Microwave holographic- imaging remote objects using light-modulated scattering technique
3. Modulated Scattering Microwave Holographic System
A schematic diagram of the system is shown in Figure 2. Experimental positions arrangement of the pyramidal microwave antenna, dipole scaterrer and object are shown in Figure 3. Both the antenna radiating field and the synthetic off-axis reference wave providing by a programmable movable short phase-shifter are fed by a single 10 mw. 9.2 GHz. Gunn oscillator. Direct illuminating wave and the object wavefield excite the 2 cm.-long MRD 721 photodiode, optically modulated at 50 KHz. via a fiber optic cable, to produce modulated scattering wave travelling back to the same antenna and combines with the reference wave at the mixer. Synchronous demodulation of the mixer signal output after amplification by a low-noise preamplifier is done by using a lock-in amplifier to produce hologram intensity signals. In this computerized-controlled prototype system,the source and scatterrer are stationary while the
Figure. 2 Schematic diagram of microwave holographic system
object is scanned 1/3 of wavelength stepwise in both x and y direction generating 128 128 sampling signals associate with each position. Fresnel diffraction approximation is used in digital reconstruction from stored hologram data to yield the two-dimensional image of the object.
Figure. 3 Relative position between microwave antenna, dipole and object
4. Experimented Results
The system was successfully used to image flat metallic objects of various shape and size at different synthesized reference wave incident angle
q and limited distance, bound by the size of the existent small anechoic chamber 3.5x3.5x3.5 m3 together with the degrading ratio S/N of the signal, but not too short to violate the Fresnel approximation. Some of the results are shown in Figure 4 for donut-shape object (for z = 288 cm., w = 211.5 cm.,
q =44.3° ) and dumbbell-shape object (for z = 288 cm., w = 249.5 cm.,
q = 44.3° ). Figure 5 shows the full width-half maximum of the image field intensity distribution of a 20 cm. diameter plate (for which z = 288 cm., w = 249.5 cm.,
q = 44.3°). Consider this case as an example, the theoretical resolution limits are 5.44 cm. and 2.92 cm. for the conventional and source-receiver type hologram respectively. The estimated difference between actual object size and full width-half maximum of 2.7 cm. implies that the resolving power of the system that has been developed is significantly better than conventional system.
(a) (d)
(b) (e)
(c) (f)
Figure. 4 Examples of the experimented results for : (a) donut-shape metallic plate objects with (b) its corresponding microwave hologram.and its image(c), (d) dumbells-shape metallic plate object with (e) its corresponding microwave hologram and its image (f).
(a)
(b)
Figure. 5 (a) microwave image of a 20 cm diameter flat metallic plate with (b) its corresponding estimate size from full width-half maximum intensity distribution.
5. Conclusions
A technique for obtaining two-dimensional microwave holographic images of objects using light-modulated scatterer has been described. Images of different target shape using the technique have been presented. The higher resolving power of the system compare with the Rayleigh limit for a conventional holography has been demonstrated.
6. Acknowledgements
The research described in this paper was carried out by the Applied Microwave Research Laboratory, King Mongkut's Institute of Technology Ladkrabang, under a contract with the Thailand National Electronics and Computer Technology Center (NECTEC). We are also grateful for financial assistance from the NECTEC.
Reference
-
Born, M., and Wolf, E ., 1964. Principle of Optics., New York.
- Cullen, A.L., and Parr, J.C., 1955. A new perturbation method for measuring microwave field in free space., Proc. IEE, 102B, pp.836-844.
- Hajnal, J.V.,1987., Compound modulated scatterer measuring system., Proc. IEE, 134H(4), pp. 350-356.
- Orme,R.D., and Anderson,A.P., 1973. High resolution microwave holographic technique : application to the imaging of objects obscured by dielectric media., Proc. IEE, 120(4),pp. 401-406.
- Richmond J.H., 1955. A modulated scattering technique for measurement of field distributions. IRE Trans. on Microwave Theory and Techniques., ;V. MTT-3,pp. 13-15.
- Tricoles, G. and Farhat, N. H. , 1977. Microwave holography: applications and techniques, Proc. of the IEEE.,65(1),pp. 108-120.
