| Conventional optics only discusses the impact of phase difference on the energy distribution of diffraction facula in condition of uniformly distributed light beams in diffraction space. Laser bean has asymmetric distribution in energy and the energy distribution of diffraction facula is determined by the distribution of complex amplitu- de which is composed of phase distribution and amplitude distribution. Laser optics which combines the matrix optics method with optical diffraction theory studies the rule of propagation and transformation of laser bean in space. Based on laser optics in theory, the thesis introduces the matrix optics method and transformation matrix of common optical elements, the basic characteristic of the important object in laser optics-Gaussian beam and the transformation rule when passing through lens, especially discusses the beam quality parameter which is important in describing the transform characteristic and compares different methods of evaluation and measurem- ent. The idea based upon the product of space and space frequency bandwidth made by ISO is to choose Gaussian beam of which the product is the smallest as standard to evaluate beam quality and use intensity second moment which satisfies propagation equation to express beam width. According to the standard, the author developed related theoretical and experimental work. The application of laser of which the high-power density characteristic is more utilized has gone deep into produce and science or many other aspects in modern times. The characteristic makes laser widely applied in industrial machining, high-tech weapon guidance, micro fabrication, photolithography, medical treatment and other fields. Along with that higher requirements are proposed. Facula form, size and power density are the most concerned parameters. The following problems are how to describe and evaluate the quality of emergent beam, how to measure the factual beam effectively and give the reasonable adjustment and control.Many significant physical quantities which are used to value beam quality have been set up such as spatial coherence, far-field angle of divergence, size of focal spot, times-diffraction-limited factor, Strehl ratio, power ratio BQ in bucket, M~2 quality factor and so on. However, those characterizations are partial and only satisfiy special need. M~2 quality factor is proposed by A.E.Siegman in 1988 and have been accepted by ISO. Its definition is:M~2 factor describes both far and near field properties. The product can reflect beam quality better and satisfy the propagation equation:The integral of beam width based on the definition of second moment reflects the distribution of intensity. It is widely used and suitable for theoretical research and optical design. However, when M~2 factor is used to evaluate beam quality, the intensity of beam section must be continuous and no steep edge, so M~2 factor can not used everywhere. In addition, M~2>1 is correct in certain conditions while M~2<1 often exist in self-focusing effect of nonlinear dielectric. To conclude, although M~2 factor is predominant over other parameters, it also has limitations, so that proper parameters should be chosen according to actual situations.There are a few methods of measurement against various evaluation parameters including adjustable diaphragm, moving knife-edge, CCD detecting method, fiber probe scanning and otherwise. The method for measuring beam parameters put forward by ISO demands photodetectors with high SNR, high resolution, spatial uniform response and linearity, beam transformation system with free aberration and undistorted optical attenuator and so on. Also the error of measuring beam width should be smaller than 1%. Lens must be fixed and the measurement is accomplished by moving the detector. Meanwhile, the calculation must be based on second moment method. The measurement system is a precision measurement system integrating with optics, machine, electron and computer.A system for measuring beam parameters which accorded with the standards above basically was built our laboratory conditions in being. Schlieren was designed for light path to extend the measurement range. A self-make background defilade could effectively reduce stray light. The hyperbolic fitting method was used in experiment. Multipoint measurement as well as one background and many approximate pictures in every point were done in near and far fields. The facula collected in every point was preprocessed mainly by denoising based on background-subtraction and multi-average eliminating random signal-jitter with MATLAB. From the image after preprocessing, the beam width was calculated by integration based on the definition of second moment. Then beam widths and measuring positions from all of the faculas were Hyperbolic Fitting, and we got beam waist and its position, Rayleigh size, angle of divergence and M~2 factor. Stereogram of intensity distribution was plotted using MATLAB and the contour of the facula was detected. The uniformity of light intensity was calculated quantificationally and useful information could be extracted. Against to the problem of CCD being susceptive to light intensity and easily got into saturation distortion, Gaussian-fitting was adopted in the program, resulting in restoring the facula's real intensity information better. The techniques mentioned above which were more reasonable and exact compared to original four-point method and more general in processing facula took full advantage of the existing instrument. The whole measurement equipment was simple in structure and convenient in operation while integrating with optics, machine and electron. The set of facula processing software was self-exploited with MATLAB. Once equipped, it could be used in actual measurement and experiment teaching with low cost, real time designing. Moreover, appending a pulse synchronization collector and multiple-CCD, the system could be used to measure pulsed light.Being effected by measurement environment, aberration of optical elements, resolution of CCD, A/D conversion efficiency and resolution of grab card,limitations of computational model, the measurement accuracy of the system can not reach too high. To meet high requirements in measurement,image grab care with high performance, CCD with scientific grade and undistorted optical system are needed. Those can efficiently improve the precision of measurement. |
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