GB2135471A

GB2135471A - Electro-optic light shutters

Info

Publication number: GB2135471A
Application number: GB08211923A
Authority: GB
Inventors: Brian Asplin; Christopher John Kirkby
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1982-04-24
Filing date: 1982-04-24
Publication date: 1984-08-30

Abstract

An electro-optic light shutter comprising PLZT material connected to a drive circuit which includes means for reducing oscillations (ringing) so that no light is transmitted after the drive circuit has been energised. The drive circuit includes a thyristor and the means for reducing oscillation comprises a transformer primary connected in series with the PLZT material. The transformer secondary winding is short circuited. The shutter may be used in PLZT flash protection goggles. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electro-optic light shutters The present invention relates to Electro-Optic Light Shutters and more particularly to light shutters using switching of a PLZT ceramic to eliminate the passage of light.

PLZT ceramic materials from the "slim-loop" region of the phase diagram exhibit a strong quadratic electro-optic birefringence and, by virtue of their pseudo-cubic symmetry at zero field, offer good extinction in the closed state when operated between crossed polarizers. It has been shown that a substantial fraction of the total electro-optic effect is derived from strain-optic interactions driven by the electrostrictive strain induced in these materials by electric fields. Since this strain comprises a bulk mechanical distortion of the modulator material the response of this component of the electro-optic effect to a stepfunction change in electric field is relatively slow.

More importantly, in certain practical device configurations mechanical considerations result in under damped relaxation of the strain with a consequent oscillation of light transmission through the modulator.

This oscillation, or "ringing" may continue for many tens of microseconds after switching from the open to the closed state with an initial ringing amplitude of as much as 20% of the open state transmission.

It is an object of the present invention to substantially reduce this ringing effect.

According to the present invention there is provided an Electro-Optic Light Shutter including a sheet of PLZT material connected to an electrical drive circuit and including means for reducing the oscillation or ringing effect produced when the PLZT material changes state such that substantially no light is transmitted by the shutter after the electrical drive circuit has been energised.

In a preferred embodiment the electrical drive circuit comprises a thyristor and the means for reducing the oscillation comprises a primary winding of a transformer connected in series with the PLZT material, the secondary winding of the transformer being terminated with a short circuit or low resistance load.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawing which shows an electro-optic shutter drive circuit according to the present invention.

The present invention provides a technique for driving a PLZT electro-optic modulator in such a way that the light-transmission ringing effect is eliminated, or substantially reduced, without significantly slowing the overall device switching response.

With reference to the figure a transformer is included in the modulator drive circuit, with its primary winding in series with the modulator itself; the secondary winding is terminated with a short circuit or a low resistance load. In this configuration the modulator (electrically equivalent to a capacitor of several nano Farads) and the transformer primary form a resonant circuit. A typical circuit implementation is shown in Figure 1 in which a trigger circuit T.C. is used to control the conduction of a thyristor T.H. and in which the Thyristor T.H. is connected via a resistor R to a voltage rail H.T. and to the PLZT device.

On switching, an oscillating voltage appears across the modulator. By suitable choice of transformer inductance and turns ratio it is arranged that the oscillation frequency corresponds to the mechanical oscillation frequency associated with the under-clamped strain relaxation. The stored electrical energy in the capacitative modulator is then smoothly coupled out via the transformer and dissipated in the short circuit load. In this way, reduction of amplitude of oscillation by a factor of 40 or more is achieved.

The immediate application of this technique is in PLZTflash protection goggles. The same basic principle is, however, applicable to stabilisation of the response of other electrostrictive or piezoelectric modulator materials and possibly to classes of device other than light modulators based on these materials, CLAIMS:-- (Filed on 22/4/83) 1. An electro-optic light shutter including a sheet of PLZT material connected to an electrical drive circuit and including means for reducing the oscillation or ringing effect produced when the PLZT material changes state such that substantially no light is transmitted by the shutter after the electrical drive circuit has been energised.

2. An electro-optic light shutter as claimed in claim 1 in which the electrical drive circuit includes a thyristor and in which the means for reducing the oscillation comprises a primary winding of a transformer connected in series with the PLZT material, the secondary winding of the transformer being terminated with a short circuit or low resistance load.

3. An electric-optic light shutter substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to electro-optic light shutters The present invention relates to Electro-Optic Light Shutters and more particularly to light shutters using switching of a PLZT ceramic to eliminate the passage of light. PLZT ceramic materials from the "slim-loop" region of the phase diagram exhibit a strong quadratic electro-optic birefringence and, by virtue of their pseudo-cubic symmetry at zero field, offer good extinction in the closed state when operated between crossed polarizers. It has been shown that a substantial fraction of the total electro-optic effect is derived from strain-optic interactions driven by the electrostrictive strain induced in these materials by electric fields. Since this strain comprises a bulk mechanical distortion of the modulator material the response of this component of the electro-optic effect to a stepfunction change in electric field is relatively slow. More importantly, in certain practical device configurations mechanical considerations result in under damped relaxation of the strain with a consequent oscillation of light transmission through the modulator. This oscillation, or "ringing" may continue for many tens of microseconds after switching from the open to the closed state with an initial ringing amplitude of as much as 20% of the open state transmission. It is an object of the present invention to substantially reduce this ringing effect. According to the present invention there is provided an Electro-Optic Light Shutter including a sheet of PLZT material connected to an electrical drive circuit and including means for reducing the oscillation or ringing effect produced when the PLZT material changes state such that substantially no light is transmitted by the shutter after the electrical drive circuit has been energised. In a preferred embodiment the electrical drive circuit comprises a thyristor and the means for reducing the oscillation comprises a primary winding of a transformer connected in series with the PLZT material, the secondary winding of the transformer being terminated with a short circuit or low resistance load. Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawing which shows an electro-optic shutter drive circuit according to the present invention. The present invention provides a technique for driving a PLZT electro-optic modulator in such a way that the light-transmission ringing effect is eliminated, or substantially reduced, without significantly slowing the overall device switching response. With reference to the figure a transformer is included in the modulator drive circuit, with its primary winding in series with the modulator itself; the secondary winding is terminated with a short circuit or a low resistance load. In this configuration the modulator (electrically equivalent to a capacitor of several nano Farads) and the transformer primary form a resonant circuit. A typical circuit implementation is shown in Figure 1 in which a trigger circuit T.C. is used to control the conduction of a thyristor T.H. and in which the Thyristor T.H. is connected via a resistor R to a voltage rail H.T. and to the PLZT device. On switching, an oscillating voltage appears across the modulator. By suitable choice of transformer inductance and turns ratio it is arranged that the oscillation frequency corresponds to the mechanical oscillation frequency associated with the under-clamped strain relaxation. The stored electrical energy in the capacitative modulator is then smoothly coupled out via the transformer and dissipated in the short circuit load. In this way, reduction of amplitude of oscillation by a factor of 40 or more is achieved. The immediate application of this technique is in PLZTflash protection goggles. The same basic principle is, however, applicable to stabilisation of the response of other electrostrictive or piezoelectric modulator materials and possibly to classes of device other than light modulators based on these materials, CLAIMS:-- (Filed on 22/4/83)

1. An electro-optic light shutter including a sheet of PLZT material connected to an electrical drive circuit and including means for reducing the oscillation or ringing effect produced when the PLZT material changes state such that substantially no light is transmitted by the shutter after the electrical drive circuit has been energised.