Study Of Evaluating And Measuring Laser Beam Quality

In the process of designing and manufacturing high power laser system, besidesinvestigating the technical scheme to increase the laser’s output power and improve itsbeam quality, it is enormously important for the estimation of overall performance oflaser system to evaluate reasonably and measure reliably, thus, people pay more andmore attention to the issues on laser beam quality. This paper places the beam quality ascore issue and is devoted to study three kinds of researches, i.e., the evaluation ofperformance of different laser system, error analysis of calculating laser beam qualitybased on far-field spot measured with CCD sensor, and enhancement of dynamic rangeof CCD sensor to improve the measurement precision of far-field spot and beam quality,in which the methods of academic analysis, numerical simulation and experimentalverification are used.First, according to application purposes concerning focusability of laser energy,three kinds of lasers, i.e., the annular lasers generated from unstable resonators,Gaussian lasers generated from stable resonators and combined lasers of severalGaussian lasers are taken as analyzing objects, the advantages and drawbacks of beamquality parameters available are discussed, and some key issues on definition of beamquality are calculated and analyzed. The results show that, the parameter of laserbrightness or laser energy density on the target is a very important standard forevaluating the performances of laser systems, which is suitable for measuringperformance of the same system, as well as different systems objectively. Furthermore,the laser energy density is used to investigate the influence of inherent factors, such aslaser wavelength, size of emission window, central obscuration ratio, shape of emissionwindow for single laser beam, and the number of sub-beam, duty ratio, truncation ratiofor coherently combined laser beam, on the far-field focusablity of different lasersystems.Residual phase structure function method is used, and a new method of calculatingthe modulation transfer function and far-field irradiance profile of a truncated Gaussianlaser and annular plane laser through atmospheric turbulence is proposed, the effects ofseveral lower-order aberration modes corrections on the spot quality in the receivedplane are investigated, the results derived in this paper agree well with the results ofcorrection reference, moreover, the complexity of this new method used here is superiorto the method used by correlation reference. Besides, the optimal truncation of theemission window to maximize the system performance is investigated by numericalsimulation, and the relationships between the coefficient of the optimal emissionwindow truncation and the turbulence strength are presented, which agree well with thereal situation over a considerable turbulence regime and the accuracy is better than7%. In the aspect of error analysis of measuring laser beam quality, taking the far-fieldGaussian spot as an example, the calculation error formulas of peak Strehl ratio,encircled energy Strehl ratio and beam quality factor β are derived in the present ofvarious error sources, such as background photon noise, CCD pixel’s non-idealsampling, CCD dark voltage, read-out noise of CCD, signal photon noise andmeasurement window size. The calculation errors of laser beam quality changing withvarious error sources are presented in theory and the corresponding numericallysimulated results are given. Theoretical and simulated results show that when thecalculation threshold is the sum of average value of the error sources and mean-squarevalue of the fluctuation in error sources, the lowest calculating error of laser beamquality could be achieved.A new method of calibrating2-D imaging device based on1-D linear diffractiongrating is proposed, and the theoretical foundation and calibration step are givenaccordingly. On the one hand, this new calibration method is extension of double-slitmethod and has higher light available ratio, on the other hand, the syncretization ofmultiple sets of measurement data expands the calibrating range of CCD camera andincreases the sampling data. As an application example, a series of1-D lineardiffraction gratings are designed and an infrared CCD camera with an unknowncharacter is calibrated.The far-field propagation model of annular plane beam through Kolmogorovatmospheric turbulence is presented and the nonlinear response characterization, such asso called dead zone and nonlinear saturation effect of CCD sensor are considered aswell, the influences of CCD’s nonlinear response characterization on the measurementresults of laser beam quality are investigated. The results show that, CCD’s dead zonecauses compression of center core of far-field spot and nonlinear saturation effectcauses loss of far-field intensity information with minor amplitude, and in order toreduce their influence, it is neecessary to calibrate the response characterization of CCDbeforehand and expand CCD’s measurement dynamic range.In the aspect of how to improve the measurement precision of far-field spot and itsbeam quality, firstly, the drawbacks and applicability of different method for expandingdynamic range of CCD sensor are analyzed, then, according to the far-field focal opticalsystem available in the laboratory, we choose diffraction grating as light splitting deviceand give a new method for measurement of far-field focal spot. Theoretical analysis,numerical simulation and experiment method are used to verify the validity of this newmethod. Furthermore, a new method for focal spot reconstruction and far-field focalmeasurement based on the orthogonal wedges is proposed, the beam quality factor aretaken as a useful evaluating measure, the theoretical model of intensity profiles of thedefocus spot is built and the laboratory scheme accordingly is implemented, theexperiment results show that the method given here is practicable.