GB2218822A - Electro-optic device - Google Patents

Abstract

An electro-optic device in which an active electro-optic element carries a surface coating of a moisture-impermeable material. In one embodiment, a Kerr cell construction includes a modulator element 1 provided with a vapour deposited polymer coating 18. The coating prevents a build-up of space charge effects which can cause a premature ageing by growing unevenness of the light transmission through the device when in use.

Description

ELECTRO-OPTIC DEVICE This invention relates to an electro-optic device. It relates particularly to a device such as an electro-optic modulator in which the intensity of light transmitted through the modulator can be varied over a wide range by the application of an electric field.

When an electro-optic device with an internal active element is operated in a humid environment, there is a possibility that after a few hours of use, moisture particles will penetrate to the element surface and cause a deterioration in the performance of the device.

If the material of the element includes a body of lead lanthanum zirconate titanate (PLZT) this may be encapsulated in a soft silicone gel supporting medium between crossed polariser filters to form a Kerr cell construction. The supporting medium serves to hold the element without causing uneven mechanical pressures. Under humid conditions, moisture is still likely to be attracted to the element surface and when this surface supports electrodes a space charge is likely to build up in the electrode vicinity. The presence of this space charge acts to distort the applied electric field and this can give rise additionally to distortion of the uniformity of light transmission through the device.

In order to reduce the effects of moisture, it has sometimes been necessary to store the device in a desiccator when not actually required for use. The device is then able to be used for only perhaps four or six hours before this 'ageing' effect occurs and it becomes essential to return the device to the desiccator in order to regain the full operating efficiency after some hours of dry storage.

The present invention was devised in an attempt to prolong the useful life in service of the electro-optic device. If the effect of the onset of 'ageing' could be delayed by only a few hours, this should considerably improve the period of usefulness of the device before it became necessary to return it to the desiccator.

According to the invention, there is provided an electro-optic device in which an active electro-optic element carries a surface coating of a moisture-impermeable material. Preferably the surface coating is of a polymer material. One example of a suitable polymer coating material is a 'Parylene' (Registered Trade Mark) composition.

The invention further comprises a method of constructing an electro-optic device in which an electro-optic element is supported in an encapsulant body, the method comprising the step of coating the element with a moisture-impermeable material before the encapsulation stage. The coating may be formed in a vapour deposition process.

Preferably the coating material is a polymer composition which is polymerised directly on the surface to be coated.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 shows an electro-optic modulator structure including interdi gitated electrodes, Figure 2 is an end view of an electro-optic element with a surface interdigitated electrode structure, Figure 3 is a similar view of an electro-optic element with a buried electrode structure, and, Figure 4 is a view partly cut away of a complete electro-optic modulator.

As shown in Figure 1, the modulator structure comprises an electro-optic modulator element 1 which is located between a polariser filter 2 and an analyser filter 3. The modulator element 1 is a slice of an electro-optic ceramic material. The polarisation directions of the polariser filter 2 and analyser filter 3 are indicated by the arrows 4. The alignment of the arrows 4 show that the polarisation directions are located at right angles to one another to form a crossed polariser arrangement.

The modulator structure is arranged such that a light beam 6 which enters from the left will follow a straight line path and, under suitable conditions, will emerge at the right hand side of the Figure.

The modulator element 1 is formed of a body 7 of electro-optic material in the form of a plate, and one surface of this plate carries an interdigitated electrode 8 structure. Connection wires 9 to the electrodes permit these to be raised to suitable potentials so that an electric field can be established through the material of the body 7.

In operation of this device, the electric field applied to the electro-optic material of the element 1 serves to modulate the birefringence of the material and this affects the polarisation of the light which passes through it. In the absence of the electric field, the amount of light able to emerge from the analyser filter 3 will be zero since the polarisation directions are crossed. Any change in the polarisation direction which occurs as a result of the electric field applied to the element 1 will thus modify the intensity of light emerging from the analyser filter 3.

In an alternative embodiment, the polarisation directions of the polariser and analyser filters 2,3 could be arranged parallel to one another, if this was required.

The particular electro-optic ceramic material of the element 1 is chosen to have a strong quadratic electro-optic effect and one suitable material is a lead lanthanum zirconate titan at (PLZT) composition. The device is termed a Kerr cell.

In the embodiment of Figure 1, the element 1 carries the electrode structure on only one major surface of the plate-like slice.

In an alternative embodiment, electrode structures could be carried on both sides of a plate. Figure 2 shows an end view of such a element 1 which has interdigitated electrode structures on both sides. The electrodes in this instance have been formed by depositing lines of electrically conductive material on the two surfaces of the slice constituting the element 1. An alternative way of creating the electrode structures would be to form grooves by making saw cuts in the slice surface and then fill these grooves with the electrode material. Figure 3 is a similar view of an element 1 with the surfaces carrying grooves and the walls of the grooves supporting layers of electrode material.Since the material of the element is a dielectric, the embodiment of Figure 3 can be preferred where it is required to use a comparatively high electrical potential between the electrode fingers on the element surface.

Figure 4 shows in a partly cutaway drawing a complete encapsulated Kerr cell structure. A modulator element 1 provided with buried electrodes is encapsulated in a body 11 of a supporting medium which is located between two glass plates 12,13. The supporting medium body 11 is a soft silicone gel which serves to hold the element 1 in the zero mechanical stress condition which is required for correct operation of the device.

The first glass plate 12 is provided with an external hard antireflection coating 14. Internally, this glass plate supports a high gamma retarder film 16 which is in contact with the polariser filter 2. The second glass plate 13 supports the analyser filter 3 on its internal surface and on its external surface it also carries a hard antireflection coating 17.

In operation of this assembly, after a period of several hours, and particularly in the presence of a humid ambient atmosphere, charged species in the encapsulating silicone gel will migrate under the influence of the electric field and cause a space charge to build up in the vicinity of the electrodes 8. This space charge distorts the electric field and thus causes the light transmission through the Kerr cell to be non-uniform, giving rise to a distinctive 'barred' appearance with alternate light and dark regions. It becomes necessary, consequently, to limit the time for which the electric field voltage is applied to the modulator, and to keep the device stored under desiccated conditions.

In published attempts to overcome this problem, it has been proposed to provide a hermetically sealed enclosure for the modulator element 1. The methods suggested in order to achieve this result involve technically demanding multi-step processes such as employing a metal foil and glass-to-metal seals.

The present invention proposes the use of a surface coating on the modulator element which has been found to be capable of giving a useful degree of environmental protection whilst still being able to be applied in a single process step. It is recognised that a surface coating of a polymer material may not be capable of providing complete long-term protection against moisture but it may still enable the useful life of the modulator to be extended.

One suitable polymer material for the surface coating is a poly (para-xylylene) polymer which is condensed as a polymeric coating directly on the surface to be treated. This material is commercially available as a 'Parylene' (Registered Trade Mark) Conformal Coating from Nova-Tran Electronics Limited, 62 Bunting Road, Northampton NN2 6EE, Great Britain. Process details for forming thin pin-hole free coatings are also available.

Using the manufacturers recommended method, a surface coating constituted by a two-micron thick layer 18 (Figure 4) of 'Parylene' was deposited on the modulator element 1. In order to measure how effective such a thin coating might be, it had been noted that the uncoated modulator element would show significant space charge effects after only a single damp heat cycle in accordance with the British Standard conditions (BS 2011 pt 2.1dB). A modulator element with the two-micron coating was found not to show any significant space charge deterioration after being subjected to as many as five such damp heat cycles.

Since the refractive index value of the 'Parylene' coating lies between those of PLZT and silicone gel, the coating will also act as an anti-reflection coating. Calculations have shown that the overall modulator transmission can be increased by a factor of 11. Some other conventional anti-reflection coatings have been shown to be ineffective in this role because they can tend to aggravate the space charge effects.

One application of a electro-optic modulator in which the present invention has been used is in the construction of eye protection goggles, such as welding goggles.

Use of the surface coating of the invention on the modulator element has been found to make a significant improvement in the time available for use of the electro-optic device before 'ageing' occurs and the device must be returned to the desiccated storage container. A surface coating of 'Parylene' very much thicker than the two micron layer specifically described did not bring any significant advantage since this can give undesirable optical effects due to the intrinsic birefringence of the coating material.

There are three currently available types of 'Parylene' designated N, C and D and any one of these has been found to be suitable for the purposes of the present invention. The invention could alternatively make use of other suitable polymers that were capable of being deposited from the gaseous state in vacuum systems. The deposition could be effected, for example, by an RF excitation process.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, it is not essential that the electro-optic device to which the element coating is applied should be a Kerr cell. In a different embodiment, an alternative kind of device such as an optical waveguide could be used. Since the use of the element coating process can bring two separate benefits, the reduction in space charge effects and an improvement in the modulator transmission factor, the advantages can still be significant even in the absence of any silicone gel supporting medium.

Claims (8)

1. An electro-optic device, in which an active electro-optic element carries a surface coating of a moisture-impermeable material.

2. A device as claimed in Claim 1, in which the said surface coating is of a polymer material.

3. A device as claimed in Claim 1 or 2, in which the said surface coating is of a 'Parylene' (Registered Trade Mark) composition.

4. An electro-optic device substantially as hereinbefore described, with reference to the accompanying drawing.

5. A method of constructing an electro-optic device in which an electro-optic element is supported in an encapsulant body, comprising the step of coating the element with a moistureimpermeable material before the encapsulation stage.

6. A method as claimed in Claim 5, in which the coating is formed in a vapour deposition process.

7. A method as claimed in Claim 6, in which the coating material is a polymer composition which is polymerised directly on the surface to be coated.

8. A method of constructing an electro-optic device substantially as hereinbefore described.