GB2374158A

GB2374158A - Optical switching with feedback and controllably adjustable mirror

Info

Publication number: GB2374158A
Application number: GB0108812A
Authority: GB
Inventors: Robert Charles Theobald
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-04-07
Filing date: 2001-04-07
Publication date: 2002-10-09
Also published as: GB0108812D0

Abstract

An optical switch, for switching an optical signal (12) from one optical fibre (10a) to another optical fibre (10b) in a bundle (10) comprises a (preferably) curved two axis positionally adjustable mirror (28) which reflects the optical signal (12). A secondary light source (22) reflects secondary light from the mirror onto a four quadrant photo detector (26) which provides input to a position control servomechanism (36) which, in turn, drives (preferably) magnetic actuators to correct the position of the mirror (28) .The mirror (28) may be suspended on three flexible wire struts (30).

Description

OPTICAL SWITCHING This invention relates to optical communication systems and, in particular, to apparatus for switching optical signals within fibre optic networks and the like.

Many different optical fibre systems and networks are known. In such systems and networks it is necessary to provide switching means for switching an optical signal from an input optical fibre to one of a plurality of output optical fibres. Several mechanical switches have been developed for this purpose, which are relatively low in cost, provide low insertion loss, and which are compatible with the bandwidth of optical fibres. However, with switching speeds of around 5 to 50ms, such switches are unsuitable for use in high speed optical fibre networks, such as asynchronous transfer mode (ATM) packet switching systems, which require optical switching speeds of 10 s or less. Such switching speeds can be achieved using some types of electrical or solid-state switches.

However, not only are such switches relatively expensive, but they can also experience losses that can affect network function.

International patent application WO-A-95/33219 describes an optical switch for switching an optical signal from an input optical fibre to one of two output optical fibres located substantially perpendicular to the input optical fibre. The optical signal is collimated and then reflected by a flat mirror in one of two positions before being decollimated prior to output.

I have now devised an improved arrangement. In accordance with the present invention, there is provided optical switching apparatus for directing an optical signal from one optical fibre to one of a plurality of optical fibres in a bundle, the apparatus comprising at least one reflective member including at least one adjustable reflective member, means for adjusting the position of said reflective member, a secondary optical source arranged for reflection by said adjustable reflective member and an optical detector arranged to receive the beam from said secondary optical

source reflected by a reflective member and to detect the position of said at least one adjustable reflective member accordingly, and control means arranged to adjust the position of said adjustable reflective member if necessary.

In a preferred embodiment, only one reflective member (which is positionally adjustable) is used, so that the same reflective surface is used for feedback control of the reflective member alignment as is used for coupling the fibres, thereby allowing optical misalignment to be effectively and accurately corrected.

In a preferred embodiment, the control means is arranged to compare the actual and desired position of said reflective member and to adjust the position of the reflective member if the actual and desired positions differ at all or by a predetermined amount.

The reflective member may comprise a concave spherical or ellipsoidal mirror, which is preferably rotatable about one, and more beneficially two, axes. Alternatively, the reflective member may comprise a plane mirror which is tiltable about at least one axis, a refractive member, such as a lens, preferably being provided between the input optical signal and the mirror.

The ends of the bundle of optical fibres are preferably polished to provide a surface which is preferably curved, but may be flat. A refractive member may be provided between the optical fibres and the reflective member, even if the reflective member is a concave spherical or ellipsoidal mirror, which is especially beneficial if the polished surface of the optical fibre bundle is flat.

The secondary optical source preferably comprises a light source and mode scrambler and the optical detector preferably comprises a quadrant photodetector or the like, to provide the position of the reflective member in X, Y coordinates. The reflective member is preferably adjustable about both the X and Y axes.

The control means preferably includes a servo-mechanism for automatically adjusting the position of the reflective member if

required. In a preferred embodiment, the reflective member is a concave spherical or ellipsoidal mirror, and its desired position is such that the centre of the optical fibre ends to be coupled is substantially at the centre of curvature of the mirror. If the reflective member is a plane mirror, its desired position is beneficially such that the same effect is achieved.

In general, the present invention provides significant advantages over known arrangements because the light path, particularly with single mode fibre, may only be 10, um in diameter and must be controlled to an accuracy of around +/-lm. Position can be measured optically to this accuracy (as in the present invention) whereas it is very difficult to design a micro positioner with adequate positional accuracy (as required in the prior art).

Further, because variations due to drift in the X/Y directions are compensated in the present invention only the (less critical) focus distance needs to be held stable.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a simplified schematic plan view of an optical switch according to an exemplary embodiment of the invention ; Figure 2 is a cross-sectional view taken on line A-A in Figure 1 ; Figure 3 is a cross-sectional view taken on line B-B in Figure 1 ; Figure 4 is a side view of the optical switch of Figure 1 with the spacers omitted for clarity; Figures 5A-5D are schematic side views of various exemplary embodiments of the invention; Figure 6 is a perspective view of optical elements of an optical switch according to an exemplary embodiment of the invention; and

Figures 7A-7C are rear, elevation and plan views of the arrangement of Figure 6.

In general, the function of the apparatus of the invention is to couple light from the end face of one fibre of a bundle of fibres into the end face of the same fibre or another fibre in the same bundle. In a first exemplary embodiment, the light from the end face of a first fibre is projected onto a concave mirror. When the mirror is appropriately positioned, the reflected light converges to a focus to enter the second fibre.

To couple light efficiently, the spherical mirror is preferably tilted so that the centre of curvature of the mirror lies midway between the two fibre ends on the end face of the fibre bundle.

Any pair of fibres in the bundle may be optically coupled by tilting the mirror as described.

A position control mechanism is used to accurately control the mirror position. Mirror angle is measured by reflecting light from a light source, via the same mirror surface, on to a position sensitive detector. Alternatively, a distinct but mechanically attached mirror surface may be used for this purpose. The measured position is compared with the required position and the error is used to control positioning actuators which adjust the mirror angle to cancel the error.

In one exemplary embodiment, the switch consists of two preferably metal housing blocks separated by spacers. The first block carries the fibre bundle, the light source and the detector ; the second block carries the mirror, supported by wire flexures, and actuators to tilt the mirror. optical fibres are normally covered with protective secondary coating and sleeving. These are removed so that fibres may be made into a bundle. This is bonded into a ferrule and the end of the bundle is polished to an optically smooth surface.

The bundle is preferably polished at an angle. Most of the light power (typically more than 95%) is projected towards the mirror.

The small proportion of light power (typically less than 5%) which is reflected from the end face is mainly angled outside of the acceptance angle of the fibre and cannot propagate back down the same fibre. (Without this angled polishing the back-reflection can have adverse effects on other optical components.) The bundle is angled so that the polished front face is parallel to the face of the block. The centre of rotation of the mirror is on a line normal to this surface, ensuring that the ends of all fibres are at the same distance and remain in focus. (The centre of rotation of the mirror is where the nominal axes of the three support wires intersect.) The light source may use a large diameter step-index optical fibre as a mode scrambler: light enters the fibre and angled rays are internally reflected different numbers of times along different paths. This produces an evenly illuminated end face. The light is projected, via reflection at the mirror, to produce a circular spot within the active area of the photosensitive diode.

The relative intensity of light falling on the diodes enables precise measurement of the spot position and therefore of mirror position. (Other types of optical position detector may be used with different principles of operation.) For single mode optical fibre, the centre of curvature of the mirror is preferably positioned with an accuracy of 1 or 2 microns.

This is beyond the capability of most mechanical positioning devices-due to friction, mechanical hysteresis etc. Mounting the mirror using elastic support wires in a preferred embodiment overcomes these problems.

The support wires are mutually at right angles. The point where the axes of the three wires intersect has the property that force applied to it in any direction is resolved into tension or compression forces shared between the three wires. Because the

wires are stiff in compression and tension, but bend more easily, this point becomes the centre of rotation when rotational forces are applied by the magnetic actuators.

Passing a current through either solenoid actuator of a preferred position control mechanism produces a force on the mirror support plate and causes rotation about the point described above. The orthogonal movements due to the two actuators enable the mirror to be tilted to any desired angle.

Each position of the mirror has a pair of (X and Y) positional outputs from the quadrant photo-detector, in one exemplary embodiment. A position control circuit compares the measured position signals with the required position signals. The error signals are fed to correction amplifiers which drive the actuator coils to cancel the error. The mirror is placed at the required angle by applying the corresponding X and Y position signals.

Referring to Figure 1 of the drawings, an optical switching apparatus according to an exemplary embodiment of the invention comprises an optical fibre bundle 10 through which is transmitted an optical signal 12. The bundle of optical fibres 10 is held together by means of a ferrule 11 or the like, and is mounted within a groove 14 in the top surface of front block 15 which forms a front portion of switch housing 16. The groove 14 is at an angle relative to the longitudinal axis of the apparatus, and tapers down to a narrow end 18 positively locating the fibre bundle and leaving a relatively very small aperture 20 in the block 15 through which the optical signal 12 is transmitted from the ends of fibres 10.

The optical fibre bundle 10 is slidable along the groove 14 to adjust the focus of the optical signal 12, and is fixed in the desired position within the groove 14 by a clamp (not shown). In this position it is aligned with the centre of a curved mirror 28, further described below, which is mounted on a rear block 31 which forms a rear portion of the switch housing 16.

Light source 22 projects light into a large diameter glass fibre 25 in block 15. Glass fibre 22 is mounted in channel 23 parallel to

the longitudinal axis of the apparatus and in alignment with mirror 28. Light is guided by internal reflection in the glass fibre to produce a uniformly illuminated disc at the far end face 24. The end face 24 of fibre 22 is above and spaced apart from the end aperture 20 of the groove 14.

A quadrant photodetector 26 is mounted on the inner face of block 15 below the outlet 20 of the groove 14 and facing mirror 28. The mirror images the illuminated end face disc 24 on to the quadrant photodetector.

Mirror 28 is a concave spherical mirror which is adjustably mounted on rear block 31. It is located and mounted such that its centre of curvature lies midway between the ends of the optical fibre (s) in bundle 10 that are to be coupled. The mirror 28 is mounted on a substantially frictionless suspension mechanism including wire flexure support elements (for example, three flexible wire struts mutually at right angles) or suspension springs 30 which are clamped to the front wall of the rear portion 31 of the switch housing 16 via a clamp plate 34. In one embodiment, each wire strut forms part of the flexure to control the focus (Z) distance.

In this case, the axis of the strut should be aligned to the end face of the fibre and normal to its centre. The front 15 and rear 31 blocks of the switch housing 16 are connected together by means of spacers 35.

Housed within the rear block 31 of the switch housing 16 is a positional control mechanism 36 which facilitates positional control of the mirror 28 which can be rotated about a horizontal and vertical axis 39a, 39b (see Figure 6), as required, with the centre of rotation being the intersection of axes 39a and 39b. The positional control mechanism 36 includes two actuators 40 (for example, magnetic (solenoid) piezo-electric or magneto-strictive actuators) positioned to cause orthogonal tilting of the mirror 28 about its centre of rotation.

The ends of the optical fibre bundle 10 are polished to give a surface which may be flat or curved, and may be angled relative to

the perpendicular plane of the bundle 10 (see Figure 7C), to prevent propagation of some back-reflected light back down the fibre from which it is transmitted.

In use, as stated above, the spherical concave mirror 28 is positioned so that the centre point of its curvature lies midway between the polished ends of the cores of a pair of fibres to be optically coupled. The cone of light 12 transmitted from one fibre is reflected back by the mirror 28 to converge on the core of the other fibre of the pair. The light source and mode scrambler 22 directs a beam of light 42 at the same area of the mirror 28.

According to the tilt of the mirror 28, light from source 22 illuminating the end disc 24 of fibre 25 is reflected by the mirror 28 and is detected by the photodetector 26 to produce an output indicative of the orientation of the mirror 28. This output is compared with the required position of the mirror 28 to produce a positional error signal indicating the difference between the actual and desired mirror positions. The positional error signal is suitably amplified and used to drive the actuators 40 in a closed-loop position-control servo-mechanism. The wire flexure mirror supports or suspension springs 30 facilitate the control of the locus of the centre of curvature of the mirror 28. In one particular exemplary embodiment, the actuators 40 and wire flexure supports or suspension springs 30 act at 45'to each other, and are driven in-phase for vertical and anti-phase for horizontal deflection.

Referring particularly to Figures 5A-5D, and as stated above, the ends of the optical fibres 10 may be polished to produce a flat surface (see Figure 5A). As shown, the cone of light 12a from the top fibre lOa does not fully overlap the acceptance cone 12b of the bottom fibre lOb, which results in coupling loss. Such coupling loss is increased as fibre separation is increased.

Referring to Figure 5B, in order to reduce or eliminate coupling loss, the ends of the optical fibres 10 may be polished to a curved or spherical surface, thereby causing the cone of light 12a from the top fibre lOa to substantially overlap at the mirror 28.

Alternatively, referring to Figure 5C, a lens 50 can be place in the light path to reduce the cone angle (numerical aperture) and cause the cones to overlap at the mirror 28.

Referring to Figure 5D of the drawings, in another alternative exemplary embodiment of the invention, a refracting element such as a lens 52 and tilting plane (or concave) mirror 54 can be used instead of the rotating, concave spherical mirror described above.

The refracting element has advantages with wider beam angles and can provide better coupling between fibres in larger bundles. For best coupling the fibre end lies at one focal point of the lens and the mirror at the other.

Embodiments of the invention have been described above by way of examples only, and it will be apparent to persons skilled in the art that modifications and variations can be made without departing from the scope of the invention.

For example, although the ideal reflecting surface of the mirror is spherical, especially if the input optical signal is to be transmitted back to the fibre from which it is emitted, the reflecting surface may under some circumstances be ellipsoidal.

Further, although it is highly advantageous to rotate the mirror about a point behind it, other forms and methods of position manipulation are envisaged, provided the centre of curvature of the mirror scans properly across the face of the fibre optic bundle.

The apparatus may include a pair of rotating shallow wedges to offset the light beam before it strikes the mirror. Further, a separate input fibre adjacent to multiple output fibres could be used to prevent coupling between fibres, other than those intended to be connected. Still further, a second reflective surface could be used to measure the mirror angle.

Claims

CLAIMS 1. An optical switching apparatus for directing an optical signal from one optical fibre to another optical fibre in a bundle, said apparatus comprising: reflection means, including an adjustable reflective member; means to adjust the position of said adjustable reflective member; a secondary optical source arranged for reflection by said adjustable reflective member ; an optical detector arranged to receive the beam from said secondary optical source reflected by said adjustable reflective member, said optical detector being operative to provide output indicative of the position of said adjustable reflective member ; and control means, responsive to said output of said detector to adjust the position of said adjustable reflective member towards a desired position.
2. An apparatus, according to claim 1, wherein said adjustable reflective member is further employed to reflect said optical signal from said one optical fibre to said another optical fibre.
3. An apparatus, according to claim 1, wherein said reflection means comprises a coupled reflector, coupled to move with said adjustable reflector and operative to reflect said optical signal from said one optical fibre to said another optical fibre.
4. An apparatus, according to any one of the preceding claims, wherein the end of said one optical fibre and the end of said another optical fibre are polished.
5. An apparatus, according to claim 4, wherein said ends are polished to a planar finish.
6. An apparatus, according to claim 4, wherein said ends are polished to a curved finish.
7. An apparatus, according to any one of the preceding claims wherein said adjustable reflective member comprises a plane mirr 8. An apparatus, according to claim 7, wherein said plane mirror is tiltable about one axis.
8. An apparatus, according to claim 6, wherein said plane mirror is tiltable about two axes.
9. An apparatus, according to claim 6,7 or 8, comprising a refractive member in the path of said adjustable reflective member.
10. An apparatus, according to claim 9, wherein said refractive member comprises a lens.
11. An apparatus, according to any one of claim 4,5 or 6, comprising a refractive member in the path of said adjustable reflective member.
12. An apparatus, according to claim 11, wherein said refractive member comprises a lens.
13. An apparatus, according to any one of the preceding claims, wherein said adjustable reflective member comprise a concave spherical mirror.
14. An apparatus, according to claim 13, wherein said concave spherical mirror is rotatable about one axis.
15. An apparatus, according to claim 13, wherein said concave spherical mirror is rotatable about two axes.
16. An apparatus, according to any one of claims 1 to 12, wherein said adjustable reflective member comprises a concave ellipsoidal mirror.
17. An apparatus, according to claim 16, wherein said concave ellipsoidal mirror is rotatable about one axis.
18. An apparatus, according to claim 6, wherein said concave ellipsoidal mirror is rotatable about two axes.
19. An apparatus, according to any one of the preceding claims, wherein said control means comprises: means to compare the actual and desired position of said reflective member ; and means, operative if said actual position differs from said desired position, to apply control signals to adjust the actual position of the reflective member towards said desired position.
20. An apparatus, according to claim 4, wherein said desired position comprises positional components expressible in two axes, wherein said optical detector is operative to sense the position of said adjustable reflective member on said two axes; wherein said means to compare said actual position of said reflective member with said desired position is operative on said two axes; and wherein said means to apply control signals to adjust the position of said adjustable reflective member is operative on said two axes.
21. An apparatus, according to any one of the preceding claims, wherein said secondary optical source comprises a light source and wherein said optical detector comprises a quadrant photo detector to provide the position of the adjustable reflective member in X-Y co-ordinates on X-Y axes.
22. An apparatus, according to claim 21, wherein said adjustable reflective member is adjustable in said X-Y axes.
23. An apparatus, according to any one of the preceding claims, wherein said control means comprises a feedback servomechanism.
24. An apparatus, according to any one of the preceding claims, wherein the distance of the end of said bundle from said

reflective means is adjustable to ensure focus of said optical signal.
25. An apparatus, according to claim 24, when dependent upon claim 13, wherein the end of said bundle is substantially at the centre of curvature of said concave spherical mirror.
26. An apparatus, according to any one of the preceding claims, wherein said adjustable reflective member is suspended on at least three wire struts.
27. An apparatus, according to any one of the preceding claims. including a secondary reflective surface to direct said light from said secondary light source towards said optical detector.
28. An apparatus, according to any one of the preceding claims. wherein said one fibre and said another fibre are separated in said bundle.
29. An apparatus, according to claim 28, wherein said one fibre and said another fibre are separated by a non active optical fibre.
30. An apparatus, according to any one of the preceding claims, wherein said adjustable reflective member is adjustable using magnetic actuators.
31. An apparatus, according to any one of claims 1 to 30, wherein said adjustable reflective member is adjustable using one or more piezo electric actuators.
32. An apparatus, substantially as described, with reference to the appended drawings.