GB2227853A - Spatial light modulator - Google Patents

Abstract

A spatial light modulator includes a liquid crystal cell 2 in which the liquid crystal molecular orientation is modulated according to an input signal, the liquid crystal having a front face at which the liquid crystal molecules are orientated in a first direction and a rear face in which the liquid crystal molecules are orientated in a second direction at an angle of twist alpha to the first direction, and means for directing polarized coherent read light at the front face of the liquid crystal cell with a direction of polarization which makes an angle of alpha with the first direction. <IMAGE>

Description

OPTICAL APPARATUS This invention relates to optical apparatus of the type comprising a spatial light modulator, and in a particular aspect to an optical correlator.

There has been considerable interest in recent years in the development of spatial light modulators (SLMs) for use in applications such as correlators, neural nets and displays. The devices have been implemented in a number of different ways, one of the most common being the photoconductor/liquid crystal sandwich. See, for example, A.

Fisher and J. Lee, "The Current Status of Two-dimensional Spatial Light Modulator Technology", SPIE 634, pp 352-371 (1986). The most successful of these devices have used a photoconductor linked to a 45 twisted nematic liquid crystal cell. These can offer high resolution and good grey scale with TV frame rates. Higher frame rates are available with smectic liquid crystals, which have binary output. See D.

Williams et al, "An Amorphous Silicon/Chiral Smectic Spatial Light Modulator", Journal of Physics D21, lOS, pp S156-S159 (1988).

In the case of a SLM operating with an analogue output at TV frame rates, each application has different performance requirements. For example, to use an SLM as an optical input device for real world images for a correlator, the sensitivity and dynamic range of an SLM are important performance parameters. In order to meet different requirements from a particular photoconductor/liquid crystal system the structure of a device or its electrical drive may be changed. See, for example, W. R. Roach, "Resolution of Electro-optic Light Valves", IEE Trans ED21, pp 453-459 (1974), and R. Buzzard and J. Sloan, "Application of a Liquid Crystal Light Valve (LCLV) in a Coherent Optical Correlator.", SPIE 684, pp 101-107 (1986).

In EP-A-O 023 796 there is disclosed a liquid crystal light valve image display system in which the polarization direction of the read beam is orientated to fall within the 45 angle of twist, typically at 22.5 to 25- to the orientation of the crystal molecules at the first incident face of the cell. The magnitude of the electric field across the liquid crystal is then adjusted so that, for example, predetermined intensity levels of the input image modulate the output or read beam to produce either a simultaneous display of colour symbols and achromatic grey scale images or a separate display of either of these.

It has now been found that by changing the polarization direction of the incident read light so that it makes an angle with the orientation of the crystal molecules at the first incident face of the cell, a device with enhanced sensitivity is produced.

Accordingly, the present invention provides optical apparatus comprising a spatial light modulator which includes a liquid crystal cell in which the liquid crystal molecular orientation is modulated according to an input signal, the liquid crystal having a front face at which the liquid crystal molecules are orientated in a first direction and a rear face in which the liquid crystal molecules are orientated in a second direction at an angle of twist a to the first direction, and means for directing polarized coherent read light at the front face of the liquid crystal cell with a direction of polarization which makes an angle of a with the first direction.

The polarizing means suitably comprises a polarizing beam splitter, the angle of the spatial light modulator suitably being adjusted relative to the fixed plane polarization of the incident read light from the beam splitter.

Preferably, the liquid crystal cell is a 45 twisted nematic liquid crystal cell, and the light is polarized at an angle of 450 to the first direction. The first direction is typically the direction in which the front read substrate of the liquid crystal cell is brushed during manufacture to align the molecules. For such a cell, the thickness is preferably at least 2.0cm, more preferably 2.4 to 5.5 Hm.

It has been surprisingly found that, by rotating the input polarization from 0' to 45' to the front read substrate brushing direction for a 45* twisted nematic cell (in reflection) (1) the insertion loss of the spatial light modulator could be reduced by a factor of 2; (2) the 10% and 90% saturation levels increased from 0.01 and 1 mW to 0.03 and 1.2 mW respectively, giving an increase in dynamic range; and (3) the sensitivity increases. By selection of the direction of input optical polarization, it is possible to optimise the spatial light modulator performance for a specific application.

The optical apparatus of the invention is suitably an optical correlator.

Reference is made to the drawings, in which: Figure 1 is a diagrammatic sectional view of a spatial light modulator of the type used in an optical apparatus according to a preferred embodiment of the invention; Figure 2 is a simple equivalent circuit of the spatial light modulator shown in Figure 1; Figure 3 is a diagram illustrating the twist of the molecules in the liquid crystal cell forming part of the spatial light modulator illustrated in Figure 1, in the off-state; Figure 4 is a graph of theoretical and experimental reflected light intensity against applied voltage for a typical 45- twisted nematic liquid crystal cell of the type forming part of the spatial light modulator illustrated in Figure 1, but with the incident read light polarized parallel to the alignment layer;; Figures 5 to 8 are graphs of predicted liquid crystal response at cell thicknesses of 2.0, 2.4, 3.0 and 4.0 Hm, respectively, with polarization parallel to the alignment direction of the cell according to the prior art; Figures 9 to 14 are graphs of predicted liquid crystal response at cell thicknesses of 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 Hm, respectively, with input polarization rotated 45 e from the alignment direction, in accordance with the invention; Figures 15 to 17 are graphs of measured liquid crystal response at cell thicknesses of 1.9, 3.2 and 3.8 ym, respectively, with input polarization rotated 45" from the alignment direction, in accordance with the invention; ; Figure 18 is a graph comparing experimental input/output curves for 0" and 45 rotated input light; Figure 19 is a graph comparing experimental sensitivity curves for 0 and 45' rotated input light; and Figure 20 is a diagrammatic representation of an optical correlator in accordance with one aspect of the invention.

An optically addressed SLM transfers information from one beam of light (called the write light) to a second beam of light that may be of a different coherence, wavelength or intensity (called the read light). A typical device structure is shown in Figure 1. It consists of three main parts, the photoconductor 1, the modulating medium (nematic liquid crystal) 2, and the light blocking layer/mirror 3, which are sandwiched between two transparent conducting electrodes 4 and 5. The light blocking layer acts to separate the read and write beams, reducing the effect of the read beam on the photoconductor. See, M. Powell, C. Powles and J. Bagshaw, "Effect of Read/Write Isolation on the Resolution of the Marconi Spatial Light Modulator", SPIE 936, pp 68-75 (1988).

The role of the photo conductor and liquid crystal can be understood by consideration of a simple equivalent circuit as shown in Figure 2. The photoconductor, light blocking layer and liquid crystal are in series between the two electrodes. The impedance of the light blocking layer and liquid crystal are effectively constant at a particular drive frequency. A change in impedance of the photoconductor changes the voltage split between the photo conductor and light blocking layer and cell. The a.c. voltage across the two electrodes is held constant, so the change in impedance of the photo conductor is reflected in a change of voltage across the liquid crystal.

The response of the nematic liquid crystal cell to an applied voltage is for a re-orientation of the molecules to occur. This can be described in simplified terms as follows. In a twisted nematic cell the cell walls are coated with an alignment layer which forces the liquid crystal molecules to lie parallel to the cell walls with their optical axes along the alignment direction in the "off" state. In the case of the 45 twisted cell, the two alignment directions are at 45* to each other, and the planes of the molecules twist between them, as shown in Figure 3. In this state, light incident with polarization along the front alignment direction is guided by the molecules and emerges with the same polarization after reflection from the dielectric mirror.When a voltage is applied, the molecules in the centre of the cell re-orientate to align (tilt) with the applied electric field, and take up the twist between the cell walls, leaving the molecules near the cell walls along the alignment directions. In this orientation, the molecules in the centre of the cell no longer appear birefringent to the incident light and the guiding of the polarization breaks down. The light now only experiences a birefringent region near each wall of the cell. These regions rotate the polarization of the light so that it no longer emerges with the original polarization. An analyzer is then used to convert the polarization modulation to amplitude modulation. This provides the modulation between the "on" and "off" states of the device.

The 45' twisted nematic liquid crystal cell has been modelled by a number of authors who indicate how the molecules will twist and tilt with applied voltage. See, for example, D. Berremann, "Optics in Smoothly Varying Anisotropic Planar Structures: Application to Liquid Crystal Twist Cells", J. Opt. Soc. America 63.11 pp 1374-1380 (1973), and references therein, and H. Ong, "Original Characteristics of the Optical Properties of General Twisted Nematic Liquid Crystal Displays", J. App. Phys. 64(2), pp614-628 (1988). Figures 5 to 14 were prepared using a model based on the work by Berremann, which predicts the molecular orientations and analyses the interaction of polarized light with the liquid crystal structure. The results of the model were confirmed by comparison with experiment, as shown in Figure 4 for incident polarization parallel to the alignment layer.There is general agreement between the predicted and measured values for the peak and threshold voltages, with a decrease in peak transmitted intensity caused by losses in the optical system.

First the standard polarization direction was modelled for a 45' cell in reflection, with the input polarization parallel to the alignment layer on the input face. A series of curves showing the effect of increasing cell thickness on optical output as a function of voltage are shown in Figures 5 to 8. All the curves show a broad region of low transmission at low voltages, rising to a peak of about 80%.

The characteristic curves obtained from similar cells with an input polarization of 45' to the front surface alignment direction are shown in Figures 9 to 14, for a range of thicknesses. Experimental results obtained from SLMs are shown for comparison in Figures 15 to 17, for thicknesses of 1.9, 3.2 and 3.8 pm, respectively. This series of curves shows several new features not present in the 0 case. The most striking feature is that the curve breaks up into a series of peaks and troughs and that the number of peaks is a function of cell thickness. The second feature of interest is that the output may be very high when no voltage is applied, depending on the cell thickness, but there is a low output null at a voltage below the response peak.The most important aspect is that the gradient of the response is very high, i.e. that a large change in output is obtained for a small change in voltage across the cell.

This effect can be used to adjust the performance of the SLM. In the two orientations, different performance trade-offs come into play. In an SLM based on an amorphous photoconductor such as amorphous silicon, there are two sets of conditions to be met in device design. The first is that all layers are kept as thin as possible to reduce charge spreading effects that reduce the resolution. The second is that the voltage change across the liquid crystal (for a change in light level on the photoconductor) should be enough to fully modulate the liquid crystal (which is necessary to maximise the device contrast and minimise its insertion loss).

These considerations result in a trade-off in SLM performance between very high resolution and contrast. An additional degree of freedom is introduced by selection of the input polarization which allows both high resolution and contrast to be maintained. This may be seen from the following.

An SLM is operated.with an a.c. drive voltage that is defined as the voltage of the null (or output minimum) before the main response peak, when there is no writing light. This is marked on Figure 14 for a typical SLM. The simplest figure of merit to describe the liquid crystal response is then given by AV/Vdrive where AV is the difference between the voltages at the peak and null, and Vdrive is the drive (or null) voltage. The values of EV/VdrjVe for each of the theoretical curves is given in Table 1.

Table 1 Variation of characteristics with cell thickness.

For polarization parallel to L. C. director alignment direction on the input side.

Cell Thick- AV (volts) Vdrive (volts) nV/Vdrive ness (irk) 2.0 0.8 0.6 1.30 2.4 0.7 1.1 0.63 3.0 1.0 1.2 0.83 4.0 0.9 1.5 0.60 Polarized at 45 to L.C. director alignment direction on the input side direction

Cell Thick- AV (volts) Drive (volts) AV/Vdrive ness (pm) 1.5 1.3 0.4 3.25 2.0 0.7 1.3 0.54 2.5 0.6 1.7 0.35 3.0 0.7 1.8 0.39 3.5 0.7 2.0 0.35 4.0 0.3 1.3 0.23 The 45 orientation has much lower values of AV/Vdrive than the 0 orientation for the cells with thickness above 2.4pom. This suggests that for devices with a thin photoconductor layer, (producing an inferior impedance match but improved resolution) rotating the SLM by 45' should produce an improvement in contrast, sensitivity and insertion loss over the 0 orientation.

SLMs were tested to compare each performance parameter at 0 and 45 input polarization.

There are two responses to write light that are important in systems using SLMs. The first, the input/output curve, describes the response of the SLM to a uniform write light. The output light intensity (image plane) is plotted as a function of the input (write) light intensity. This is shown for the two orientations in Figure 18. The devices were not set in such a way as to produce gain. In both cases the output rises from zero to a saturation value. The saturation occurs as the maximum voltage across the liquid crystal, determined by the thicknesses and properties of the layers, is reached. The gradient of the 45' curve is greater than that of the 0' case because of the sharper characteristic of the liquid crystal cell. Hence the 45' orientation is more sensitive to input light level.

The second response to write light is concerned with the transfer of information. This is described as the sensitivity. A diffraction grating (at 10 lp/mm, which is much less than the limiting resolution of approximately 100 lp/mm) is used as the input and the diffracted intensity is measured as a function of the input intensity. This is shown in Figure 19 for a write wavelength of 514 nm and a read wavelength of 633 nm. The diffracted intensity reaches a peak before charge effects in the photoconductor start to reduce the modulation in the image.

The high gradient of the 45' cell characteristic near the null voltage is expected to affect the read light power handling capability. In an SLM a small percentage of the read light passes through the mirror and affects the photoconductor. It can be considered as a uniform intensity or d.c. term over the device added to the write intensity.

For a sharp characteristic, a small d.c. term produces a large change in the output, whereas for the lower gradient 0 case, a similar change in voltage due to the read light does not produce such a large change in the optical output.

A set of performance parameters for each of the two orientations is compared in Table 2. The 45' orientation has a better sensitivity, lower insertion loss (higher saturated output) and greater diffraction efficiency.

Table 2 Comparison of performance at the two orientations.

Parameter 0' 45' Sensitivity 10% response} 0.2 mWcm'2 0.03 mwcm-2 } below peak 90% response} 0.7 mWcm'2 0.5 mWcm-2 90% response above peak 1.4 mWcm'2 1.2 mWcm-2 Output intensity at satura tion (for 633 nm read) 0.9 mWcm'2 3.4 mWcm-2 Maximum diffraction efficiency few percent+ 40%+ Switching speed 5 ms 8 ms * The sensitivity was measured with 514 nm write light and a 10 lp/mm grating on the input.

+ A second device was used to measure this parameter.

The device had a thinner photo conductor and more pronounced difference between 0' and 45'.

The optical correlator shown diagrammatically in Figure 20 uses a spatial light modulator 20, for example of the type described with reference to Figure 1, as the input device, for example receiving on the write side a "real world" image via a camera lens (not shown). The correlator uses degenerate four wave mixing whereby three input waves mix within a non-linear medium (a bismuth silicon oxide crystal) 21 to generate a fourth wave. A HeNe laser 22 provides the three input beams to the crystal 21 via beam splitters 23 and 24 and associated optics, and a polarizing beam splitter 25 which is arranged such that the direction of polarization of light falling on the liquid crystal cell of the spatial light modulator 20 is at 45- to the cell alignment layer at the input face. A reference image 26 is provided in the path of one beam. A TV camera 27 receives the image at the output plane 28.

By setting the spatial light modulator 20 in this fashion, the sensitivity of the correlator is substantially increased in comparison with the conventional arrangement in which the direction of polarization of the read light is parallel to the initial liquid crystal molecular orientation.

Claims (8)

1. Optical apparatus comprising a spatial light modulator which includes a liquid crystal cell in which the liquid crystal molecular orientation is modulated according to an input signal, the liquid crystal having a front face at which the liquid crystal molecules are orientated in a first direction and a rear face in which the liquid crystal molecules are orientated in a second direction at an angle of twist a to the first direction, and means for directing polarized coherent read light at the front face of the liquid crystal cell with a direction of polarization which makes an angle of a with the first direction.

2. Optical apparatus according to Claim 1, wherein the spatial light modulator comprises an amorphous silicon photoconductive layer in electrical contact with the. liquid crystal cell.

3. Optical apparatus according to Claim 1 or 2, wherein the liquid crystal cell is a 45e twisted nematic liquid crystal cell.

4. Optical apparatus according to Claim 3, wherein the liquid crystal cell has a thickness of at least 2.0 ijm.

5. Optical apparatus according to Claim 4, wherein the liquid crystal cell has a thickness of 2.4 to 5.5 pm.

6. Optical apparatus, substantially as described with reference to Figures 1 to 3,and 9 to 17 of the drawings.

7. An optical correlator comprising optical apparatus according to any preceding claim.

8. An optical correlator, substantially as described with reference to Figure 20 of the drawings.