Methods of driving an array of optical elements

Details Methods of driving an array of optical elements Methods of driving an array of optical elements

2002-01-23

2004-07-13

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S224200, C359S230000

Reexamination Certificate

active

06762873

ABSTRACT:

This application in the U.S. national phase of International Application No. PCT/GB99/04275, filed Dec. 16, 1999, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of driving a array of optical elements. It has particular but not exclusive relevance to the driving of a spatial light modulator.
2. Discussion of Prior Art
The spatial light modulator to be described in relation to a preferred embodiment in this specification is a in the form of a smectic liquid crystal layer disposed between an active semiconductor backplane and a common front electrode. It was developed in response to a requirement for a fast and, if possible, inexpensive, spatial light modulator comprising a relatively large number of pixels with potential application not only as a display device, but also for other forms of optical processing such as correlation and holographic switching. Other aspects of this device are dealt with in our copending International Patent Applications of even filing and priority dates (PCT/GB99/04285, U.S. Ser. No. 09/868,219, priority GB9827952.4; PCT/GB99/04286 and PCT/GB99/04276, U.S. Ser. No. 09/868,230 and U.S. Ser. No. 09/868,220, both priority GB9827965.6; PCT/GB99/04282, U.S. Ser. Nos. 09/446,325 and 10/084,652, priority GB9827900.3; PCT/GB99/04279, U.S. Ser. No. 10/085,140, priority GB9827901.1; PCT/GB99/04274, U.S. Ser. Nos. 09/868,218 and 10/094,958, priority GB9827964.9; and PCT/GB99/04260 and PCT/GB99/04277, U.S. Ser. No. 09/868,242, both priority GB 9827944.1).
During the course of development of this spatial light modulator, a series of problems were encountered and dealt with, and the solutions to these problems (whether in the form of construction, function or method) are not necessarily restricted in application to the embodiment, but will find other uses. Thus not all of the aspects of the present invention are necessarily limited to liquid crystal devices, nor to spatial light modulators. Nevertheless, it is useful to commence with a discussion of the problems encountered in developing the embodiment to be described later.
The liquid crystal phase has been recognised since the last century, and there were a few early attempts to utilise liquid crystal materials in light modulators, none of which gave rise to any significant successful commercial use. However, towards the end of the 1960's and in the 1970's, there was a renewed interest in the use of liquid crystal materials in light modulating, with increasing success as more materials, and purer materials became available, and as technology in general progressed.
Generally speaking, this latter period commenced with the use of nematic and cholesteric liquid crystal materials. Cholesteric liquid crystal materials found use as sensors, principally for measuring temperature or indicating a temperature change, but also for responding to, for example, the presence of impurities. In such cases, the pitch of the cholesteric helix is sensitive to the parameter to be sensed and correspondingly alters the wavelength at which there is selective reflection of one hand of circularly polarised light by the helix.
Attempts were also made to use cholesteric materials in electro-optic modulators, but during this period the main thrust of research in this area involved nematic materials. Initial devices used such effects as the nematic dynamic scattering effect, and increasingly sophisticated devices employing such properties as surface induced alignment, the effect on polarised light, and the co-orientation of elongate dye molecules or other elongate molecules/particles, came into being.
Some such devices used cells in which the nematic phase adopted a twisted structure, either by suitably arranging surface alignments or by incorporating optically active materials in the liquid crystal phase. There is a sense in which such materials resemble cholesteric materials, which are often regarded as a special form of the nematic phase.
Initially, liquid crystal light modulators were in the form of a single cell comprising a layer of liquid crystal material sandwiched between opposed electrode bearing plates, at least one of the plates being transparent. Such cells were slow to operate and tended to have a short life due to degradation of the liquid crystal material. Quite early on it was recognised that the application of an average dc voltage to the liquid crystal cell was not beneficial, and at least in some cases produced degradation by electrolysis of the liquid crystal material itself, and schemes were evolved to render the average dc voltage to zero (dc balance).
It is now appreciated that other effects are also at work when a dc voltage is applied. When driving liquid crystal electro-optic devices for any length of time, a phenomenon known as image sticking may occur. Although the precise cause of this effect is unknown, there are theories that ions are trapped or a space charge is induced within the material in response to an overall dc field, and this results in a residual field even when the external dc field is removed. Whether to avoid electrolytic breakdown, or to avoid image sticking, it is evidently desirable that the time averaged voltage (that is, the average over the time that the voltage is actually being applied from an external source to the liquid crystal) applied to a liquid crystal material is zero.
The thickness of the liquid crystal layer in nematic cells is commonly around 20 to 100 microns, and there is a correspondingly small unit capacitance associated with a nematic liquid crystal cell. Furthermore, the switching time from a wholly “OFF” state to a wholly “ON” state tends to be rather long, commonly around a millisecond. Relaxation back to the “OFF” state can be somewhat longer, unless positively driven, but the “OFF” state is the only stable one.
At the same time, electro-optic nematic devices comprising a plurality of pixels were being devised. Initially, these had the form of a common electrode on one side of a cell and a plurality of individually addressable passive electrodes on the other side of the cell (e.g. as in a seven-segment display), or, for higher numbers of pixels, intersecting passive electrode arrays on either side of the cell, for example row and column electrodes which were scanned. While the latter arrangements provided considerable versatility, there were problems associated with cross-talk between pixels.
The situation was exacerbated when analogue (grey scale) displays were required by analogue modulation of the applied voltage, since the optical response is non-linearly related to applied voltage. Addressing schemes became relatively complicated, particularly if dc balance was also required. Such considerations, in association with the relative slowness of switching of nematic cells, have made is difficult to provide real-time video images having a reasonable resolution.
Subsequently, active back-plane devices were produced. These comprise a layer of liquid crystal material disposed between a back plane and a spaced opposed substrate. The backplane comprises a plurality of active elements, such as transistors, for energising corresponding pixels. Energisation normally involves cooperation with one or more counterelectrodes disposed on the opposed substrate, although it would be possible to provide counterelectrodes in the backplane itself for fields generally parallel to the plane of the liquid crystal layer.
Two common forms of backplane are thin film transistor on silica/glass backplanes, and semiconductor backplanes. The active elements can be arranged to exercise some form of memory function, in which case addressing of the active element can be accelerated compared to the time needed to address and switch the pixel, easing the problem of displaying at video frame rates.
Active backplanes are commonly provided in an arrangement very similar to a dynamic random access memory (DRAM) or a static random access memory (SRAM). At each one of a distributed array

Affiliated with

Also associated with

Rating

Methods of driving an array of optical elements does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Methods of driving an array of optical elements, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods of driving an array of optical elements will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3221063