2. The damage probability curve of multiple pulse is extremely steep. The defect density in the irradiated area is quite high, a large amount of pin-point or cone-shaped pits are observed. The absorption coefficient and defect density increase a lot under multiple pulse irradiation. Material modification of the precursor under multiple pulse is thought as the main reason of high absorption coefficient. Once damage occurs, the damage increases fast and soon grows catastrophic damage. The initial damage morphologies are similar, pin-point or cone-shaped pits, which indicates the initial precursor or defect of multiple and single pulse are the same.
355nm UV lidars are of high spectral resolution and are demanded in space-borne applications. As a result of the vulnerability of the coatings in the laser systems and coatings irradiated by 355nm UV laser are more prone to be damaged than visible or infrared lasers. Thus, the study of the processing on substrate surface will contribute to the improvement of the performance of optical coatings. The dual ion beam sputtering can provide compact and stable layers for space applications. Thus, Al2O3/SiO2 high reflection(HR) and anti-reflection(AR) coatings deposited on acid etched substrate were prepared by dual ion beam sputtering. Damage characteristics on the surface were analyzed after 355nm laser irradiation on the coatings. The damage morphology measured by focused ion beam-field emission scanning electron microscopy (FIB-FESEM) and depth measurement of damage pits hint that the damage of HR coatings occurs in the coating layers rather than the substrate. The HR and AR coatings deposited on acid etched substrate are of larger changes on laser-induced damage thresholds(LIDT) than these on the non-etched substrate. The damage mechanisms of coatings are attributed to thermal absorption and the damage area is a molten pool surrounded by mechanical ejection. The etched area with nicks caused the damage on the HR coatings deposited on etched substrate. And due to the elimination of defects on the subsurface of the AR coatings by etching method, the LIDT of AR coatings was improved.
We present the design and fabrication approach of a rugate narrow band minus filter. A method for the fabrication of graded-index coatings by rapidly alternating deposition of low (SiO2) and high (Al2O3) refractive index materials is introduced, and this technology was used to fabricate a rugate structure. This paper mainly discusses about rugate narrow band minus filter design and fabrication approach. The experimental results show the measured transmittance spectra are in good agreement with the designed value. The laser-induced damage threshold (LIDT) of the narrow band rugate minus filter is measured.
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