**Surface Characterization by the Use of Digital Holography** Surface Characterization by the Use of Digital Holography

Dahi Ghareab Abdelsalam, Takeshi Yasui, Takayuki Ogawa and Baoli Yao Dahi Ghareab Abdelsalam, Takeshi Yasui, Takayuki Ogawa and Baoli Yao

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66082

#### Abstract

Digital holography (DH) is an attractive measuring optical technique in the fields of engineering and science due to its remarkable accuracy and efficiency. The holograms are recorded by an interferometer and reconstructed by numerical methods such as Fresnel transform, convolution approach, and angular spectrum. Because harmful coherent noise often arises when long coherent lengths are used, bright femtosecond pulse light with ultrashort coherent length may be the solution to reduce both spurious and speckle noises. Since the usual DH uses a visible light, it is difficult to visualize 3D internal structure of visibly opaque objects due to their limited penetration depth. The terahertz (THz) radiation has a good penetration capability; thus, 3D visualization of both surface shape and internal structure in visibly opaque object can be achieved via THz-DH technique.

Keywords: digital holography, numerical reconstruction, 3D surface metrology, femtosecond DH, terahertz DH

#### 1. Introduction

Various optical techniques have been developed for measuring 3D shape from a single position. The height distribution of the surface of the object is encoded into a deformed fringe pattern, and then shape is directly decoded by one of those optical techniques. DH is considered one of those optical techniques that have received much interest for surface characterization. DH has been heavily developed over recent years because of newly available high-resolution charge-coupled device (CCD) cameras and advances in digital and automated image processing techniques. Performing Fourier transforms and spectral filtering without the need for additional optical

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components for reconstruction has given advantages of digital holography over conventional optical holography. DH enables the extraction of both amplitude and phase information of objects in real time with high resolution. DH technique is less sensitive to external perturbations and has long-term stability in object measurement. DH has merits of wide applications covering particles, living cells, and 3D profiling and tracking of micro-objects or nano-objects. In this chapter, we present recent developments in DH techniques carried out by the author. In Section 2, the principle of holography with highlights on three numerical reconstruction methods, namely, Fresnel approximation, convolution approach, and angular spectrum, is explained. In Section 3, the impact of slightly imperfect collimation of the reference wave in off-axis DH is presented. In Section 4, low-coherence, off-axis digital holographic microscope (DHM) by the use of femtosecond pulse light for measuring fine structure of in vitro sliced sandwiched biological sarcomere sample is described. In Section 5, off-axis terahertz DH using continuous-wave radiation generated from cascade laser source for testing a letter T from paper is described. Section 6 gives concluding discussions and remarks.
