| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 99 === The thesis presents how to apply the Digital Holographic Microscopy (DHM) with its non-contact, non-invasive and non-destructive characteristics to measure the physical characteristics of the optical components quantitively. In contrast with lensless digital holography, DHM uses an Objective Lens (OL) to collect the object wave and magnify the dimensions of object under test. When the object wavefront passes OL and propagates forward, then object wavefront will contain a quadratic phase term, the so called defocus aberration. The propagating object wavefront will interfere with the reference wave at the hologram plane and the interference pattern be captured by CCD. Thus the holograms recorded with DHM setup will not only the information of the zero order, object wavefront and its conjugate term, but also a quadratic phase term. Even the blur, i.e. zero order and wavefront conjugate terms, had been removed; if the hologram is directly reconstructed using numerical algorithms, then the reconstructed object wavefront will still include a quadratic phase term (defocus aberration). Since the phase information of the object wavefront contains a quadratic phase term, thus the absolute phase distribution of object wavefront had been masked, therefore the shape, profile and surface roughness of the object under test can not be reconstructed.
To remove the defocus aberration due to OL and reconstruct the exact phase distribution of the object wavefront, we use a physical mean to compensate the quadratic phase term due to OL and then the physical characteristics of the optical components can be measured quantitively. The proposed scheme is that an offset lens is inserted after OL, the OL and offset lens are separated such that their foci are common and together (confocal), to physically compensate for the quadratic phase term due to OL, thus the recorded holograms which were captured by CCD will not contain the defocus aberration no longer. Meanwhile, through the analysis of the optical operator, it can be shown that the inverted and magnified object wavefront, which has passed through the OL and compensation lens configuration, will not carry and contain the quadratic phase term anymore.
We exploit the Mach-Zehnder interferometry to implement the in-line and off-axis DHM optical setups, and the arbitrary phase-step digital holography (APSDH) to suppress the blur, i.e. zero-order and twin-image terms. For the off-axis configuration, we use Chen et al. method which is the second reconstruction plane added to spatially filter the unwanted terms, and then numerically reconstruct the object wavefront at the original plane. For demonstrating the effect of the phase compensation, the optical experiments without and with phase compensation (modified) DHM scheme are conducted, and verified that the modified scheme can satisfactorily compensate for the defocus aberration due to OL. After using the numerical algorithms to eliminate those blur, obviously the reconstructed magnitude- and phase-contrast images do present no defocus aberration and blur. Moreover, optical characteristics and reconstruction results which were obtained from in-line and off-axis modified DHM are briefly reviewed and discussed. Furthermore, we also demonstrate how to implement off-axis setups, without cutting (destruction) the whole piece LLA (lenticular lens array), to measure the pitch of a 62 LPI (lenticular per inch) LLA, and the error is about 3.17% when compared the nominal value. The suggested scheme can be applied to DHM, phase object measurement, and optical metrology.
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