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Study On Thin Films Technology For Mid-Far-Infrared Laser Coatings

Posted on:2006-01-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:W HuangFull Text:PDF
GTID:1118360155963719Subject:Optics
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The optical thin film coatings are the important parts in the laser systems. For the high-power laser systems, optical thin film coatings usually have the lower damage threshold related to the other optical components. It is the important factor that limits the development of high-power lasers and becomes the important basis and the maximum limitation for the design of laser systems. Thus, it is of important value to improve the damage threshold of the thin film coatings for the development of high power laser systems. Since some Mid-Far-Infrared chemical systems were demonstrated successfully in the late of 1960's, it developed very fast. The requirement for the optical thin film coatings becomes higher and higher as the output power is higher and higher.With the requirements of high power laser technology, the key technology for the optical thin film coatings of Mid-Far-Infrared laser with high damage threshold have been studied in details, which includes the optimal design, the fabrication technique and the parameter testing. The main results obtained here can be summarized as follows: 1. The basic design theory and methods for the optical thin film coatings for the high power Mid-Far-Infrared laser have been given. The keytechnology for the fabrication of the optical thin film coatings, such as the choice of substrate materials, polishing and cleaning techniques, the choice of optical thin film materials, deposition method, as well as the determining the fabrication parameter, such as substrate temperature, deposition velocity and vacuum pressure, were discussed in details. Consequently, the foundation for the design and the fabrication of the optical thin film coatings for the Mid-Far-Infrared lasers has been established.2. The physical model of the temperature field of multiplayer films was built. The corresponding thermal conducting equations were given. The numerical calculation code was built up by using alternating direction-implicit technique and confirmed by the calculating results of the Stanford University, showing the validity and credibility. The temperature field distributions of the thin film coatings with high reflectivity on the Si substrate, the infrared antireflection coatings on CaF2 window and the coatings of the aperture shared components irradiated by Mid-Infrared laser have been simulated numerically for the first time in China. The variation of the temperature fields along the radial and the thickness directions, as well as the variation of the temperature field distributions with the different surface thermal exchange coefficients and the irradiating time of Mid-Infrared laser have been studied in details. The damage mechanism of optical thin film coatings irradiated by cw lasers has been also analyzed and discussed. Furthermore, by using the Shack-Hartmann wavefront sensor, the thermal distortion of the cavity mirrors with high reflectivity irradiated by Mid-Infrared laser were studied experimentally for the first time in China.3. The classification, the properties, the cause and the effects of the defects in optical thin film coatings have been discussed. The morphology of defects of the ZnS and YbF3 single-layer dielectric thinfilms, and the YbF3/ZnS multilayer dielectric thin films fabricated by different technique have been detected by using the atomic force microscopy. The numbers of the defects in the different optical thin film coatings have been taken count of statistically. The effective methods for decreasing the defects have been proposed, such as laser preprocessing, the choice of deposition method, the optimization of deposition parameters, the choice of source materials, as well as the filtering of ejecting particles.4. The experimental setup for the measurement of the weak absorption of the optical thin film coatings has been built in the use of the surface thermal lensing technology. The weak absorption of different optical thin films, including the single-layer ZnS and YbF3 dielectric thin films, as well as the different multilayer thin films (YbFs/ ZnS) deposited on Si fabricated by different technique, irradiated by Mid-Infrared laser has been measured for the first time in China. The relation between the absorption of the thin film coatings and the fabrication technique has been discussed in details. The experimental results show that the lowest absorption measured by our experimental system is 4.57x10" . The sensitivity of the measured system is about 105. Moreover, the experimental setup for the measurement of the extremely high reflectivity of the optical thin film coatings has been built in the use of the cavity ring-down method. The highest reflectivity measured by the system is 99.91%. The precision of the measured system is about 105.5. The stress of the infrared optical thin film materials, such as YbF3, ZnS and ZnSe, has been measured in the use of the Vecco interferometer for the first time in China. The optimal combination of the thin film materials for the stress matching of the Mid-Infrared laser has been discussed.6. With the especial requirements for the high reflectivity coatings of the cavity mirror, the infrared antireflection coatings on CaF2 window andthe coatings of the aperture shared components of high power Mid-Infrared laser, the optimal design, the fabrication technique and the parameter testing of the thin film coatings have been discussed in details. The optical thin film coatings for the high power Mid-Infrared laser with high damage threshold have been fabricated for the first time in China. The performance and the damage threshold can meet the requirements for practical uses. It can be shown that the absolute reflectivity of the cavity mirror at the triple frequency is 99.816%, which means that the reflectivity at the central wavelength would be higher. The absorption of the cavity mirror is 4.57*10'4. The thermal distortion under 21kw/cm2 power density is about 0.3/1 (/l=632.8nm), and the lowest thermal distortion is 0.164A (A=632.8nm). The damage threshold of the cavity mirror is higher than 40kw/cm2. It also can be seen that the average transmittance of the infrared antireflection coatings on CaF2 window in the wavelength region of Mid-Infrared laser is high than 99%, and the peak transmittance is 99.8%. The high-order reflection region in the visible region of the aperture shared components has been removed, and the average transmittance in the visible and near infrared region ( 0.40.9jo.m) is higher than 85%.The study results obtain in this thesis would be of practical interest in the optimal design, fabrication technique and the parameter testing of the optical thin film coatings of Mid-Infrared. It would not only enrich the contents of the mid-infrared optical thin film technology, but also would drive and accelerate the development of the mid-infrared optical thin film technology in China.
Keywords/Search Tags:Mid-Infrared, optical thin film coatings, high damage threshold, cavity mirrors with high reflectivity, antireflection coatings on CaF2 window, defects, stress
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