| Summary: | 博士 === 國立臺灣大學 === 機械工程學研究所 === 105 === The Shack-Hartmann wavefront sensor uses the microlens array splitting the incident light into several sub-light beam, and converged into focused-spots. Using the image analysis methods, the orientations of the focused-spots are tracked. The wavefront aberration of the incident light is calculated. However, the common problem of the wavefront sensor is the insufficient dynamic range, which limits the maximum detectable range of the sensor.
This research introduces artificial neural-network to pattern recognition, in order to find the best-suit contour. Accompanying with the Hough transform, the modified Hough spots-centroiding algorithm is proposed to locate the position of each focused-spot. The algorithm does not need any subaperture as compared with the present centroiding algorithm to calculate the centroid position, therefore the SNR and the dynamic range can be extensively increased. There are two primary numerical algorithms: zonal and modal wavefront reconstruction. This research proposed the Haar wavelet zonal reconstruction to increase the accuracy. As for the modal reconstruction, Gram-Schmidt process is used to modify the orthogonality of the discrete Zernike polynomials and reduce the error. The accuracy of the algorithms is verified through test data, and the analysis results show that the accuracy of the modified Hough centroiding algorithm is 0.007 pixel. When the wavelength of the laser is 532 nm, the reconstruction mean RMS error of the Haar wavelet algorithm is 0.0589 λ. The rank of the Grant matrix is used to analysis the ability Gram-Schmidt process to the Zernike polynomials. For a 7-th order Zernike polynomial sets, the rank Grant matrix can be fixed to full fank.
In order to analysis the performance of the proposed algorithms, optical elements and automation platform are used to build-up wavefront measurement experiment equipment. The wavefront sensor WFS20 from Thorlabs Inc. is introduced as a measurement reference, in order to verify the dynamic range, accuracy and repeatability of the system. The accuracy and repeatability are 0.0625 λ and 0.00363 λ respectively. The measured dynamic range is 1.52 times the range of the WFS20 sensor. This research constructs the SHWS system using the developed modified Hough spots-centroiding algorithm without installing any special element to increase the dynamic range of the sensor, while the Haar wavelet zonal reconstruction algorithm is used to maintain the accuracy. With the property of highly-dynamic range, the application for the wavefront sensor on the high order aberration measurement can be increased.
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