Rearch On The High-precision Measuring Technology For The Laser Beam’s Far-field Parameters

After the atmospheric transmission of the laser beam, the high-precision measurement of its far-field parameters(mainly refer to intensity temporal-spatial distribution, power, centroid, encircled size, etc.) plays an important role in the accurate assessment of the laser’s atmospheric transmission effect and the laser system’s overall performance. This dissertation, which focuses on the measuring technique for the laser beam’s far-field parameters, comprehensively analyzes the influential factors of the measurement of the laser beam’s far-field parameters and thus obtains the high-precision measurement’s basic requirements on the measuring equipment. On the basis of these requirements, the paper explored the measuring methods and techniques for the high-precision measurement, designed sampling target based on diffuse transmission, explored the technology of the array detectors composed of the diffuse transmission sampling and photodiode arrays or CCD image sensors, developed corresponding measuring equipments with obtained high-precision measurement results. This dissertation can be divided into four parts.The technical requirements of measuring laser beam’s far field parameters with high precision were analyzed. The intensity temporal-spatial distribution of laser beams is the basis of the measurement of the other far-field parameters. From the perspective of measurement methods of the laser beam’s far-field intensity distribution, the uncertainty of the measurement of the intensity temporal-spatial distribution and its sources were analyzed. From the perspective of the sampling methods and process, the measurement requirements of laser beam’s far-field parameters on active aperture,dynamic range, spatial resolution and spatial duty ratio of array detectors were exhaustively analyzed through computation or numerical simulation. From the perspective of the measuring and computing process(the mathematical expressions), the uncertainty of the measurement of laser beam’s far-field parameters were analyzed. Based on the above analyses, the technical requirements of the high-precision measurement of the laser beam’s far-field parameters were obtained.According to the measurement requirements for high precision, diffuse transmission sampling method was explored based on the principle of diffuse transmission. Diffuse transmission sampling target was designed, which can spatially sample lasers with high spatial resolution, high spatial duty ratio, keeping the power sampled nearly the same while incident angle changes from-30° to +30°. The angular response feature of the diffuse transmission material was studied with the optical tracing method, and the conclusion that the distribution of the diffuse transimission light has nothing to do with the incident regular was drawn, thus the diffuse transmission sampling method was explored. A series of experimental studies on the sampled materials’ angular distribution feature of diffuse light and its feature of angular response and linear response were carried out. A theoretical analysis and numerical simulation on the angle response features of the unit construction of common samplings was conducted, the optimal combination of the sampling holes and theirs surface characteristic was obtained. The design of the sampling target was explored, and the feature of angular response of the sampling target was analyzed, through which the expression of the maximum sampling error of laser power with the laser’s incident angle, spatial resolution and duty cycle was obtained.The technique of the array measuring equipment, consisting of diffuse transmission sampling target and photodiode arrays, for the high-precision measurement of high-power lasers, was studied. 1) The study on the attenuation techniques of sampled high-power lasers. The attenuation unit construction of the sampled light was designed, the calculation formula of attenuation ratio was deduced, the effect of the intensity distribution within one sampling unit on the responses of the photodiode was analyzed through theoretical computation, the influence of the attenuators’ side walls on the received light of the detectors was discussed under the optical tracing tool Tracepro, and an experimental verification for the calculation formula of attenuation ratio was performed, which proves that the experimental result matches with the above-mentioned theoretical derivation. Attenuation of any ratio can be realized by modifying the construction or combination of the attenuator, these analyses provides the design of similar sampling attenuators with guidance. 2) The study on the techniques of resistance to strong light damage. High-temperature resistant diffuse transmission materials were applied to sample incident high-power lasers. The method to apply the pure copper sand-blasted gold-plated technique to perform diffuse reflection with high reflectivity on these strong lasers which are ineffective on laser beam sampling was proposed, which greatly improves the resistance to strong light damage of related equipment. Experimental researches on the scattering characteristics and reflectivity of the sand-blasted and gold-plated pure copper were performed. A simulated calculation of the temperature field distribution of the sampled target surface with the finite element analysis software Ansys was performed, through which structural parameters were optimized. 3) The uniformity correction method and the power coefficient calibration method were designed. The non-uniformity coefficient uncertainty of and the power coefficient uncertainty were analyzed. 4) A high-precision strong light array detector was developed with a spatial resolution of 5mm(effective aperture of Ф300mm), a spatial duty cycle of 20%, a dynamic range of more than 104, a usable angle of ±30°(the density response value changes within 2.5%), a non-uniformity of within 1%(rms), a power repeatability error of about 1%, a power error of within 3.0% compared with that of Chinese first-order optics measurement center, a maximum measuring energy density of over 10kJ/cm2.The technique of the imaging-model array detectors, consisting of diffuse transmission sampling target and CCD imaging devices was studied. 1) Researches on system integration technology, the measuring system’ components were integrated with plane mirror imaging principles. 2) Researches on the correction technologies of laser spot’s geometric distortion. The laser spot’s geometric distortion was analyzed, the method of accurately correcting laser spot’s geometric distortion in real time by building the mapping relations between the detecting units — the corresponding pixels of the sampling units(the sections used to sample laser beam within the detecting units)—the output laser spot units was put forward, and an experiment was conducted to validate the correction method. 3) Researches on the correction technologies of laser spot’s intensity distortion. The laser spot’s intensity distortion was analyzed, the analysis of the irradiance formula of image face on the basis of the imaging principle was emphasis. Uniform faculae was adopted to carry out the full-system correction of the laser spot’s intensity distribution distortion, and the non-uniformity is less than 1%(rms) after the correction. An experimental validation was conducted on the correction method. 4) The method of calibrating power coefficient was presented. 5) The application of CCD imaging-model array detectors in the measurement of multiple wavelength laser parameters was provided, which can realized the measurement of multiple wavelength laser parameters at the same time. 6) A high-precision CCD imaging-model array detector was developed with a spatial resolution of 3mm(effective aperture of Ф198mm) and a spatial duty cycle of 34.9%, experimental results were achieved with a power measurement error of 5% or below.The dissertation’s research results have been successfully applied, they have provided several important scientific experiments with large amount of scientific data, and they improve the measurement capability and the measurement accuracy of the laser beam far-field parameters. Therefore, they are of great significance for analyzing laser atmospherics’ transmission characteristics as well as diagnosing and evaluating the laser system’s comprehensive performance. At the same time, they play important roles in promoting the development of the relevant laser systems.