Abstract:
A quadrant detector includes a holographic optical element and a plurality of detector elements. The holographic optical element is designated to have four quadrants, each of which directs radiation incident thereon to an associated detector element. Signals from the detector elements are processed individually to optimise the performance of each detector element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to improvements in or relating to detectors, and is more particularly, although not exclusively, concerned with quadrant detectors. 
     BACKGROUND OF THE INVENTION 
     Quadrant detectors are used in many applications, but their usefulness can be limited by their operating wavelength which is dependent on the semiconductor material which is used for the detector elements. For example, detectors operating at a laser wavelength of 1.064 μm tend to comprise silicon based materials which are used close to their long wavelength limit. It is also known to ‘extend’ the laser wavelength range of a silicon based detector by heating the detector to around 70° C. This ‘thermally’ reduces the band gap. 
     It will readily be understood from the above discussion that silicon based detectors cannot be extended to ‘eye safe’ laser wavelength of 1.5 μm as is now required, and other detector materials will need to be utilised, for example, Si:Ge, InGaAs and other Group III/V materials. 
     Furthermore, producing a detector to operate at two or more wavelengths simultaneously, for example, at 1.064 μm and 1.5 μm, will require specialised materials and devices, such as sandwich structures, which are inherently difficult and expensive to manufacture into quadrant geometries. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a detector arrangement which overcomes the problems discussed above. 
     In accordance with one aspect of the present invention, there is provided a method of detecting radiation from a scene using a quadrant detector, the method comprising: 
     providing separate quadrant separation function and optical detection functions. 
     In accordance with another aspect of the present invention, there is provided a quadrant detector comprising: 
     means for providing a quadrant separation function; and 
     means for providing an optical detection function, the means for providing the optical detection function being separated from the means for providing the quadrant separation function. 
     It will readily be understood that the terms ‘quadrant separation function’ relates to dividing incoming radiation to produce four output signals and the term ‘optical detection function’ relates to the process of receiving radiation at a detector and its subsequent conversion to an electrical signal by the detector. 
     Adavantageously, the quadrant separation function is carried out by a holographic optical element, and the optical detection function by a plurality of detector elements, each detector element being associated with a quadrant formed by the holographic optical element. 
     Preferably, the holographic optical element has multiple wavelength capability. In this case a set of detector elements are provided for each wavelength and each detector element is associated with a quadrant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which: 
         FIG. 1  illustrates one embodiment of a holographic based quadrant detector in accordance with the present invention; and 
         FIG. 2  illustrates a second embodiment of a holographic based quadrant detector having multiple wavelength capability in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, a quadrant detector is provided in which the quadrant separation function is separated from the optical to electrical detection function. This is achieved by using a holographic optical element (HOE) which ‘focuses’ or directs light incident on the HOE onto physically separated detectors. This is shown in  FIG. 1 . 
       FIG. 1  shows a holographic optical element (HOE)  10  which comprises quadrants  12 ,  14 ,  16 ,  18  as shown. Associated with each quadrant  12 ,  14 ,  16 ,  18  is a respective detector  22 ,  24 ,  26 ,  28 . Each detector  22 ,  24 ,  26 ,  28  is a single wavelength detector and is separated from each of the other detectors. Each detector  22 ,  24 ,  26 ,  28  is connected to its own individual processor (not shown) which receives signals from its associated detector and provides an output signal indicative of the radiation received by the associated quadrant  12 ,  14 ,  16 ,  18  of the HOE  10 . 
     The output signals received from each of the detectors  22 ,  24 ,  26 ,  28  are processed using conventional processing techniques to provide information relating to where an object in a scene is relative to the quadrants  12 ,  14 ,  16 ,  18 . For example, a difference between the sums of vertical quadrant pair  12 ,  14  and quadrant pair  16 ,  18  provides azimuth information relating to the object relative to the centre of the element  10 , and a difference between the sums of horizontal quadrant pair  12 ,  18  and quadrant pair  14 ,  16  provides elevation information relating to the object relative to the centre of element  10 . 
     It will be appreciated that the HOE  10  provides a quadrant separation function which could be derived from, for example, a computer generator pattern. 
     It will be appreciated that, by separating the quadrant separation function from the optical to electrical detection mechanism, that is, the optical detection and subsequent conversion to an electrical signal, each detector and its associated processor can be optimised without compromising other detectors. 
     Naturally, although individual processors are described above, a single processor having four separate areas could also be used. 
     The embodiment described with reference to  FIG. 1  operates at a single wavelength. However, the HOE may be made to operate at more than one wavelength. Such a HOE is shown in  FIG. 2 . 
       FIG. 2  shows a HOE  30  similar to HOE  10  of  FIG. 1  but which has multiple wavelength capability. HOE  30  has quadrants  32 ,  34 ,  36 ,  38  each of which has a first detector  42 ,  44 ,  46 ,  48  operating at a first wavelength λ 1  and a second detector  52 ,  54 ,  56 ,  58  operating at a second wavelength λ 2  associated with it. As an example, λ 1  may be 1.064 μm and λ 2  may be 1.5 μm. As shown, the second detectors  52 ,  54 ,  56 ,  58  are spatially separated from the first detectors  42 ,  44 ,  46 ,  48 . Again, each of the first and second detectors  42 ,  44 ,  46 ,  48 ,  52 ,  54 ,  56 ,  58  can be optimised without comprising any of the other detectors. 
     The HOE  30  shown in  FIG. 2  has the advantage that images at two different wavelengths, λ 1  and λ 2 , can be formed. It will be appreciated that a HOE may be designed to operate at more than two wavelengths and that a set of detectors is provided for each extra wavelength. 
     Holographic optical elements can be made in large sizes and at low cost, and therefore are viable alternatives to known quadrant detectors. 
     Apart from use in a quadrant detector as described above, HOEs may also have application in other electro-optic systems.