Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1997-07-24
1999-07-27
Henry, Jon
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 9, 359 10, 359 25, 385 37, 349201, G02B 532, G02B 637, G03H 108, G02F 113
Patent
active
059300124
DESCRIPTION:
BRIEF SUMMARY
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. Nematic 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 2.theta.. 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 holographically 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 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 2.theta.=90.degree.. 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.degree. 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 r
REFERENCES:
patent: 4703993 (1987-11-01), Hinton et al.
patent: 5159473 (1992-10-01), Feldman
patent: 5182665 (1993-01-01), O'Callaghan et al.
O'Callaghan, M.J., et al., "Diffractive Ferroelectric Liquid-Crystal Shutters for Unpolarized Light," Optics Letters, May 15, 1991, vol. 16, No. 10, pp. 770-772.
O'Brien, D.C., et al., "A Holographically Routed Optical Crossbar: Theory and Simulation," Optical Computing & Processing, Jul.-Sep., 1991, vol. 1, No. 3, pp. 233-243.
Warr, S.T., et al., "Polarisation Insensitive Operation of Ferroelectric Liquid Crystal Devices," Electronics Letters, Apr. 27,1995, vol. 31, No. 9, pp. 714-716.
Mears Robert Joseph
Warr Stephen Thomas
Cambridge University Technical Services Ltd.
Henry Jon
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