This invention relates to optical computers and to interconnections between optical arrays.

In optical computers, light is. used to transmit signals representing the state of one elerent or a plurality of elements to another element or plurality thereof. The elements may be non-linear optical logic gates. In particular, though not exclusively, the invention relates to self-adaptive digital optical computers and has particular relevance to those incorporating neural networks.

Examples of a digital optical computer and an interconnection system are given' in our published European Patent Application EP-A-0268382 , the contents of which are incorporated herein by reference.

Existing proposals for optical processors operate on digital principles; the signals are either ."on" or "off". However, particularly for the types of processors discussed above, it is desirable to provide systems in which the signals may be weighted with variable weightings because this allows complex mathematical functions to be applied between arrays. Such systems may

be of particular benefit where threshold devices are used which open or close dependent on the number of inputs incident thereon.

Accordingly, in one aspect this invention provides an optical interconnection system comprising first and second arrays of optical computing or logic elements, wherein said first array is mapped onto said second array in a predetermined interconnection pattern, and modulation means are disposed in the optical path between said first and second arrays for applying respective modulations to the optical interconnections between each of the elements in said first array and the associated mapped element or elements of the second array.

In another aspect, this invention provides an optical system comprising an input array, an output array and transmission means disposed in the optical path between said input and output arrays, said transmission means comprising a set of elements each of selected optical density for adjusting the amplitude of the signal passed thereby.

In a further aspect, this invention provides an optical processor comprising an input array, an output array, and a holographic array of holographic elements together defining a plurality of interconnections between said input and said output arrays, wherein the

contrast of said holographic elements is selected to provide a predetermined weighting to the signals passed thereby.

Further preferred features of the invention are apparent from the subsidiary claims.

The invention will now be described by way of non- limiting example, reference being made to the accompanying drawing which is a schematic illustration of an optical processor in accordance with this invention.

The drawing shows in simple schematic form an input array and an output array of logic elements, or other non-linear optical elements, which form part of a digital optical computer capable of performing a large number of interrelated logic operations in parallel.

In the example shown in the drawing, an input or first array 10 of NxM elements (where ,M are integers ^1)supplies light to an NxM holographic array 11 of holographic elements which provides interconnects between the input array 10 and an output or second array 12 of PxQ elements (where P,Q are integers >1 ) . The holographic array applies a mapping function which maps the outputs of the input array 10 onto the outpMt array 12, thus providing the required interconnection pattern. The mapping function may be one to one, many to one, one to many etc. Indeed, in the simplest case, the

holographic array could map each element of the input array onto the correspondingly positioned element of the output array. By contrast, the mapping may be much more complex, for example as shown in EP-A-026838, where groups of outputs are configured to form a "cell" which may be mapped in cell to cell fashion or cell-shifted fashion onto one or more cells of inputs.

It should be noted that, in many applications the input array and the output array comprise the inputs and the outputs respectively of an array of optical logic gates, so that the input and output arrays form the front and rear surfaces of the array of logic gates. Alternatively, the input and output arrays may be physically separate, and the inputs may be mapped in one-to-one fashion onto corresponding elements of the output array without requiring special imaging or deflection apparatus.

A variable transmission filter 13 is disposed adjacent the side of the holographic array 11 that faces the output array 12. The filter 13 may comprise an array of elements of independently controllable transmission characteristics. The layout of the elements may correspond with the layout of the elements of the input and/or output arrays. The variable transmission filter may have a wide range of transmission so that it may act as a switch in the light path when operated at extreme

points of the range of transmission. In addition, it may act as a variable control of the transmission strength when operated at intermediate conditions. This latter state would also apply for filters that had only a limited range of transmission.

It is preferred for the variable transmission filter 13 to be a spatial light modulator, i.e. an essentially two-dimensional element in which the transmission characteristics of different parts of the element can be controlled independently of each other. The spatial light modulator may be placed in the path of the signals interconnecting two arrays of an ^' optical computer to change the configuration - or control the interconnections - of the computer by changing the transmission of the appropriate region of the spatial light modulator.

The control of the spatial light modulator may be electrical or optical. In the electrical case, the pattern required may be set by electrical signals on a grid of intersecting electrodes. It can readily be seen how a variety of electrical signals may be used to modulate the various beams between the arrays so that the computing elements may be interconnected or reconfigured as desired. The control may also be optical, by projecting a distribution of light over the surface of the modulator to control the transmission.

The wavelength of the light which "writes" the image onto the spatial light modulator may be the same or different as that of the light which subsequently reads the image. The light signals may be provided by imaging an array of active elements within the computer on to the spatial light modulator.

The configuration of the computer may thus be adapted by the state of parts of the computer system. This control may be either switching, in which the archi¬ tecture may be changed, or variable, where the weights of various summing elements can be varied, or a combination of both.

Our published European Application EP-A-0268382 discloses an example in which the configuration of the optical computer is determined by an array of holographic lenses. If, in such an arrangement, the array has holographic lens elements corresponding to more than one interconnection arrangement, a spatial light modulator adjacent to the holographic lens array may be used to select which holographic lens elements are active (and thus the mapping or interconnedction applied) .

In the arrangement illustrated in the drawing, the variable transmission element 13 is adjacent the outlet side of the holographic array 11 so that it modulates the light beams as they leave the holographic array 11.

If the variable transmission element 13 is located adjacent the output array 12, it will modulate .the light beams immediately before they are incident on the output array. In this way it is possible to select different modulation effects dependent on whether the beams are to be modulated before or after they have passed through the interconnection matrix defined by the holographic array.

In some applications it may be beneficial to dispose the variable transmission element 13 part way, between the holographic array 11 and the output array 12, so that, for example, a cell of the element 13 modulates beams which intersect each other as they pass from the holographic array 11 to the output array 12. Similarly, the variable transmission element 13 may be disposed either adjacent the input array 10, . or adjacent the input side of the holographic array 11, .or p-att way therebetween. - '

The arrangement described above may be modified by replacing the array of holographic elements by an array of refractive elements such as lenslets. *• *

In a further modification, the optical combination of the input and output arrays 10 and 12 may be controlled without the use of a separate Variable transmission element 13, by providing a holographic array 11 of holographic elements in which the contrast

of the hologrraphic elements is adjusted. In particular, where the holograms are produced in "real time" by mixing beams in a dynamic or reprogrammable optical medium, the contrast of each holographic element can be varied by varying the strength of one of the beams, e.g. the reference beam, used to form the holographic element.

In one particular example, the input array and the output array could be interconnected via a real time holographic array generated in a dynamic optical storage medium by a first "write" optical beam interfering with a reference beam of which the intensity is spatially modulated in accordance with the signals present on a further array. In this way a logical network may be constructed.

In a modification of this invention, the effective dynamic range of an array of output detectors having a relatively low dynamic range and used within an electro- optic system may be increased to match or compensate for the relatively high dynamic range of an input signal from an input array by interposing between said input and output arrays a variable transmission element in the form of a photochromic film for modulating the transmitted light in accordance with the intensity thereof, or alternates said light.

The above arrangements introduce a transmission

element between two arrays of optical computing or logic elements which form part of an optical computer. It should be appreciated however that the arrangements are not limited to use in optical computers and _. may be used in hybrid electronic/optical systems. Similar arrangements may be provided to interconnect elements of conventional (i.e. electronic) computers, transputers or other data processing/transmission equipment. For example an upstream piece of equipment may supply electric signals to a converter array which converts the electric signals into light signals which are supplied via a transmission element which controls the interconnections, and possibly via a mapping element, to an optical/electrical converter array which supplies electrical signals to a downstream piece of equipment.