Research On Some Key Technologies For Robust Design, Parameters Characterization And Reverse Engineering Of Thin Film Optical Coatings

As vital components, thin film optical coatings support or assure the success ofvirtually any modern optical system, among which the optical coating softwaretechnologies had played and will go on to play a critical role. The dissertationinvestigates basic theories and application technologies in details of some front or keyaspects with especially practical significance of optical coating software technologies.The research contents include the core mathematical algorithms determining opticalcoating design software’s calculation speed, the robust design methods of multilayeroptical coatings aiming for high manufacture yield, the spectral measurement data errorstreating techniques of optical parameters characterization of thin film materials, and thereverse engineering algorithm development of multilayer optical coatings. Research onthese key technologies can help to improve the algorithm level and performancespecifications of home-made optical coatings software, and can make the softwaretechnologies play a more prominent role in the manufacture and measurement of opticalcoatings. In a word, they might help to conquer the urgent problems of cost control ofindustrial mass production and spectral quality requirement of high-level applications.The main contents and contributions of this dissertation include:(1) This dissertation theoretically establishes the core mathematical algorithms ofoptical coating design software, i.e. the analytical computation models of multilayersystem’s spectral coefficients’ first and second order partial derivatives with respect tolayer parameters, which also result in the analytical computation expressions of groupdelay and group delay dispersion. The analytical model of spectral coefficients’ partialderivatives is applicable to any isotropic homogeneous thin film system. And it isconcise in expressions, strict in mathematics, fast in calculation algorithms whileprogramming, and universal in all kinds of application fields of optical coatings. Allthese characteristics enable it to be a powerful research tool for optical coatingengineers to investigate the performance analysis, design, optical characterization andreverse engineering of thin film optical coatings.(2) Based on the above analytical model of thin film spectral coefficients’ partialderivatives, this dissertation presents accurate calculation model and fast realizationalgorithm of thin film design merit function’s gradients and Hesse matrix with respectto design parameters. Numerical experiments had been conducted to testify theadvantage of the analytical model over the finite differentiating approximation model incalculation amount, accuracy and time of thin film design merit function’s gradients andHesse matrix. And it is best suitable to use second order optimization methods to speedup thin film design process with the established analytical model, where theimprovement in design speed is especially significant when the layer number is large. (3) Based on the above analytical model of thin film spectral coefficients’ partialderivatives, this dissertation solves the deficiency problem in acoustic-optic sensitivityof an optical multilayer hydrophone with a plate glass substrate. A novel concept opticalmultilayer ultrasonic hydrophone is proposed with the sensing film deposited on atriangular pyramid glass substrate. This dissertation presents reasonable method andadjusting strategy for the optimum working point selection of the ultrasoundmeasurement. Besides of all the other merits of a plate glass substrate optical multilayerhydrophone, the novel hydrophone possesses about8.8-time improved detectionsensitivity without adding the layer number of the sensing film. Moreover, it ischaracterized with more stable optical path alignment and adjustment, longermeasurement time without distortion, and simplified spatial spot area correction, whichcan decrease the difficulty of high frequency signal circuits and will contribute to theaccurate calibration of the hydrophone’s wideband frequency response.(4) Based on the active control concept of thin film’s sensitivity to layerparameters’ errors, this dissertation presents a novel robust design method of multilayeroptical coatings. And the analytical model and fast calculation algorithm have beenestablished. A thorough comparison in calculation accuracy and time had beenconducted among the presented robust design method, traditional deign method andother robust design thoughts of thin film systems, which demonstrated the advantage ofthe presented robust design method in both aspects. We demonstrated its effectivenessin errors control by applications into a variety of optical coatings, such as broadbandantireflection coating, dual-band antireflection coating, neutral beam splitter and linearramp transmittance filter. It shows that the presented robust design method ownsinherent fast computation characteristic and the designed film is insensitive tomonitored layer parameters’ errors in deposition process, which is of practicalsignificance to improve the mass production yields and repetitive production of highquality optical coatings.(5) Based on the above robust design method of multilayer systems, the spectralcharacteristic responses to layer parameters’ errors were researched comparativelyamong three different application solutions of a high-performance thin-film polarizingbeam splitter (PBS) used in a novel orthogonal polarized dual-frequency laser. Theinherent factor of laser PBS’s errors sensitivity had been found by elaborately arrangednumerical robust design experiments. A robust thin film structure had been designed forthe high-performance laser PBS with moderate layer number and easy-manufacturablelayer thicknesses. This PBS design solution is very suitable for mass production at lowcost and high yield, which will play a positive role in the development, engineering andapplication of the orthogonal polarized dual-frequency laser.(6) Based on the different characteristic analysis of systematic and random errorsin spectral measurement data, this dissertation presents a novel errors treating technique of optical parameters characterization of single-layer thin film materials. In order tominimize the characterization deviations of thin film optical parameters from real valuescaused by systematic errors hard to eliminate, it is advised to select measurement dataused in optical characterization from spectral bands characterized with opposite signs orsingle zero of first-order spectral coefficients’ partial derivatives with respect to layerthickness and refractive index for most measurement incident angles, and to excludespectral bands characterized with the same signs or both zeros of spectral coefficients’first-order partial derivatives for all measurement incident angles. The essence of thistechnique is to minimize the errors transfer effect of spectral measurement data on thinfilm optical parameters characterization through spectral measurement band selectionby first-order partial derivatives analysis. In order to minimize the characterizationuncertainties of thin film optical parameters from real values caused by random errorsimpossible to exclude, it is creatively presented to inject independent random noise withthe same distribution law and magnitudes into actual measurement data, which areutilized for many times as target spectral coefficients of the investigated film in theprocess of optical characterization. The statistics mean values of all fit opticalparameters obtained each time by the characterization algorithm are then chosen asestimates of the real optical parameters of the investigated film, whose uncertainiesdecrease to one of square root of characterization times equal parts of the initial valueswithout random errors injection. Numerical simulations had been comparably conductedwith both photometric and ellipsometric data to research the applicability and selectionskills of the range of measurement incident angles. The reliability of this technique issupported by resumable numerical experiments’ results and reasonable theoreticalexplanations. It shows this novel errors treating technique might play a practicalsignificance in in-situ characterization and thickness monitoring of thin films.(7) Different local optimization algorithms in the reverse engineering of multilayeroptical coatings have been compared with characteristics of search ability, multiplesolutions treatment, escaping possibility of local extremum and influence of therestriction condition settings. Numerical experiments show that there is a reasonablelocal optimization method based on the Levenberg-Marquardt algorithm which is bestsuitable for reverse engineering algorithm development of multilayer optical coatings.And the limited applicability of the local optimization algorithm is investigated in termsof reverse engineering quality by numerical simulated experiments. A hybrid reverseengineering algorithm is presented based on genetic algorithm with elite selectionstrategy and nonlinear least-squares method with Levenberg-Marquardt algorithm. Thegood reliability, superiority and universality of the hybrid reverse engineering algorithmis tested and supported by resumable numerical reverse engineering experiments’ resultsof a12-layer infrared antireflection coating, a19-layer quarter-wave mirror and a29-layer quarter-wave narrow band filter under intentionally simulated layer thicknesses errors. As for actual reverse engineering experiments, four multilayer optical coatings,including a15-layer red band-pass filter, a31-layer infrared mirror, a34-layerpolarizing beam splitter and a unknown mirror made in America, were used to prove thereliability and significance of the presented hybrid reverse engineering algorithm withphotometric measurement data by Lambda950spectrometer. Physically sensibledispersion relations of thin-film material’s refractive indices in multilayer coatings arefirstly determined according to the wavelength positions deviations of extremum in themeasured spectral curves. And reliable thicknesses information is secondly determinedby tying to recur the special characteristics in the measured spectral curves which aretypically caused by systematic or random thickness errors. The closeness of fitted modelspectral curves to measured curves and the recurrence of special characteristics inmeasured curves both indicate that the reverse engineered results are reliable both frommathematical and physical point of view.