| Summary: | 碩士 === 國立交通大學 === 光電系統研究所 === 107 === In laser micromachining applications, the quality of the microstructural dependents on intensity and phase stability of the laser source. Recently, the Shack-Hartmann wavefront sensor (SHWS) has been widely used as the wavefront measuring technique. The laser beam incident to a microlens array (MLA) and is focused individually. By analyzing the shift of each focused points, SHWS can derive wavefront information of the incident laser beam.
In this thesis, an MLA is directly fabricated on the protection window of a board-level camera via multiphoton polymerization (MPP) mechanism in the ultrafast laser system. With the combination of embedded platform and Wi-Fi transmission, miniaturized SHWS is achieved and can be used for laser source monitoring. The excitation source for MPP is an ultrafast laser of 780nm center wavelength. The objective lens is 20x, air-type, and with a numerical aperture of 0.75. By curing the optical adhesive, Norland Optical Adhesive 81 (NOA81), with the above system, the fabrication of MLA is realized. In femtosecond lasers, there is sufficient photon energy density in the focal volume. Therefore, energy transfer into monomers and produce free radicals. Through the connection of covalent bond, NOA81 is transferred into a transparent 3D solid structure inside a specific micrometer region.
The miniature SHWS is equipped with a 5-megapixel (2592 × 1944) camera module. The design of MLA is based on Fresnel lenses rather than conventional plano-convex lenses. Thus, nearly 90% of fabrication time can be saved. Each sub-lens of the MLA is 61 μm in diameter and 5.98 μm in height. Currently, the miniature SHWS has a dynamic range of ± 14.29 π, a resolution of ±0.06 π, and a sensing range of 0.5 × 0.5 mm2. According to the lab-developed algorithm, the focal spots size is about 9.852 μm, which can achieve the requirement of the wavefront reconstruction algorithm and can be used in wavefront measurement.
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