GB2092770A - Adjustable mirror mounting device - Google Patents

Abstract

An adjustable mounting device for a mirror, in e.g. a Fabry-Perot elaton, comprises four piezo-electric elements 21a-21d arranged on edge on a common base support 23, each element having conductive electrode films on both major surfaces thereof. The elements may all be electrically energised in a common mode to achieve axial movement of a mirror 22 affixed to the ends of the elements remote from the base. Alternatively the elements may be differentially energised in pairs to achieve tilting of the mirror. The support 23 may itself be a mirror. In another embodiment the elements instead of being arranged radially about a central axis are in the form of a square section tube (Fig. 3 not shown). In this case the elements are provided on at least the outer surface or the inner surface of the tube with a resistive electrode coating, electrical contacts being made to the corners of the tube. <IMAGE>

Description

SPECIFICATION Adjustable mounting device This invention relates to adjustable mounting devices, such as may be used for mirrors in electro-optics appiications.

Adjustable mountings for mirrors find application as modulator devices in optical communications systems, in interferometer devices etc.

It is known to use piezoelectric materials in adjustable mirror tilting devices. Such devices have previously used stacks of elements parallel to the mirror to achieve the necessary movement.

These stacks are difficult to construct and may require a large number of piezoelectric elements whilst at the same time the stack adjustment may be limited to one direction of movement.

According to the present invention there is provided an adjustable mounting device comprising a plurality of plates of piezoelectric material arranged on edge on a base support member, each plate having conductive electrode films on both major faces thereof whereby each plate may be subjected to an applied electric field to cause the dimensions of the plate to alter.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates the effect of an applied electric field on a piezoelectric element; Fig. 2 illustrates one form of mounting device according to the invention, and Fig. 3 illustrates an alternative form of mounting device according to the invention.

In Fig. 1 there is illustrated a piezqelectric element in the shape of a rectangular plate 11 having length /, width wand thickness t. The two faces of the plate are each provided with a metallised electrode layer 12, 13, to which leads 1 2a, 1 3a are attached respectively. When an electric field is applied via the leads in such a manner as to cause the thickness t to decrease then the length land width w will simultaneously increase. If t is small compared with /then the increase in /will be many times greater than the change in t.

Conversely, for a small increase in t there will be a proportionately greater reduction in /.

Fig. 2 shows an arrangement of four piezoelectric elements 21 a - 21 d each similar to the element of Fig. 1 and each provided with metallised electrode layers (not shown) on its two faces. The elements are symmetrically disposed radially about an axis z with their major length / parallel to the axis. Two circular partial mirrors 22, 23 are mounted on the ends of the four elements, e.g. to form an optical cavity for a Fabry-Perot etalon. The electrical connections of the four elements depend upon the movement required.

For example, if the mirrors are required to remain parallel whilst the length of the cavity is altered all four elements will be connected in parallel to an adjustable supply. Thus all four elements will increase, or decrease, in length I simultaneously and by the same amount. If a tilting movement is required opposite pairs of elements are connected, only the connections for each pair are such that when one pair is energised, e.g. 21 a, 21 c, one element 21 a increases in length whilst the other element 21 c of the pair decreases in length, or vice versa. Elements of 21 b and 21 d are not energised. If mirror 23 is fixed to a base the mirror 22 will then be tilted about an axis x which is colinear with the end edges of elements 21 b, 21 d.

Conversely if elements 21 b and 21 d are differentially energised the mirror 22 will tilt about axis.

In the alternative arrangement shown in Fig. 3 the four elements 31 a - 31 d are disposed in the form of a square tube. This allows mirrors to be affixed to the ends of the tube with a much greater region of contact than is possible with the arrangement of Fig. 2. The inside of the square tube of piezoelectric material is provided with a conductive metallised coating 32, whilst the outside surfaces of the tube are provided with a uniform resistive metallised coating 33. Electrode stripes 34a - 34d are provided on each longitudinal corner of the tube. The inner conductor is grounded and differential voltages are applied to the adjacent pairs of corner electrode stripes 34a, 34b and 34c, 34d respectively.The action of the resistive coatings 33 is to cause a drop in the locally applied voltage, between corners 34a and 34d and corners 34b and 34d. Element 31 a will contract uniformly whilst element 31 c will expand uniformly.

Because the voltage between the grounded inner conductor and the resistive outer conductor on each of the elements 31 b, 31 d varies iinearly from corner 34a to corner 34d and corner 34b to corner 34c respectively these elements will be subjected to a differential change of shape. Corners 34a and 34b will shorten and corners 34c and 34d will lengthen. Thus the end of the tube will tilt whilst remaining flat so far as the attached mirror is concerned. Alternatively, if the same voltage is applied to all four corners the tube length will increase or decrease accordingly.

In either embodiment if the two mirrors are partial reflectors the device forms a rigid but electrically adjustable Fabry-Perot etalon.

It is apparent that various modifications and/or alterations to the above embodiments are possible. Thus in either case the number of piezoelectric elements may be changed, e.g. to three radially spaced elements in the case of Fig. 2 and to a triangular tube in the case of Fig. 3 if orthogonal axes of movement are not required.

Also in the case of the tubuiar embodiment of Fig.

3 it is possible to reverse the conductor arrangement, with the resistive coating on the inside and the grounded conductor on the outside.

In yet another modification to the tubular device if two independent sets of inputs are required, e.g.

one for manual adjustment and one for automatic adjustment, then both the inside surfaces and the outside surfaces may be covered with resistive coatings, contact being made to the internal corners for one input set and to the external corners for the other. In the case of the embodiment of Fig. 2 it is useful, espeically for tilting applications, to mount at least one mirror on the ends of the piezoelectric elements by compliant means such as an O-ring. This acts as a grimbal. The radius of the '0' ring, or equivalent mirror contact points, together with the extension sensitivity of the piezo devices, determines the angular sensitivity.

Claims (14)

1. An adjustable mounting device comprising a plurality of plates of piezoelectric material arranged on edge on a base support member, each plate having conductive electrode films on both major faces thereof whereby each plate may be subjected to an applied electric field to cause the dimension of the plate to alter.

2. A device according to claim 1 including a mirror affixed to the edges of the plates remote from the base support member.

3. A device according to claim 1 or 2 wherein the plates of piezoelectric material are symmetrically disposed radially about an axis normal to the plane of the support member.

4. A device according to claim 3 in which the rectangular plates have their major length parallel to the axis.

5. A device according to claim 3 or 4 comprising four plates of piezoelectric material disposed about the axis.

6. A device according to claim 3 or4 comprising three plates of piezoelectric material disposed about the axis.

7. A device according to claim 5 or 6 wherein all the plates are electrically connected in parallel to an adjustable electrical supply whereby all the plates will be subject to the same simultaneous change in dimensions.

8. A device according to claim 5 or 6 wherein the plates are electrically connected to adjustable sources of supply whereby simultaneous differential changes in dimensions may be effected in some or all of the plates.

9. A device according to claim 8 having four plates in which the plates are connected such that differential changes in dimensions may be effected separately in each diametrically disposed pair of plates.

10. A device according to claim 1 or 2 wherein the plates of piezoelectric material are disposed in the form of a tube.

11. A device according to claim 10 wherein the plates adjoin one another along the corners of the tube, either the inside or the outside surfaces of the tube being provided with a uniformly resistive electrode film terminating at each corner of the tube in a longitudinal electrode stripe, the outside or inside surface of the tube having a conductive film overall.

12. A device according to claim 11 in which the electrode stripes are connected in parallel to an adjustable electrical supply whereby all the plates will be subject to the same simultaneous change in dimensions.

13. A device according to claim 11 in which the electrode stripes are parallel connected in adjacent pairs to separate adjustable sources of electrical supply whereby the pairs of plates will be subject to simultaneous differential changes in dimensions.

14. A device according to any preceding claim wherein the base support member is a mirror.

1 5. A device according to claim 14 wherein the base support member is a partially reflecting mirror.

1 6. An adjustable mounting device substantially as described with reference to Fig. 2 or Fig. 3 of the accompanying drawings.

1 7. An interferometer incorporating a device as claimed in any preceding claim.