GB2026188A - Optical switches - Google Patents

Abstract

A switch for multi-mode optical fibres comprises a ferro-electric element through which the light passes. Electrodes are formed on the element such that an electric field can be applied perpendicular to the light path in the element to thereby scatter the light and disconnect the input from the output for the "OFF" condition. The element may be provided with focussing means, as by forming it from a quadrant of material 1 wherein focussing reflections occur at the curved surface 21, or guiding means for the light, as by forming it as a rectangular sectioned cylindrical waveguide to increase the transmission of the switch in the "ON" condition. The ferro-electric material is preferably PLZT 9/65/35, with a grain size greater than about 3 mu m. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical switches The invention relates to optical switches, and in particular to switches for use with multimode optical fibres.

A requirement for modern data highway systems is fast switching of multimode optical fibres. Fast single mode switches are available which typically have extinction ratios of about 25dB between the 'on' and 'off' conditions. There are however no satisfactory means to switch high numerical aperture multimode fibres, the choice being almost entirely limited to relatively slow mechanical switches. The object of the present invention is to provide an electrical switch for multi-mode optical fibres.

The present invention consists of a muiti-mode optical switch comprising a ferro-electric element providing a transparent light path connection between a light input and a light output, means to guide light from the input to the output and means to apply an electric field in a direction perpendicular to the light path so that on application of the field domains in the material align with the electric field direction thereby scattering light away from the light path to effectively disconnect the light output from the input.

Preferably the ferro-electric element is provided with two opposed parallel faces and the electric field means comprises electrodes which are attached to the respective parallel faces. In one form the electrodes may be applied such that the electric field can be switched from a direction parallel to the light path to a direction perpendicular to the light path. By this means scattering of light is reduced to a minimum when the light is in the 'on' condition.

The ferro-electric element may be a ceramic of the perovskites class, advantageously the ceramic is PLZT with a composition denoted by PLZT 9/65/35. In addition, it is preferable to manufacture the PLZT such that the material has a slim hysteresis loop and a grain size greater than about 3Fum, the switching action being determined by bulk scattering of light.

In one advantageous arrangement the ferroelectric element is in the form of a sectored disc, the light input being connected to one radius edge and the light output being connected to the second radius edge in such positions that, in the absence of an electric field, light from the input is reflected from the curved edge of the disc to the output.

Preferably the light input and output comprise optical fibres and the thickness of the ferro electric element is chosen to be substantially the same as the core diameter of the optical fibres. It has been found that optimum focussing for a sectored disc occurs for a specific sector angle and specific position of the fibres which are butt jointed (not necessarily perpendicularly) to the respective radius edges. The sector angle is found to vary from 121" to 127 as the thickness of the optical fibre is varied from 80 lito 400cm. In order to provide support, the PLZT material can be supported between two similarly shaped glass plates or attached to a supporting substrate.Reflection of the input light by the curved surface may be either by total internal reflection or the surface may be mirrored. Preferably the light path length in the PLZT material is at least 10 mm in order to obtain a good extinction ratio.

In order to reduce the capacitance and hence increase the switching speed of the device the electrode areas can be reduced so as to cover only the envelope of light paths within the ferro-electrical material.

Preferably the electrodes are formed by deposition of aluminium on the PLZT material and the glass supporting plates are attached to the respective electroded surfaces by means of a conducting adhesive, the plates being provided with a respective hole there-through so that electrical connections to the electrodes can be made through said holes.

Embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings of which: Figures 1 and 2 iliustrate schematically the princi ple of light scattering: Figure 3 shows one example of a light switch according to the invention; Figure 4 shows a second exampie of the light switch.

A known light switch device applying the principle of light scattering is illustrated in Figures 1 and 2. An optically transparent sample 1 of a ferro-electric ceramic of about 0.2 mm thickness has solid aluminium electrodes 2, 3, 4 and 5 deposited on the opposed major surfaces 6 and 7 so as to form a co-linear 150 m wide transparent window 8 extending perpendicular to the plane of the drawings.

Arranged co-linearly with the window 8 is an aperture 9 in a field stop 10. When an electric field of 20 - 25 kV/cm is applied as in Figure 1 in a direction parallel to the optical axis 11 through the window 8 and aperture 9, the domains in the ferro-electric ceramic sample 1 align themselves with the optical axis. Light from an unpolarised He-Ne laser when directed along the optical axis 11 is transmitted through the ceramic sample 1 with substantially no scattering, the switch thus being in the 'on' condition. On switching the direction of the electric field such that the direction of alignment of the domains rotates through 90 as shown in Figure 2, light is scattered away from the optical axis 11 of the system and is not transmitted through the aperture 9, the switch now being in the 'off' condition.

In the realisation of this device according to the invention a PLZTsample 1 comprising Pb, La, Zr and Ti having a substitution of 9 atom % La for the Pb and a Zr/Ti ratio of 65/35 - denoted PLZT 9/65/35 was polished on both major faces using standard lapping and final stage mechanical/chemical polishing techniques. The chemical composition and method of fabrication of the ceramic were chosen so that the ferro-electric material had a slim hysteresis loop and a typical grain size of 8Fm. The optimum grain size was determined by the requirement of bulk scattering, necessitating a grain size greater than about 3pm.

When a multi-mode fibre having a core diameter of 80Fm was butt-jointed to the output face of the PLZT (between the electrodes 3 and 5) an on-off extinction ratio of 4dB was obtained. By experimenting with the thickness of the ceramic sample 1 and testing the effect of polarisation of the incident laser light, it was found that the extinction ratio could be improved by increasing the active scattering length and that the scattering was sensitive to polarisation, although high extinction ratios of the order of 20dB were possible for the worst case polarisation. It was found necessary to increase the scattering length, to about 10 mm, in order to obtain a good extinction ratio. Such known devices however have been found to be impractical for use with multimode optical fibres since a low insertion loss cannot be obtained.

Some form of focussing or guiding is desirable if low loss is to be achieved A practical example of the light switch according to the invention is shown in Figure 3. The PLZT ceramic 1 is provided in the form of a sector having an angle 12 of 121" and a uniform radius of 12 mm, and having electrode material deposited over its major faces. The ceramic 1 is sandwiched between two similarly shaped supporting glass plates 13 and 14 through each of which a hole 15 is provided so that electrical connections can be made therethrough between electrical leads 16 and the surface electrodes. An input optical fibre 17 having a diameter approximately equal to or siightly smaller than the thickness of the ceramic 1 is connected in perpendicular abutting relationship to the mid-point of one radius edge 18 of the sectored ceramic 1.In a similar manner an output optical fibre 19 is connected to the mid-point of the second radius edge 20 of the sectored ceramic 1. Light from the input optical fibre 17 entering the edge 18 of the sectored ceramic 1 has a uniform angular spread about an axis colinear with the fibre 17. Hence, in the plane of the sectored ceramic a diverging light source is incident on the curved surface 21 of the sectored ceramic. The refractive index of PLZT, viz 2.5, ensures that the condition for total internal reflection at the curved surface 21 is satisfied for a satisfactorily large bundle of diverging light rays. After reflection, the light is focussed to a small area which includes the mid-edge loation at which the output optical fibre 19 is positioned.

The light switch thus described is in the 'on' condition when, with no electric field applied, light reflected from the curved surface 21 of the sectored ceramic is collected by the output optical fibre 19. In the 'off' condition, when the electric field is applied between the electrodes on the major faces of the sectored ceramic, the domains are aligned perpendiocular to the light path through the ceramic and therefore cause the light to be defocussed and scattered away from the output optical fibre.

The electric field in the ceramic is provided by aluminium electrodes which are evaporated onto both major faces of the PLZT sectored ceramic 1.The electrodes are evaporated so as to leave a margin of about 1 mm around the edge of the device in order to prevent arcing. The glass support plates 13 and 14 are bonded to the ceramic 1 by means of silver loaded epoxy glue which is highly conducting, thereby enabling electrical contact to be made between the external leads 16 and the aluminium electrodes.

The sector angle 12 is chosen such that the light internally reflected from the curved surfae 21 of the sectored ceramic 1 is optimally focussed on to the output optical fibre 19. Providing that the angular spread of the light emerging from the input optical fibre 17 is less than about 0.4 radians in the sectored ceramic lit can be shown that there is little abberration of the focus of the light after reflection by the circularly curved surface 21. In the chosen example, the input and output optical fibres 17 and 19 have a core diameter of 80Zm and the thickness of the sectored ceramic 1 is made equal to the thickness of the optical fibres in order to minimise loss of light in a direction perpendicular to the plane of the ceramic 1.It has been found that optimum focussing for a sectored disc occurs for a specific sector angle and specific positions of the optical fibres, the angle being dependent on the core diameter (hence sector thickness) of the optical fibres.

The procedure for fabricating the device will now be described. A sawn PLZT disc of the chosen diameter (25 mm) and 1/2 mm thick is first polished on one major face using standard carborundum and chemical/mechanical polishing techniques. The disc is then cleaned and an aluminium sector (1 21 )elec- trode is evaporated onto the polished surface. A glass support disc ( 2 mm thick, 25 mm diameter) is stuck onto the electrode PLZT face using silver loaded conducting epoxy. At the edges the conducting epoxy is 'diluted' with normal epoxy to prevent electrical breakdown. The hole 15 in the glass support disc is positioned near the central area of the electrode and is of such size that the conducting cement can squeeze through for an external contact with a lead 16.The second major face is then polished until the material is of the required thickness. After cleaning, the second sectored electrode is deposited so that it is coincident with the first electrode and a second glass support disc is glued there-over, ensuring that the electrical contact holes 15 are not exactly coincident. The circular edge of the sandwich is then polished to an optical finish.

The 121" sector is cut out of the sandwich using the sectored electrodes for positioning the saw-cuts.

Finally, the two radii faces are polished to an optical finish and the input and output optical fibres butt jointed to respective faces using an optical coupling cement.

Using this type of PLZT switch an extinction ration of up to 44 dB has been obtained between the 'on' and 'off' conditions of the switch. In an alternative form the curved surface 21 of the sectored ceramic 1 could be metalized to form a conventional concave mirror.

A second example of the light switch is shown in Figure 4. A length of PLZT material 22 of rectangular cross section is cut and has all its surfaces polished using conventional and ion etching techniques.

Electrodes (not shown) are deposited on two opposed major surfaces. The PLZT length 22 is attached by epoxy glue to a substrate support 23 and the input and output optical fibres 24 and 25 are both jointed to the respective minor faces of the PLZT material 22. In this example, no focussing is required of the light in the PLZT material 22 as the light will be guided along the length of the material by successive total internal reflections. The numerical aperture of the PLZT unit can be reduced to match that of the fibre by cladding the PLZTwith a suitable dielectric material. On applying an electric field perpendicular to the length of the PLZT material 22 bulk scattering of the light in the material results in light being transmitted out through the polished non-electroded major faces of the material.This example of the invention will have a lower material loss, lower field matching loss and lower capacitance than the first example described. It has been found that the waveguiding property of the PLZT cylinder can be improved by diffusing titaium into the surface layer.

Other examples of light switches falling within the scope of the invention will be apparent to those skilled in the art, using, for example, different transparentferro-electric ceramic materials. There are many ferro-electric ceramics of the ABO3 perovskites class, typified by BaTiO3 and Pb (Zr, Ti) O3 - PZT by its trade name. the optically transparent PLZT used in the examples of the invention is one such ceramic. The curvature of the reflecting surface 21 (Figure 3) could also be made non-circular so as to improve the focussing of the light onto the output optical fibre 19. In addition to incorporating means to focus or guide the light it is possible to include electrodes in such form that the electric field can be switched from a direction parallel to the light path to a direction perpendicular to the light path.

Claims (13)

1. A multi-mode optical switch comprising a ferro-electric element providing a transparent light path connection between a light input and a light output, means to guide light from the input to the output and means to apply an electric field in a direction perpendicular to the light path so that on application of the field domains in the material align with the electric field direction thereby scattering light away from the light path to effectively disconneck the light output from the input.

2. A multi-mode optical switch as claimed in claim 1 wherein the ferro-electric element is provided with two opposed parallel faces and the electric field means comprises electrodes which are attached to the respective parallel faces.

3. A multi-mode optical switch as claimed in claim 1 or 2 wherein the electrodes are applied such that the electrical field can be switched from a direction parallel to the light path to a direction perpendiculartothe light path.

4. A multimode optical switch according to any one preceding claim wherein the light input and output comprise optical fibres and the thickness of the ferro electric element is chosen to be substantially the same as the core diameter of the optical fibres.

5. A multimode optical switch according to any one preceding claim wherein the ferro electric material has one plane surface which is supported on a substrate.

6. A multimode optical switch according to any one of claims 1 to 4 wherein the ferro electric material is provided with two opposed parallel plane surfaces and a support glass plate is attached to each opposed surface.

7. A multimode optical switch according to any one preceding claim wherein the ferro electric element is a ceramic of the perovskites class.

8. A multimode optical switch according to claim 7 wherein the ceramic is PLZTwith a composition denoted by PLZT 9165135.

9. A multimode optical switch according to claim 8 wherein the PLZT material is manufactured such that the material has a slim hysteresis loop and a grain size greater than about 3y m.

10. A multimode optical switch according to any one preceding claim wherein the ferro electric element is in the form of a sectored disc, the light input being connected to one radius edge and the light output being connected to the second radius edge in such positions that, in the absence of an electric field, light from the input is reflected from the curved edge of the disc to the output.

11. A multimode optical switch as claimed in claim 10 wherein the curved edge of the sectored disc is mirrored.

12. A multimode optical switch substantially as described with reference to Figure 3 of the accompanying Drawings.

13. A multimode optical switch substantially as described with reference to Figure 4 of the accompanying Drawings.