WO1990004803A1

WO1990004803A1 - Optical switch

Info

Publication number: WO1990004803A1
Application number: PCT/AU1989/000459
Authority: WO
Inventors: Robert Brunswick; Pieter Albert Otten
Original assignee: Australian Optical Fibre Research Pty Limited
Priority date: 1988-10-24
Filing date: 1989-10-23
Publication date: 1990-05-03

Abstract

An optical switch includes a body (14) for locating an end of an optical fibre, and a member (20) providing a reflective face (21) disposed to reflect at least a proportion of light received from the end of the fibre back along the fibre. This member (20) is movable by an externally applied magnetic, electric or electromagnetic field, relative to the fibre location body (14), to in turn relatively move the reflective face (21) between two different positions relative to the end of the fibre. In a method of assembling such a switch, the member (20) is adjusted to vary the displacement of the reflective face (21) from the end (13) of the fibre (12) until a desired higher value of reflected signal is monitored in the fibre. A second member (22) is then placed on the fibre location body (14) with a spring or the like to bias the reflective face (21) to maintain the aforesaid higher value. An external magnetic, electric or electromagnetic field is applied to move the reflective face (21) to a new position with respect to the end (13) of the fibre, and the second member (22) is adjusted relative to the fibre location body (14) to vary this new position until a desired lower value of reflected signal is monitored in the fibre.

Description

OPTICAL SWITCH

Field of the Invention

This invention relates to an optical switch and to a method of assembling such a switch, and more particularly provides a switch which is selectively adaptable for response to a variety of kinds of actuator to generate ON/OFF signals in a fibre optic sensing network.

Background Art

A known class of optical switches relies on movement of a reflective element relative to the mouth of an optical fibre. The element may move laterally of the light path or along the light path to intersect an incident diverging cone of light from the fibre and so vary the proportion of received light which is reflected back along the fibre. Several practical difficulties remain with these devices. They are relatively complex and generally rely on sliding sealing elements such as elastomeric 0-rings. Because the reflection signal, at the ON position in particular, is very sensitive to the precise position of the

SUBSTITUTESHEET reflector and because it is also important to avoid cyclic contact with the end of the fibre even though the reflector may necessarily be very close to the end of the fibre, precision manufacturing is required to tolerances of the order of 10 microns. Because the reflector is usually in contact with the air, the need to avoid condensation on the reflector limits the range of applications for switch devices of the kind in question.

Summary of the Invention

It is an objection of the invention to at least in part alleviate one or more of these practical difficulties. The invention accordingly provides, in a first aspect, a method of assembling an optical switch, comprising:- securing a body to an end of an optical fibre; disposing a reflective face to reflect at least a proportion of light received from said end of the fibre back along the fibre, which face is provided on a first member which is moveable relative to said body by an externally applied magnetic, electric or electromagnetic field to in turn move said reflective face relative to said end of the fibre; adjusting said first member to vary the displacement of the reflective face from said end of the fibre until a desired higher value of reflected signal is monitored in said fibre; placing a second member on said body with means biasing said reflective face to maintain said higher value; applying an external magnetic, electric or electromagnetic field to move said reflective face to a new position with respect to said end of the fibre; and adjusting the second member relative to said body to vary said new position until a desired lower value of reflected signal is monitored in said fibre. The second member is preferably a cover defining a cavity for the first member and the biasing means and the method advantageously further includes sealingly securing the cover along the body after said adjustment of the second member. Moreover, securement of said body to said end of the fibre preferably includes sealingly securing the body about the fibre. These two sealing securements may together provide an assembly in which the end of the fibre, the first member and the biasing means are disposed in a gastight enclosure: the method may further include carrying out said assembly in a substantially moisture-free atmosphere, e.g. of nitrogen, whereby said enclosure contains a moisture-free atmosphere on completion of said assembly. The first member advantageously includes a plunger slideable in a complementary bore in said body, and having an end surface which provides said reflective face. The invention also affords an optical switch when assembled or manufactured by the aforesaid method. In a second aspect, the invention further provides an optical switch comprising: means to locate an end of an optical fibre; and a member providing a reflective face disposed to reflect at least a proportion of light received from said end of the fibre back along the fibre and movable by an externally applied magnetic, electric or electromagnetic field, relative to said fibre location means, to in turn relatively move said reflective face between two different positions relative to said end of the fibre. The switch according to the second aspect of the invention preferably further includes means, e.g. a helical spring, to act on said member to bias said reflective face to one of said positions. A cover may be further provided to support said biasing means and this cover is preferably sealingly secured to a body providing said fibre locating means. The said member advantageously includes a plunger slideable in a complementary bore in said body, and having an end surface which provides said reflective face. The body may be sealingly secured about the fibre. The end of the fibre, said member and the biasing means may be disposed in a gastight enclosure defined by the cover and body: this enclosure preferably contains a substantially moisture-free atmosphere, e.g. of nitrogen. In both aspects of the invention, the moveable member, plunger, cover and optical fibre are conveniently co-axial.

Brief Description of the Drawings

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a magnetically operated optical switch according to the invention; Figure 2 is an axial cross-section of the switch, shown in its normal ON position; Figure 3 is a view similar to Figure 2 but shows the switch in its OFF position; and Figures 4a and 4b schematically show stages in a preferred assembly of the switch of Figures 1 to 3 in accordance with the method of the invention.

Best Mode(s) of Carrying out the Invention

The illustrated optical switch 10 is shown in situ at the end of a length of optical fibre 12 and includes an annular body 14 with an axial bore 15, secured to fibre 12, a two-part moveable member 16 comprising an annular slug 18 and a solid cylindrical plunger 20 slideable in bore 15, and a cover 22. The end of plunger 20 remote from slug 18 is polished to provide a reflective face 21 opposite the open end 13 of fibre 12. Body 14 is counterbored at 23 to receive a standard ferrule connector 24 fitted to the segment of fibre 12 so that the fibre extends through a capillary bore in the connector. The end 13 of the fibre is flush with the inner end of ferrule connector 24 and is displaced from the inner end of counterbore 23. Fibre 12 is typically shielded by a reinforced sheath 12a including a threaded socket 26 which threadingly engages a spigot portion 14a of body 14. Cover 22 has a disc base 28 and a relatively thin annular skirt 30 which is an interference fit at 25 about the outer peripheral surface of body 14 and thereby defines with the end of body 14 a cavity 32. Cavity 32 houses slug 18 and a helical compression spring 34. Spring 34 is disposed between the underside of cover base 28 and an annular shoulder seat 36 on the slug to bias the slug against the inner end face of body 14. Plunger 20 has a forward portion 23a of slightly reduced diameter (e.g. about 50 micron) relative to rear portion 23b, which is an interference fit in an axial bore 19 in the slug. Forward reduced diameter portion 23a is a close sliding fit in bore 15 of body 14. In the spring biased position of slug 18 in abutment with body 14, reflective face 21 is very close to but not touching end 13 of fibre 12 - the ON position in Figure 2. The two components of moveable member 16 are magnetically susceptible by being formed in stainless steel. If a magnet 8 of sufficient power is brought up to the outer face of cover base 28, it will attract slug 18 and plunger 20 and thereby move them against spring 34 until the outer end of slug 18 strikes the base of the cover. Reflective face 21 has thereby moved to a second, more remote position in front of the end of fibre 12 - the OFF position of Figure 4. It will be immediately appreciated that the switch is substantially bistable: once slug 18 begins to move under the influence of the magnet, it will certainly complete its travel against the spring, of substantially constant spring constant, to the OFF position. Magnet 8 is conveniently a small rare-earth permanent magnet, or perhaps an electromagnet. In the simple case of linear motion sensing, the magnet might be carried by a moving component whose motion is to be sensed. It might alternatively move past the switch assembly rather than to and from. A magnet to operate switch 10 may be added to known kinds of switch actuators, e.g. conventional industrial microswitches, temperature sensors, pressure sensors, level switches, magnetic proximity switches and pressure gauges.

SUBSTITUTESHEET The method of assembling switch 10 in accordance with the preferred practice of the invention will now be reviewed. The assembly is carried out in a moisture-free nitrogen atmosphere. A fibre 12 is prepared and first connector 24, then body 14 are applied to the fibre: the body is sealingly secured about connector 24, e.g. by epoxy cement. Plunger 20 is polished at its front end to form reflective face 21. The plunger is then forced from the rear by a suitable tool 7 right through bore 19 of slug 18 and its projecting end slid into bore 15. At this initial stage, the plunger is set too far from the fibre end 13 for the ON position. The forward portion 23a of the plunger provides a sliding bearing surface in bore 15 and the advantage of forming this portion with a reduced diameter is that it is thereby substantially not scratched or otherwise damaged as it is polished through slug 18. Tool 7 is then employed to push plunger 20 (Figure 4A) further in bore 19 and towards the end 13 of the fibre until a predetermined maximum reflection signal is monitored further along fibre 12 at a detector 6. This defines the ON position. Spring 34 and cover 22 are then brought into place, in the latter case by forcing the cover onto body 14. A magnet 8a is brought up to the outside of the cover to drive slug 18 against the underside of cover base 28. While maintaining the magnet, the cover is forced by a tool 7a into axial movement on the body (Figure 4B) until a predetermined minimum reflection signal is monitored on fibre 12 at detector 6. This is the OFF position. The interface between body and cover is then sealingly secured (e.g. by epoxy cement applied when the two were first brought together) to form, with the sealed securement of ferrule connector 24 in body counterbore 23, a gas-tight nitrogen-filled moisture-free enclosure housing the spring, plunger and reflector: there is no moisture present to cause condensation on the reflector in subsequent use. it will be appreciated that the techniques of assembly obviate the need for precise tolerances. The optimum inner position of the reflective face very close to (say 10 microns) but out of contact with fibre end 13 is set in a permanent calibration, as is the optimum outer position. The actual length of the plunger and other parts are not critical. Normal "high volume" engineering tolerances, an order of magnitude greater than those for adequate optical tolerance, are acceptable. The expensive steps of individual grinding and lapping are not required. Body 14 and cover 22 may conveniently be brass, or preferably any other material with a similar thermal expansion coefficient to the material of the slug and plunger. The illustrated switch has several further advantages, in addition to those already indicated. There is no need to have a precise drive linkage, as all the precision is in the sealed switch mechanism. There is also no requirement for over travel protection, as the magnet can move past the switch. The switch is of small physical size and non- intrusive. The switch element is universally applicable to many types of switch mechanism e.g. industrial microswitch, temperature switch, pressure switch, level switch, magnetic proximity switch etc.

SUBSTITUTE SHEET

Claims

1. An optical switch comprising: means to locate an end of an optical fibre; and a member providing a reflective face disposed to reflect at least a proportion of light received from said end of the fibre back along the fibre and movable by an externally applied magnetic, electric or electromagnetic field, relative to said fibre location means, to in turn relatively move said reflective face between two different positions relative to said end of the fibre.

2. An optical switch according to any preceding claim wherein said member is at least partially formed in a magnetically susceptible material.

3. An optical switch according to claim 1 or 2 further comprising means to act on said member to bias said reflective face to one of said positions.

4. An optical switch according to claim 3 further comprising a cover to support said biasing means and to form a cavity for said biasing means and for at least a portion of said member.

5. An optical switch according to claim 4 further comprising a body providing said fibre locating means and wherein said cover is sealingly secured to said body.

6. An optical switch according to claim 5 wherein said member includes a plunger which is slideable in a complementary bore in said body and has an end surface which provides said reflective face.

7. An optical switch according to claim 5 or 6 wherein said body is sealingly secured about the fibre, and wherein said end of the fibre, said member and said biasing means are disposed in a gastight enclosure defined by said cover and said body.

8. An optical switch according to claim 7 wherein said enclosure contains a substantially moisture- free atmosphere.

9. A method of assembling an optical switch, comprising:- securing a body to an end of an optical fibre; disposing a reflective face to reflect at least a proportion of light received from said end of the fibre back along the fibre, which face is provided on a first member which is moveable relative to said body by an externally applied magnetic, electric or electromagnetic field to in turn move said reflective face relative to said end of the fibre; adjusting said first member to vary the displacement of the reflective face from said end of the fibre until a desired higher value of reflected signal is monitored in said fibre; placing a second member on said body with means biasing said reflective face to maintain said higher value; applying an external magnetic, electric or electromagnetic field to move said reflective face to a new position with respect to said end of the fibre; and adjusting the second member relative to said body to vary said new position until a desired lower value of reflected signal is monitored in said fibre.

10. A method according to claim 9 wherein said second member is a cover defining a cavity for at least a portion of said first member and for the biasing means and the method further includes sealingly securing the cover to the body after said adjustment of the second member.

11. A method according to claim 9 wherein said securing of said body to said end of the fibre includes sealingly securing the body about the fibre.

SUBSTITUTESHEET

12. A method according to claim 11 wherein said two sealing securements together provide an assembly in which the end of the fibre, the first member and the biasing means are disposed in a gastight enclosure, the method further including carrying out said assembly in a substantially moisture-free atmosphere, whereby said enclosure contains a moisture-free atmosphere on completion of said assembly.

13. A method according to any one of claims 9 to 12 wherein the first member utilised includes a plunger which is slideable in a complementary bore in said body and has an end surface which provides said reflective face.

14. An optical switch when assembled or manufactured by a method according to any one of claims 9 to 13.

SUBSTI