Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-04-16
2001-02-13
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S495000
Reexamination Certificate
active
06188481
ABSTRACT:
This invention relates to interferometry and is particularly, but not exclusively, related to a single aperture quasi-optical spacial interferometer.
BACKGROUND OF THE INVENTION
The power radiated by an antenna generally varies with direction. A uniformly illuminated aperture has a radiation pattern having a power which varies with respect to angle according to the function (sin x/x)
2
as shown in FIG.
1
. The angle between half peak power points of a radiation pattern is usually defined as the angular beam-width which is shown as &thgr;. Two identical targets are said to be resolved in angle if they are separated by more than the angular beam-width. It is difficult to determine the angular location of a target to an accuracy significantly better than the angular beam-width of an antenna. This is especially the case when the signal-to-noise ratio is low.
Tracking of targets which move with angular velocities requires determination of angular location having an angular accuracy greater than can be achieved using a straightforward single antenna system. Accuracy can be improved by using the technique of spatial interferometry which relies upon having more than one receiving antenna.
In this technique, signals which are received from an object or target are collected by a number of antennae and signals from each antenna are combined by using a comparator to produce a sum channel and a difference channel. The comparator may be a hybrid or an optical circuit. The sum channel produces an output which is the combination of the signals from the antennas. The difference channel produces an error voltage (having a plus or minus sign) which is approximately proportional to the angular deviation of the target from a notional centre line which is referred to as the bore-sight. The bore-sight is the electrical axis of the antenna beam which produces a null out of the difference channel. The sign of the error voltage is determined using a phase-sensitive detector and is used to determine the direction of the angular deviation from the bore-sight.
Spatial interferometry is used in astronomy and radar systems to improve resolution, track moving objects and determine range.
One particular application of spatial interferometry is a monopulse radar system. The term monopulse refers to a radar system which can obtain angular and range information from a single pulse. Such a system has an antenna which can have any number of antenna feeds, but four are commonly used which are placed at the focus of a cassegrainian or a lens system for the reception of signals from a target, normally a moving target A side view of such a system having an antenna (aperture)
10
and an array of horns
12
is shown in FIG.
2
. The bore-sight is shown as broken line b. The radar system maintains the position of the target on bore-sight by using information provided by the difference channel to control servo-motors which move the antenna to maintain the target on bore-sight.
When a lens or reflector acts as a receiving antenna the echoes received from the target are not focussed to a point because of the wave nature of the radiation. The radiation is distributed in a diffraction pattern known as an Airy function, the exact nature of the function depending on the energy distribution in the aperture.
If an array of horns is placed in a common plane in the focal region of an antenna, the coupling of energy from free space into the cluster of horns is inefficient. This is because energy distribution in each horn is such that it is a maximum in the centre of the horn and decays towards the walls. Even when the horns are located close together the inefficiency due to coupling loss is large. An array of four horns illuminated by a single aperture has an intrinsic loss of several dB.
The array of horns
12
receives energy reflected or originating from a target. The energy distribution is maximum at the centre of the array of horns where the walls of adjacent horns meet and so coupling is inefficient.
SUMMARY OF THE INVENTION
Since antennae are reciprocal devices, in systems which use an array of horns to transmit as well as receive, the losses are further increased In such a system a separate intrinsic loss is incurred on both transmitting and receiving.
It is an object of the present invention to reduce inefficiency in coupling between focussing optics and detectors or receivers in an interferometer.
According to a first aspect the invention provides an interferometer comprising:
beam splitter means therefor having an aperature, the beam splitter means splitting an incoming beam of energy incident on the aperture into at least a first beam and a second beam, the first and second beams originating substantially from separate parts of the aperature;
first and second energy feeds for receiving said first and second beams the beam splitter means and the energy feeds being seperated by free space; and
means for detecting a phase difference between the first and second beams;
characterised in that the regions of the aperature beam from which the first and second beams originate overlap.
According to a second aspect the invention provides a method of interferometry on an incoming beam of energy comprising the steps of directing the incoming beam of energy onto an aperture splitting the incoming beam of energy into at least a first beam and a second beam, the first and second beams being substantially obtained from separate regions of the aperture, feeding the first and second beams to respective energy feeds; and detecting a phase difference between the first and second beams characterised in that the regions of the aperature from which the first and second beams originate overlap.
By having overlapping energy distributions fed to the feeds, coupling between the first and second beams and the energy feeds can be efficient.
Preferably the energy is electromagnetic energy.
Preferably the first and second beams are fed to the energy feeds which are spatially separated, that is, not adjacent.
Preferably the incoming beam is split by a beam splitter means having a non-uniform splitting ratio from one side of an energy receiving area to the other side. Preferably the splitting ratio varies from a maximum value at said one side of the beam splitter means to a minimum value at said other side of the beam splitter means. It may change at a varying rate.
Preferably the incoming beam of energy is split into more than two beams. Most preferably the incoming beam is split into four beams.
Preferably the first and second beams are obtained from opposite sides of the incoming beam.
Since the first and second beams are obtained from separate, that is different, sides of the incoming beam, any phase information embodied in the incoming beam may be obtained and analysed.
Preferably the reflectivity of the beam splitter means changes across its surface. Alternatively another characteristic may change such as phase, polarisation, a magnetic or an electrical characteristic.
Preferably the splitting ratio varies in more than one direction across the surface. It may vary in orthogonal directions.
The beam splitter means may comprise a any of a number of variants. It may comprise plurality of wires which are spaced apart in a comb-like structure. The wires may be stretched over a frame or printed on a support. The spacing between adjacent wires may increase from one side of the comb-like structure to the other. A grid-like structure may comprise an overlapping pair of two comb-like structures at an angle to each other, for example at a right angle.
In another embodiment the beam splitter means may comprise a plurality of “dots”, the concentration of dots per unit area varying across the beam splitter means. The variation may be in one or in two directions. The dots may be of any size and shape which is capable of splitting a beam when a sufficient concentration of dots is present.
The beam splitter means may be a wedge in which the splitting ratio at a point on the wedge is determined by the thickness of the wedge at that point. The beam splitter means
Frank Robert J.
Kim Robert
Kinberg Robert
Marconi Electronic Systems Limited
Venable
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