Lowering spatial frequency by using two confocal lenses in digital holography

Abstract

One of the main problems in practical application of digital holography is that the unit cell size of CCD(Charge-Coupled Device) is too large. As a result, the object size of recording of interference pattern to CCD is very limited. The angle of incoming laser light ray is reduced by the ratio of two focal lengths of two confocal lenses. This induces the spatial frequency of interference pattern to be lower. So the higher spatial frequency of interference pattern than the spatial frequency determined by the unit cell size of CCD, can be recorded. By using confocal lenses optical setup, a merit can be achieved like that the area of the 0-th order diffraction light is reduced to the square of the ratio of two focal lengths at numerical reconstruction. As a result, numerical reconstruction for larger object size compared with CCD only case, is calculated by using FFT(Fast Fourier Transform) adapted to the integral came from Huygens-Fresnel principle. The needed diameters of two confocal lenses and the position of CCD camera are calculated for recoding the interference pattern of larger object size.