OPTICAL SWITCH

Abstract: A polarisation independent optical switch comprises a light input (6), a diffractive and re-configurable liquid crystal device (8), a collimating lens (7) for distributing the input light over the liquid crystal device (8), a transform lens (9) receiving light from the liquid crystal device (8) and a light output (6), wherein no polarisers are present between the light input and output. The diffractive and re-configurable liquid crystal device (8) may be formed as a liquid crystal spatial light modulator configured as a binary phase computer generated hologram with the holograms required to form broadcast, individual or multicast switching being re-generated using an iterative algorithm and then being recalled on demand. In one embodiment the switch has a folded configuration and includes a reflector (10) so that the light input and output are both on the same side of the liquid crystal device (8).

Description: OPTICAL SWITCH TECHNICAL FIELD Switches for optical fibre networks are increasingly important. One area of active development at present is the use of liquid crystal devices such as spatial light modulators (SLMs) in such switches. Ne atic liquid crystals may be used but most of the recent work has been carried out using more modern liquid crystal materials such as ferroelectric liquid crystals (FLCs) or electroclinic liquid crystals (EC) . BACKGROUND ART Each pixel in such liquid crystal devices acts as an independent rotatable waveplate, such that an applied voltage rotates the birefringent axes through an angle 2θ . Until now, the accepted mode of operation of such devices has been to arrange and correctly orient crossed polarisers around the device. Liquid crystal spatial light modulators may be configured as binary phase computer generated holograms (CGHs) which have particular application to free space diffraction based optical switching. This is described in detail in an article entitled "A holographi- cally routed crossbar: Theory and simulation" by D.C. O'Brien, W.A. Crossland, and R.J. Mears, published in Optical Computing and processing, 1(3) :233-243, 1991. In this case, the ideal transmission of the device through crossed polarisers can be shown to be T + sin 20 sin k Δnd / 2 (1) where k is the wave number 2π/λ, Δn is the FLC birefringence and d is the FLC thickness. This is described in an article entitled "Programmable binary phase-only optical device based on ferroelectric liquid crystal SLM" by S.E. Broomfield, M.A.A.Neil, E.G.S. Paige, and G.G.Yang, published in Electronics Letters, 28(1) :26- 28, 1992. Unfortunately the intrinsic polariation sensitivity severely limits the use of such switches in normal optical fibre systems where polarisation is usually not preserved. A special situation occurs provided the device is manufactured with half waveplate thickness and an aggregate electro-optic switching angle of 2θ = 90°. This is described in an article entitled "Diffractive ferro¬ electric liquid crystal shutters for unpolarized light", by M.J.O'Callaghan and M.A. Handschy, published in Optics Letters, 16(10) :770-772, 1991. However few large angle materials exist and they are difficult to prepare; the alternate approach uses two layers of 45° material and this is described in an article entitled "Improved transmission in a two level, phase only, spatial light modulator", by M.A.A. Neil and E.G.S. Paige, published in Electronics Letters, 30(5) :445-446, 1994. DISCLOSURE OF INVENTION According to this invention a polarisation independent optical switch comprises a light input, a diffractive and re-configurable liquid crystal device, a collimating lens for distributing the input light over the liquid crystal device, a transform lens receiving light from the liquid crystal device and a light output, wherein no polarisers are present between the light input and output. Preferably, the defractive and reconfigurable liquid crystal device is formed as a liquid crystal spatial light modulator configured as a binary phase computer generated hologram with the holograms required to form broadcast, individual or multicast switching being re-generated using an iterative algorithm and then being recalled on demand. In one embodiment, preferably, the switch has a folded configuration and includes a reflector so that the light input and output are both on the same side of the liquid crystal device and the collimating lens and transform lens are common. Typically the face of the liquid crystal device remote from the input and output is formed as a mirror. In another embodiment, preferably, the switch comprises at least one reconfigurable liquid crystal spatial light modulator which is arranged so that light received at a first hologram is passed to the light output through a second hologram and wherein the second hologram is configured as the inverse of the first hologram. In a first example, preferably two reconfigurable liquid crystal spatial light modulators are provided spaced apart in parallel planes and two transform lenses arranged between the two reconfigurable liquid crystal spatial light modulators and spaced apart a distance 2f, where f is the focal length of the transform lenses. In a second example, preferably the switch has a folded configuration with the first and second holograms arranged in different regions of a single reconfigurable liquid crystal spatial light modulator and includes a reflector spaced a distance f from the transform lens, where f is the focal length of the transform lens, so that the light input and output are both on the same side of the reconfigurable liquid crystal spatial light modulator. Typically the reflector is formed as a mirror. Preferably, the light output is formed by an array of output optical fibres. The light input may be formed by a single input optical fibre where the switch has a one-to- many configuration or again by an array of input optical fibres where the switch has a many-to-one or many-to-many configuration. The least possible optical loss in the switch can be achieved by having the switching angle of the liquid crystal to be as close as possible to 90° because this maximises the phase modulation. However, it is difficult to prepare liquid crystal materials with such a large switching angle and, in general, the greater the switching angle the slower the response speed of the switch. Accordingly, because the switch is polarisation insensitive irrespective of the switching angle of the liquid crystal, it is possible to use commonly available low switching angle materials with the trade off of an increase in switching speed at the expense of some optical loss through the switch. This trade off may be extended by using electroclinic liquid crystals which, in spite of having only small switching angles, typically between 2° and 10° switch at speeds which are at least an order of magnitude greater than ferro-electric liquid crystals. BRIEF DESCRIPTION OF DRAWINGS The theory behind and two embodiments of an optical switch in accordance with this invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a diagram showing the optical indicatrix of an electric field incident upon a birefringement slab; Figures 2a and 2b are photographic representations of the replay field of the output of the FLC device both with and without polarisers present in the system, respectively; Figure 3 is an optical diagram of one embodiment of the optical switch; and, Figure 4 is an optical diagram of another embodiment of the optical switch. DETAILED DESCRIPTION The normalised complex field amplitude £, of an arbitrary electro-magnetic travelling wave E = Re{£exp(j (ωt - kz + δ) ) } , may be written in vectorised form [E x E y exp(jø)] where ø is the relative phase between orthogonal axes of polarisation (x, y) and E x2 x E y2 = 1. Thus £ encompasses all states of polarisation, i.e. any direction of plane polarised light (ø == 0 or π) ; circular polarisation (X = Y, ø = ±π/2 ) , or any other elliptical state. Figure 1 is the optical indicatrix of this field incident on a birefringement slab of material such as FLC, of thickness d (not shown). The fast and slow axes (n 0 ,n β ) of the material are inclined at an angle φ to (x, y) and the k-vector of propagation points out of the page. The Jones polarisation matrix along (n 0 ,n e ) and a rotation matrix for co-ordinate changes may be defined as: j ^ (exxppiiJJkk nndd θo\) D _ c Cc o OoSs«20 sin 2Θ _ s s s i inn 2Θ cos 29 J The input field is propagated through two FLC pixels in opposite switched states, S), and J> 2 . One state may be chosen to lie along the x axis since the input polarisation is arbitrary. Thus _o 1 = 0 and _o 2 = 2 θ , leading to two output fields arbitrarily resolved parallel and perpendicular to _ 2 with the form: + c 4 where c, to c 4 may be calculated. It may be seen that each field component consists of a phase modulated term c 2 or c 3 , and a fixed 'DC term c, or c 4 . The binary pattern displayed on the SLM and its far field replay are related by a Fourier transform containing an undiffracted zero order solution: where CGH(x, y) consists of equal numbers of ±1 representing the two FLC states and the integrated power in f(u,v) is unity. The normalised field ratios of the zero and non-zero order are derived from the propagated fields a dc = l^ (fi ι + £ 2> I and a dc = l% (fi ι " fi 2 ) i such that a 2 dc + a 2 " c =l. Observing the independentce of orthogonal polarisations: a - = {iM+ |ic 3 | 2 ) x |ι - ex UkAndtf=sin 2 29sin 2 * ^ (2) Equation (2) represents the proportion of power diffracted by the device and is identical to the binary phase transmission between polarisers, Equation (1) . Normally with crossed polarisers arranged around the SLM, the 'DC terms are removed (at the second polariser) and π- phase modulation is achieved by setting E χ = cosø, E = ±sinø and ø = 0 such that the input linear polarisation lies symmetrically between _..■ and δ> 2 . We have shown that removal of polarisers from an FLC device leaves the replay field unaltered other than the addition of an on axis zero order solution. This result is very significant for diffractive switching applications: polarisation dependence is removed; signal fading due to polarisation drift is avoided and light throughput is increased by eliminating losses in real polarising materials. For any given device, the proportion of power in the diffracted field remains constant for any input polarisation state. The efficiency may be increased by optimising the FLC thickness and switching angle. A simple experimental set up using a transmissive SLM with 128 x 128 multiplexed pixels on a 220 μ pitch has been shown to verify the above result. The input source was provided by a 633 nm HeNe laser coupled into single mode fibre. The output beam was collimated by a 125 mm lens, passed through the SLM and finally through a transform lens to form the Fraunhofer replay field on a CCD camera. With crossed polarisers correctly aligned around the SLM and an array generating CGH displayed, micro-bending induced polarisation changes in the input fibre caused the intensity of the reply field spots to fluctuate almost to zero. After removal of polarisers, the replay field was 2.3dB brighter due to removal of polarisation losses and the optical quality improved. Micro-bending the input fibre had no effect on the replay pattern, as illustrated in Figures 2a and 2b. The SLM has a measured switching angle of 28° so that the zero order spot was considerably brighter than the diffracted spots - sufficient to saturate part of the CCD array and cause charge leakage. Reduction of the input power showed this central spot to have exactly the same beam diameter as the diffracted spots. In many systems, the existence of an undiffracted zero order can simply be ignored or may even be used to monitor the input provided the CGH always directs light away from it, since the zero order remains a constant fraction of the input power. The removal of polarisation dependence makes FLC devices much more attractive in communication newtowrks where the input state is uncontrolled. A broadcast one to many single mide fibre fan out coupler using a fixed CGH has been demonstrated as described in an article entitled "Binary phase gratings for hexagonal array generation", by N.C. Roberts, A.G. Kirk and T.J. Hall, published in Optics Communications, 94:501-505, 1992. By replacing the fixed CGH with a FLC device, a one to any dynamic figre switch may be constructed which includes multicast and broadcast capability through appropriate CGH design. Figure 3 shows the basic principle of this switch. The input fibre 1 is placed on the optic axis 2 and illuminates the SLM 3 via a transform lens 4. The input may require an opto-isolator to prevent the reflected zero order destabi¬ lising the source. The output single mode fibres 5 are arranged in the same plane as the input 1 and a CGH is displayed to diffract light to them as desired. The set of CGHs required can be pre-generated using an iterative algorithm and recalled on demand. The absence of polarisers and use of a reflective SLM 3 produces a very compact collinear 2f architecture with good stability since the input/output fibre array can be constructed as a single element. As a demonstartion of switching, a 4f architecture has been constructed using the same transmissive SLM as above followed by a matching 125 mm transform lens. The CCD was replaced with two off axis single mode output fibres and the switch used to inter-connect to either, both or neither of the outputs. The experimental switch exhibits 35dB crosstalk noise rejection and has a measured insertion loss of -13.8dB. The fibre to fibre coupling loss is -3.1dB including all glass surfaces. The CGH diffraction efficiency is -4.ldB taking account of all non-zero diffraction orders and the NOT FURNISHED UPON FILING Δx is the feature and/or pixel size of the spatial light modulator 8. This ensures that all possible connection patterns may be realised. For example if B, were to be connected back to itself, the spot of light from B, formed on the mirror 10 would have to be reflected from the mirror 10 at a height just slightly less than f θ.^. above the optical axis 11. The single transform lens 9 is typically implemented as a high quality multi-element lens system and to meet the 2f θ^ x field size requirement, a telescopic magnification stage or other magnification optics may be required. Alter¬ natively, effective magnification may be built into the system by imaging a smaller field onto the mirror 10 and by curving the mirror surface. Bi-directional light from input source and output receiver B, placed at a height y, above some arbitrary reference line, is shown being collimated by a lens 7 and then passing through the spatial light modulator 8. This region of the spatial light modulator 8 is displaying hologram H 1 which deflects the beam through a small angle (not shown) such that after being refracted by the transform lens 9 and having travelled a distance f, a spot is formed at a height of (y, + y 3 ) / 2 above a reference line 12, where y 3 is the height above the reference line 12 of the current destination for B r The light spot is reflected off the plane mirror 10, travels back through the transform lens 9 and arrives at a different region of the spatial light modulator 8 which is displaying hologram H 3 . The light arrives at H 3 as a collimated beam. H 3 is configured as the inverse of H 1 such that the light is focused onto B 3 by the lens and is travelling in a direction parallel to the optical axis of the system. Accordingly, the output receiver only requires a low numerical aperture. Likewise, light from B 2 is currently destined for B 5 and so is deflected to a height (y 1 + y 5 ) / 2 by hologram H 2 . In a modification to this type of structure (not shown) , the switch comprises two defractive planes of configurable spatial light modulators with a pair of transform lenses spaced apart a distance 2f between the defractive planes. The far field diffraction distribution of the inputs is formed in the plane exactly mid-way between the lenses. In this example, light from one of an array of input sources is directed to a particular one of an array of output sources via two reconfigurable spatial light modulators, one of which is configured as the inverse hologram of the other.

Claims: C L A I M S 1. A polarisation independent optical switch comprising a light input (6) , a diffractive and re-configurable liquid crystal device (8) , a collimating lens (7) for distributing the input light over the liquid crystal device (8) , a transform lens (9) receiving light from the liquid crystal device (8) and a light output (6) , wherein no polarisers are present between the light input and output. 2. An optical switch according to claim l, in which the diffractive and reconfigurable liquid crystal device (8) is a liquid crystal spatial light modulator configured as a computer generated hologram. 3. An optical switch according to claim 1 or 2, in which the diffractive and reconfigurable spatial light modulator (8) is arranged to generate a plurality of holograms (H, to H 5 ). 4. An optical switch according to claim 2 or 3, in which the switch comprises at least one reconfigurable liquid crystal spatial light modulator (8) which is arranged so that light received at a first hologram (H,) is passed to the light output (6) through a second hologram (H 3 ) and wherein the second hologram (H 3 ) is configured as the inverse of the first hologram (H^ . 5. An optical switch according to claim 4, in which the switch has a folded configuration with the first and second holograms arranged in different regions of a single recon¬ figurable liquid crystal spatial light modulator (8) and includes a reflector spaced a distance f from the transform lens (9) , where f is the focal length of the transform lens (9) , so that the light input and output (6) are both on the same side of the reconfigurable liquid crystal spatial light modulator (8) . 6. An optical switch according to claim 4, comprising in two reconfigurable liquid crystal spatial light modulators (8) spaced apart in parallel planes and two transform lenses (9) arranged between the two reconfigurable liquid crystal spatial light modulators (8) and spaced apart a distance 2f, where f is the focal length of the transform lenses. 7. An optical switch according to any of claims l to 3, in which the switch has a folded configuration and includes a reflector (10) so that the light input and output (6) are both on the same side of the liquid crystal device (8) and the collimating lens and transform lens are common (4) . 8. An optical switch according to any preceding claim, in which the light output (6) is formed by an array of output optical fibres. 9. An optical switch according to any preceding claim, in which the light input is formed by a single input optical fibre where the switch has a one-to-many configuration, by an array of input optical fibres where the switch has a many-to-many or many-to-one configuration.

Inventors: MEARS ROBERT JOSEPH, WARR STEPHEN THOMAS, MEARS, ROBERT, JOSEPH, WARR, STEPHEN, THOMAS
Applicants: LYNXVALE LTD, MEARS ROBERT JOSEPH, WARR STEPHEN THOMAS, LYNXVALE LIMITED, MEARS, ROBERT, JOSEPH, WARR, STEPHEN, THOMAS
Application year: 1995
Main classification: G02F1
IPC codes: G02B5, G02F1