Optical Space Switch

This application is related to commonly assigned U.S. patent application Ser. No. 07/249,560, filed Sept. 23, 1988 for a BIDIRECTIONAL OPTICAL SPACE SWITCH naming Peter Healey as sole inventor now U.S. Pat. No. 4,923,269.

This invention relates to an optical space switch which is capable of use in a centralised switching system for an optical network. A centralised switching system is the simplest active network which is compatible with optical and electronic multiplexing, offers the maximum network size, range, and flexibility and is also compatible with the existing wire-network.

According to this invention an optical space switch for connecting light from one of a number of inputs to a selected one of a number of outputs includes a number of multiplexed phase volume holograms, a spatial light modulator for imposing a selected phase change on light passing through it, distribution means for coupling light from the inputs via the spatial light modulator to the multiplexed phase volume holograms, and collection means for collecting light diffracted by the multiplexed phase volume holograms, each multiplexed phase volume hologram diffracting light to a selected output in dependence upon the phase change imposed by the spatial light modulator on the light incident upon that hologram.

Preferably there are the same number of multiplexed phase volume holograms as there are inputs and the same number of holograms in each multiplexed phase volume hologram as there are outputs and the distribution means couples light from each input via the spatial light modulator to its respective multiplexed phase volume hologram and the collection means collects the light diffracted by the multiplexed phase volume holograms and applies it to its respective output. This arrangement provides total flexibility and enables each and every input to be coupled to each and every output. However, in some circumstances it is desirable to be able to arrange for two inputs to be switched together by the optical switch or, for example, two outputs to receive the same signal. In this case, there may be fewer multiplexed phase volume holograms than there are inputs and greater numbers of holograms in each multiple of phase volume hologram than there are outputs. The distribution and collection means are formed by lenses or holograms arranged to couple light emitted from a particular input to a particular multiplexed phase volume hologram via the spatial light modulator and arranged to receive light diffracted by the multiplexed phase volume holograms and couple it to the individual outputs.

The inputs may be provided by the ends of a number of optical fibre waveguides, or, alternatively, the inputs may be formed by a number of optical devices the outputs of which are modulated by the input signals. Thus, the optical inputs may all be taken from a single optical source which is split and each split component modulated in accordance with an input signal, or alternatively, the devices may be formed by a number of independent optical sources each of which is modulated in accordance with an input signal. The outputs may be formed by the ends of an array of optical waveguides leading away from the optical space switch and, in this case they are preferably formed by an array of multimode optical fibres. Alternatively, the outputs may be formed by an array of photo detectors or other active devices such as self electrooptic effect devices which detect, amplify, or regenerate the light received. Photodetectors may form part of an optical regenerator which, in turn, generates an output in the form of an optical signal. The input may be arranged in a linear array and, in this case the output may also be arranged as a linear array extending in a direction perpendicular to that of the linear input array. Alternatively, the input and/or outputs may have the form of two dimensional matrix arrays.

The spatial light modulator may be formed by a material the refractive index of which may be varied by the application of an electrical or optical signal or it may be formed from a material the physical dimensions of which can be changed selectively by the application of an electrical signal. In either case the change of phase across the wavefront of light passing the spatial light modulator can be varied by the selective application of an electrical or optical control signal to the spatial light modulator Examples of spatial light modulators are acousto-optic transducers and liquid crystals. Individual spatial light modulators may be provided to modulate the light emitted by each input or, alternatively, a single spatial light modulator may be provided which interacts with light from all of the inputs.

The multiplexed phase volume holograms may be reflection or transmission holograms or may be mixtures of both reflection and transmission holograms. The holograms may also be of the Fourier transform phase volume hologram type.

To create the multiplexed phase volume holograms an unexposed holographic recording medium is placed in position downstream of the inputs, which during creation of the holograms act as reference sources, the distribution means and the spatial light modulators. An object array corresponding to the output and collection means are located on the same side of the holographic recording medium as the input distribution means and spatial light modulator and in a laterally offset position when creating a transmission hologram. Light from a coherent source, typically a laser is applied via one of the inputs, the distribution means and the spatial light modulator to form a reference beam which impinges upon a section of the holographic recording medium which receives light from that input. Another beam taken from the same coherent light source and passing through an equivalent path length is applied via one of the objects, typically the output end of a single mode optical fibre waveguide which then, after passage through lenses forming the collection means is distributed over the same section of the surface of the holographic recording medium. Light from the same source but which has passed through these two different paths then interferes in the volume of the holographic recording medium to expose a particular pattern in that section of the holographic recording medium. Then, light is emitted from a different object and a different control signal is applied to the spatial light modulator. A further exposure of that section of the holographic recording medium is then made to expose a second pattern. This process is repeated until a pattern is exposed for each of the object sources, each one having a different control signal applied to the spatial light modulator so that the reference beam used to create the hologram has a different phase front. Thus, each reference and point source are stored in association with one another.

The next section of the holographic recording medium which receives light from the next input is then exposed by applying the reference beam to the next input and making another series of exposures with each and every object and with different control signals applied to the spatial light modulator. This process is repeated with a reference beam being applied to each and every input until the entire holographic recording medium has been exposed. The recording medium is then processed to produce the multiplexed phase volume holograms.

Thus, with this system when light from a particular input passes via the spatial light modulator to its multiplexed phase volume hologram since the light from the input forms a reconstruction beam following the path of the reference beam a virtual image is formed at the location of the object and the collection means focuses light diffracted by the hologram onto the corresponding output in dependence upon the control signal that is applied to the spatial light modulator. The light diffracted by the multiplexed phase volume hologram. The collection means focuses the light diffracted by the hologram corresponding to the required virtual image on to the output while the light diffracted by other holograms are widely dispersed over the output plane. In principle, the light dispersed from the other holograms can be made orthogonal at the point of focus on the output plane by using an orthogonal function in the spatial encoding process of the reference waves e.g. Walsh or Hadamard functions. Since the diffraction efficiency of the individual holograms falls as the number of holograms in each multiplexed phase volume hologram is increased and the power of the light incident upon the hologram is shared between the multiple diffractions the switch insertion loss grows rapidly with increasing numbers of outputs. This low can be minimised in a volume phase hologram by maximizing the angular separation between the object point sources, and hence between the outputs so that the Bragg extinction angle eliminates most of the undesired interference terms. In practice, it is possible to superimpose more than one thousand holograms of coded individual point sources on a single multiplexed hologram and it is believed that two or three orders greater than this may be possible.

A particular example of an optical switch in accordance with this invention will now be described with reference to the accompanying drawing of FIG. 1 which is a diagrammatic perspective view.

The optical switch comprises a number of inputs formed by a linear array of single mode optical fibres 1, a spherical and cylindrical lens combination 2, a spatial light modulator 3, a number of multiplexed phase volume holograms 4, a lens combination 5, and a linear array of multimode optical fibres 6 forming a number of outputs. The spherical and cylindrical lens combination 2 spreads light leaving the end of the input fibres 1 in the vertical direction as shown in the drawing to spread it over a vertical column of the spatial light modulator 3 and a similar vertical column of the hologram 4. The hologram 4 is formed by a number of vertical sections arranged side-by-side. The sections are associated with the individual input fibres 1 and light from the first input fibre is only incident upon the first section. The spatial light modulator 3 equally includes a number of vertical sections with each section only receiving light from its corresponding input fibre. The spatial light modulator 3 is formed by an acousto-optic modulator. A separate control signal is applied to each of these sections to vary the phase change that occurs on passage of light through the section of the spatial light modulator 3. The multiplexed phase volume holograms that are recorded in each section of the hologram 4 diffract the light passing through the hologram 4 in such a way that it is focused by the lens 5 onto one of the output fibres 6. Which of the output fibres 6 the light is focused upon is determined by the phase front that is imposed on the light as it passes through the spatial light modulator 3. Thus, by varying the control signal applied to the individual sections of the spatial light modulator 3 the input signal appearing on each input fibre 1 is coupled to a particular individual output fibre 6.

To create the holograms a holographic recording medium 4' is located where the hologram 4 is located and a number of object sources formed by the ends of a linear array of single mode optical fibres 7 are provided. The object sources 7 are complementary to output fibre 6 and may include a similar optical system 8 but the object fibres 7 are located on the same side of the holographic recording medium 4' as the input fibres 1. To record the holograms on the medium 4' a single coherent optical source (not shown) typically formed by a laser is applied via a beam splitter to one of the input fibres 1 and also to a selected object fibre 7. When the coherent source is applied to the first input fibre 1 a first strip of the medium 4' is uncovered, a first control signal is applied to the spatial light modulator 3, and the coherent light is also applied to one of the object fibres 7. Thus, coherent light from the coherent light source is fed firstly via one of the fibres 1, the lens system 2 and the spatial light modulator 3 and, independently, via one of the object fibres 7 and the lens system 8 and both of these beams interfere with one another in the first section of the holographic recording medium 4'. This exposes a first pattern in the first section of the medium 4'. Then, a second control signal is applied to the first section of the spatial light modulator 3 and the light from the coherent light source is then fed to the second object fibre 7 as well as the first input fibre 1. Another exposure is made of the first section of the holographic recording medium 4' and this process is repeated until a pattern is recorded in the first section of the holographic recording medium 4' for each one of the object fibres, each time a different control signal being applied to the first section of the spatial light modulator 3. Then, the coherent light source is applied to the second one of the input fibres 1, a first control signal is applied to the second section of the spatial light modulator 3 and the first section of the holographic recording medium 4' is covered up and a second section exposed. The process is repeated until all of the patterns are exposed on the second section of the holographic recording medium 4'. This process is continued to expose as many sections across the holographic recording medium 4' as there are input fibres 1. The holographic recording medium 4' is then processed in the conventional way to produce the required multiplexed phase volume holograms 4.

In preparing the holographic recording medium 4' the light from the input fibres 1 and which has passed through the spatial light modulator 3 forms the reference beam and the light from the fibres 7 forms the object. Thus, when the developed hologram 4 is illuminated by light from a source of substantially the same wavelength as the coherent light used to prepare the hologram 4, the light diffracted by the hologram 4 appears to have come from a virtual image located at the position of the object fibre 7. This light is thus focused by the optical system 5 on to one of the output fibres 6. Since a different control signal was applied to the spatial light modulator 3 when different object fibres 7 were illuminated and used as objects during the construction of the hologram, by applying particular control signals to the spatial light modulator 3 when input signals are fed from the input fibres 1 these are switched by the hologram 4 to particular of the output fibre 6 corresponding to the object fibre 7.