Digital Holography And Its Applications In Holographic Interferometry

Digital holography is a new imaging technique that uses a charge coupled device (CCD) camera for hologram recording and a numerical method for hologram reconstruction. This paper presents the concept and underlying theory for digital holography and its applications in 3-D object reconstruction and optical metrology. (1) Basic mathematical model for recording and numerical reconstruction of digital hologram is elaborated. The core of digital holography is the algorithm of numerical reconstruction. Two different approaches for numerical reconstruction of digitally sampled holograms are investigated, namely the Fresnel transform algorithm and the frequency domain algorithm, which are deduced from the diffraction theory of spherical wave and plane wave respectively. (2) The serious recording limitations due to the finite size and resolution of CCD are discussed detailedly. It's shown that the setup for recording hologram with spherical reference wave can make full use of the spatial bandwidth of CCD. (3) Various digital image-processing techniques for suppression of undesired components and enhancement of quality in numerical reconstruction images are considered, such as hologram contrast-enhancement, HRO subtraction, mean value subtraction, frequency domain filtering and spectrum expansion with conjugation symmetry property. (4) Applications of digital holography in optical metrology are briefly discussed. We also present the fundamental principles of digital holographic interferometry and the two ways to realize it. And also the difference between digital holographic interferometry and optical holographic interferometry is considered. It indicates that digital holographic interferometry is a coherent optical technique, which allows the direct access to the interference phase. The measured interference phase contains information of the object field and its changes, so we can get the parameters changes according to the measured interference phase. Finally, we successfully measure the refractive index distribution of light-induced photorefractive waveguides and the out-of-plane displacement of a deformed steel board.