# Development of Simultaneous Dual-Resolution Digital Holography System
## 2 Principle and Method  
Our objective to develop a dual-resolution system is to obtain full and local measurement results simultaneously. Figure 1 shows the advantages of a dual-spatial-resolution system for NDT applications. From Figure 1a, a whole field can be observed under the large field of view, but minor defects that can be detected under the small field of view (Figure 1b) cannot be found under the large field of view. Figure 2 shows an optical setup to achieve simultaneous dual-resolution sensitivity measurements. The setup consists of a holography system using the multiple-carrier-frequency spatial-phase-shift method combined with the polarization technique. The laser beam is split by beam splitter 1 (BS1) into two beams, an object beam and a reference beam. The object beam goes to the object's surface, then is expanded through a beam expander (BE). Here, L1 to L4 are lenses with different focus lengths. Lens 1 (L1) is the image lens. Lens 2 (L2) and Lens 3 (L3) have the same focal length and are used to enlarge the field of view [28; 29; 30]. Lens 4 (L4) can enlarge the image to obtain a small field of view. The aperture (AP), placed just before the image lens (L1), is used to control the speckle size and spectrum pattern size on the Fourier domain. Beam splitter 2 (BS2) and beam splitter 3 (BS3) are beam splitters with a 50:50 ratio. Polarized beam splitter 1 (PBS 1) is a polarized beam splitter which can generate two non-coherent polarized object beams. Polarized beam splitter 2 (PBS2) is used to obtain two non-coherent polarized reference beams. The quarter-wave plate (QWP) can convert linearly polarized light to circular polarized light. A single-mode optical coupler (SOC) 50:50 splitter is used to split the reference beam into two beams that are both circular lights. Figure 1: Measuring results under different fields of view. (**a**) Fringe pattern under large field of view. (**b**) Fringe pattern under small field of view. Figure 2: Sketch of simultaneous dual-resolution measurement system. A frequency-doubled Nd: YAG laser (532 nm green) is used in the system. The laser is separated into an object beam and reference beam by using beam splitter 1 (BS1). The object beam is expanded by a beam expander (BE), and illuminates the object. The diffused object's light is reflected from the object, which is a circular light represented by a black line, then goes through the aperture (AP), image lens (L1), and lens L2 before arriving at the PBS. Here, the object beam is divided into two non-coherent polarized beams represented in red and blue, respectively. The red beam, or S-polarized light, is reflected by the PBS and goes through L4. It is then reflected by Mirror 1 (M1) before going into PBS1 again. After being reflected by PBS1 and BS2, the light goes through L3 and BS3 and is received by the CCD camera. The blue beam, or P-polarized light, goes through PBS1, then is reflected by mirror 2 (M2) before going into PBS1 again. The light is reflected by BS2, then goes through L3 and BS3 before arriving at the CCD camera. The reference beam goes through a quarter-wave plate (QWP) and is converted into circular lights. It then goes into a single-mode optical coupler splitter fiber (OCS), in which the reference light is split into two identical circular lights represented by black lines. These two beams propagate through two optical fibers, then arrive at polarized beam splitter 2 (PBS2). Here, these two circular lights are converted to two non-coherent polarized lights which are represented by red and blue lines. These two polarized reference beams are independent. Different carrier frequencies can be introduced by adjusting the reference beams separately. The red line, or S-polarized light, is reflected by PBS2 and goes into BS3, where it is reflected before being received by the CCD camera. The blue line, representing P-polarized light, goes through PBS2 and is reflected by BS3 before arriving at the CCD camera.