Page 654 - Introduction to Information Optics

638 11. Information Display with Optics
11.3.2. OPTICAL SCANNING HOLOGRAPHY

Optical scanning holography (OSH) is a novel holographic recording
technique first suggested by Poon and Korpel [23], and subsequently for-
mulated by Poon [24], in which holographic information of an object can be
recorded using heterodyne 2-D optical scanning. Corresponding to the prin-
ciple of holography, the technique also consists of two stages: the recording or
coding stage, and the reconstruction or decoding stage. In the recording stage,
the 3-D object is two-dimensionally scanned by the so-called time-dependent
Fresnel zone plate (TDFZP) [24]. The TDFZP is created by the superposition
of a plane wave and a spherical wave of different temporal frequencies. The
situation is shown in Fig. 11.17.
The plane wave is generated at the output of beam expander BE1. Note that
the frequency of the plane wave has been up-shifted by O through the
acousto-optic frequency shifter (AOFS). The generation of a spherical wave is
accomplished by lens L following beam expander BE2. After the focusing of the
lens, we have the spherical wave. Beam splitter BS2 is used to combine the
plane wave emerging from beam expander BE1 and the spherical wave. Thus,
a time-dependent Fresnel zone plate, created by the interference of mutually
coherent spherical and plane wavefronts, is formed on the 3-D volume or the
3-D object to be inspected. The intensity pattern of the combined beam is thus
given by the following expression:

2
2
•Jk 0(x +y )/2z] exp(>j 0f) , (11.25)

M Photo
HeNe Laser Detector
/pt
* I

Blotter

012
BS2 Scanning MJ
PC/Monitor
Fig. 11.17. Optical scanning holography. BS1, 2: beam splitters; AOFS: acousto-optic frequency
shifter; Ml, 2. 3: mirrors; BE1, 2: beam expanders; ®: electronic multipliers; LPF: low-pass filter.