Microwave holographic- imaging remote objects using light-modulated scattering technique
Thammasak Vimonkiattikun1, Jirawath Panklang2, Anupong Srongprapa3
1,2Department of Electronic Engineering, Faculty of Engineering
3Applied Microwave Research Laboratory(AMRL)
Department of Applied Physics, Faculty of Science
King Mongkut's Institute of Technology Ladkrabang,
Bangkok 10520,
Tel (66-2)3269980 ext. 213/214, fax: (66-2)3269981,
E-mail: ksanupon@kmitl.ac.th,
Thailand
Keywords: Image Processing, Microwave, Holography, Digital Signal Processing, Remote Sensing,Modulated Scattering.
Abstract
Modulated scattering technique is mostly used for measuring microwave fields with less perturbation to obtain near - field antenna pattern, such a technique can also be used for microwave holography.A computerized system for recording and reconstruction of microwave hologram at 9.2 GHz X-band frequency is described. A small dipole loaded by MRD 721 photodiode is suspended by thin balsa post at some distance along symmetric Z-axis of the fixed illuminating pyramidal horn antenna. The scattering cross-section of the dipole was modulated by 50 KHz optical signals guided to the diode through a plastic optical fiber. Low-intensity microwave was transmitted to a remote object via a free space and then reflected from object surface. Both the outgoing and backward wave excited the scattering wave, originated from the dipole,then propagated back to the same antenna and mixed with the synthetic off-axis internal reference wave providing by a programmable microwave phase shifter. The object was scanned 1/3 of wavelength stepwise in both X and Y directions, perpendicular to Z-axis, generating 128x128 sampling. For each object position, the output current of the mixer was preamplified and coherently measured at the modulated frequency using a lock-in amplifier to form hologram intensity signals. Fresnel diffraction approximation is used in digital reconstruction from stored hologram data to yield the two- dimensional image of the object.The distance between antenna, scatterer and object plane was allowed to varied. The higher resolving power of system compare with the Rayleigh limit for a conventional hologram has been demonstrated.
1. Introduction
As a mean of seeing through optically opaque medium, various application areas of microwave holography (Tricole,1977) are possible such as all-weather remote sensing, medical diagnosis, buried objects locator, etc. In ordinary system the microwavefield scattered from a stationary object coherently illuminated from a stationary transmitter is mapped over a prescribed hologram recording aperture by means of a detector that is scanned over the aperture. The simplest one need a fixed reference beam to interfere with the object wave and creates wavefield pattern that can be measured by intensity sensitive detector. The more advance one uses coherent phase-locked receiver with local oscillator acting as synthetic internal reference beam. Normally, with regards to the actual size of the conventional probing detector for low intensity application, the detected wavefield is considerably disturbed.
This problem can be overcome by using modulated scattering technique, where the original ones measured the field using a scatterer which may be mechanically (Cullen and Parr,1955), electrically (Richmond,1955) or optically (Iizuka,1963) modulated with synchronous detection of the modulated scattered signal. Orme and Anderson (Orme,1973) used spinning dipole technique of Cullen and Parr in their high resolution microwave holographic imaging of the objects obsured by dielectric media. In the work described here, a system similar to Orme and Anderson's, has been used to perform two-dimensional image of remote metallic objects. Now, the scatterer,in the form of a selected high speed photodiode, is modulated optically via a plastic fiber (Hajnal,1987) and the system is computerized for creating arbitrary internal reference beam angle, scanning control, data collecting and performing image reconstruction digitally.
2. Off-Axis Scanning Source / Receiver Microwave Holography
In some situations where microwavefield can be appropriately described as scalar quantity, scalar diffraction theory of light wave can be used. It is well known in physical optics that light which passes through an aperture or reflects from a plana reflector is fully described by the phase and amplitude distribution in the aperture/or reflector area. According to The Huygens' principle, this is sufficient to reconstruct the light wave at any point behind the aperture (or infront of the reflector). If we describe phase and amplitude in the aperture plane z = 0 by the complex function F(x,y),we obtain for the wave f(x,y,z) which develops behind the aperture (Born and Wolf,1964)
f(x,y,z) = F(x,y)*G(x,y,z) (1)
where the symbol * denotes convolution and G is a sperical wave of wavelength originating
in the aperture plane. Thus
with
, which becomes in Fresnel's approximation
giving the function f(x,y,z) to be found from a Fresnel's transform of F(x,y). Figure.1 represents the general geometric set up of the scanning-source/receiver microwave hologram recording. Here, a point source, a sensor (or detector), and an object are located at 3 consequetive planes denoted by source plane, sensor plane and object plane respectively. Spherical wave emitted from point source at P
1 (x,y,z)
propagates to a point P
2 (
z,h,o) on finite extended object at object plane. The reflected wave from point P
2 propagates to sensor at position P
3 (u,v,w). If sensor and point source are scanned coincidently,(i.e. x = u , y = v ) the reflected wavefunction at sensor plane are given by
which is recognised as the diffraction field at plane z' = wz /(w+z) distant from the object for the situation of conventional holography. The Rayleigh resolution limit of the system is then given by
At the sensor position there is also reference planewave field b(x,y) whose direction of propagation make an incident angle
q to the normal of the plane.Thus
Figure 1. Recording Image data hologram of object using microwave
where
a is spatial frequency of wavefront in the x direction which is equal to
,
l = 2
p/k is wavelength , and b0 is the field amplitude .
If the total wave function is C(x,y) and regards w as a constant, the intensity at sensor is
Image reconstruction process concerns with the modification of the reconstructed reference plane wave b*(x,y) by the intensity distribution I(x,y) to be b*(x,y) I(x,y).Fresnel's transform of the term b
02a*(X,Y) corresponds to the real image wavefield of the object at the image plane.There is also a direct illuminating field E(u,v,w) which remain constant,as the sensor and source are scanned together.This term do not alter the results derived above significantly.
