WO2002016996A1

WO2002016996A1 - Optical switching apparatus

Info

Publication number: WO2002016996A1
Application number: PCT/GB2001/003761
Authority: WO
Inventors: Peter John Duthie
Original assignee: Bookham Technology Plc
Priority date: 2000-08-19
Filing date: 2001-08-20
Publication date: 2002-02-28
Also published as: GB0119995D0; GB2367145A; GB0020463D0

Abstract

An optical switching arrangement is provided with continuously rotating mirros (8, 9, 10) which are preferably micro-machined, each deflecting light from an associated optical input (2, 3, 4), preferably mono-mode optical fibres, towards optical outputs (5, 6, 7) in sequence. Further pairs of continuously rotating micro-machined mirrors (11/12, 13/14, 15/16) are each associated with respective optical outputs (5, 6, 7) to stabilise the direction of light that is to be coupled with the optical outputs so that better coupling is achieved. The optical switching apparatus provides a faster switching arrangement and lower signal loss.

Description

OPTICAL SWITCHING APPARATUS

This invention relates to optical switching apparatus for switching optical signals between one or more inputs and outputs. More especially, although not exclusively, this invention concerns optical switching apparatus for switching optical signals between optical fibres in an optical fibre communications system.

Where a group of optical fibres is used to transmit optical signals, it is desirable to cross-connect fibres selectively at a junction to route the optical signals. This may be achieved by an optical plug and socket system, whereby optical fibre links are rearranged manually. Whilst such an arrangement is acceptable for circuit switching in which it is desired to re-configure the optical fibres links at a frequency of many months or even years, it is not suited to optical switching for rapidly re-routing optical signals.

For routing optical signals it is known to carry out the switching in the electrical domain by an electrical cross-connect system. Incoming optical signals from a group of optical fibres are transduced into electrical signals, redirected by electronic switches, transduced back into optical signals and coupled to a group of receiving optical fibres. Whilst such systems can work well, the switching rate is limited by the rate at which electronic switches can operate and this can produce bottlenecks within the communications systems, especially in the current and proposed wavelength division multiplex systems.

Recently, it has also been proposed to provide a slow optical cross-connect which is capable of switching optical signals in the optical domain without the need for conversion to electrical signals. Optical signals from a group of input optical fibres, as many as one thousand, to be switched to a group of output optical fibres are each directed onto respective mirror segment of a micro-mechanical mirror assembly. As is known, micro-mechanical mirror assemblies have been developed for scanning light in overhead projectors and projection televisions. Each mirror segment is independently tiltable about two orthogonal axes through arc ranges of, for example, 2°, or in some other systems, 30°. The length of the arc swept out by the edges of the segments is approximately l-10μm. A second mirror assembly is provided for the group of receiving optical fibres having a respective mirror segment associated with each receiving fibre. The optical signal from each input optical fibre is reflected from the associated segment and is directed, in accordance with the segments' respective orientations at a given time, onto the respective mirror of the output optical fibre to which it is desired to direct the optical signal. The mirror segments of the second assembly are appropriately oriented to ensure coupling of light into the respective output optical fibre.

Typically, the mirror segments are actuated electrostatically. By suitable choice of the orientation of the segments, optical signals from the optical input fibres can be selectively directed to couple with a selected output optical fibre. Although the mirror segments are micro-machined and have a low mass, their inertia still limits their switching speed to 1 - 10 ms. This limit is affected by the period taken for the inertia of the segments to be overcome, and then be accelerated and moved to their new positions. The limit is also affected by the time taken for damping effects to settle the segments immediately after their having been moved. In the current generation of optical communication systems, such a switching rate is too low, especially in asynchronous transfer mode systems in which it would be desirable to be able to switch individual time slots.

The present invention arose in an endeavour to provide optical switching apparatus which, at least in part, alleviates the limitations of prior art switching apparatus.

Accordingly, the invention provides optical switching apparatus comprising: at least one optical waveguide input; at least one optical waveguide output; optical deflecting means associated with the or each optical waveguide input for deflecting light from the optical waveguide input in an arcuate manner to couple into the or each optical waveguide output, characterised in that the optical deflecting means is capable of being continuously rotated in a given direction thereby continuously scanning light across the, or each, optical waveguide output.

Since the deflecting means is capable of being continuously rotatable in a given direction and no mass need be accelerated or decelerated once at operating speed, this alleviates the problem of inertia associated with prior art apparatus. Consequently, the invention is able to deflect light from the, or each, optical waveguide input to the, or each, optical waveguide output in turn at a greater rate than prior art devices.

Advantageously the apparatus further comprises means associated with the, or each, optical waveguide output for converting said continuous scanning of light across the output into an intermittent scanning to increase the dwell time of the light on the output and thereby maximise the coupling of light into the output. Such means are especially advantageous when the optical waveguide output comprises an optical fibre, especially a monomode fibre in which the core has a substantially smaller diameter that the outer cladding layer, such that acceptance angle for which light can coupled into the fibre is very limited.

Preferably said means comprises second and third optical deflecting means which are capable of being continuously rotated in a given direction and which are arranged such that light which is scanned onto the second deflecting means is deflected onto the third deflecting means and therefrom into the associated output and wherein through the mutual rotation of said deflecting means light is deflected from the third deflecting means in a substantially single direction. The second and third deflecting means preferably rotate around parallel axes, and advantageously rotate with an identical constant angular velocity.

Some or all of the deflecting means are preferably reflecting means, such as multi-sided mirrors. Alternatively, they can be refracting elements such as prisms or gratings. Whilst the constituent components of the apparatus can be of macroscopic dimensions, they are preferably of microscopic dimensions. The reflectors, in particular, are preferably micro-machined reflectors, and are advantageously each single-crystal members, such as silicon, enabling the reflector to be rotated at very high speeds of the order of several tens of thousands of revolutions per minui

The apparatus finds particular application in switching light between optical fibres, but can also be used to switch light between other optical waveguide inputs and outputs, such as lasers and/or optical sensors, for example. Advantageously, particularly when the optical inputs and outputs comprise optical fibres, focussing means, preferably lenses, are provided between the fibre and the associated deflecting means to focus light from the optical input fibre, and to focus light into the output optical fibre. In an alternative arrangement, the focussing function can be provided by the first and/or third deflecting means having a curved reflecting surface.

The invention is particularly suited for use as a switching function for a plurality of optical waveguide inputs or outputs. In a particularly advantageous embodiment, the optical waveguide inputs and outputs are arranged in the same plane, with each optical input being switched to each optical output in turn in a cyclic manner. By suitable choice of the orientation of the deflecting means and the orientation of their respective axes of rotation, some or all of the optical inputs may be arranged in a first plane and some or all of the optical outputs may alternatively be arranged in a second plane. For ease of fabrication and to ensure the waveguide inputs and outputs are parallel, fixed reflecting means can be provided between each optical input and each associated deflecting means.

The present invention finds particular application as part of an optical asynchronous transfer mode (ATM) cross-connect or as a time slot interchange switch. Thus, according to a second aspect of the invention, there is provided an optical asynchronous transfer mode cross-connect incorporating an optical switching arrangement described above. According to a third aspect of the invention, there is provided a time-slot interchange switch incorporating an optical switching arrangement described above. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows a schematic representation of an optical switching apparatus in accordance with the invention at a first instance in time;

Figure 2 shows the optical switching apparatus of Figure 1 at a second instance in time;

Figure 3 shows the optical switching apparatus of Figure 1 at a third instance in time;

Figure 4 shows a part of one optical input and the effect of varying the angle of a fixed mirror relative to the optical input, and how the size and number of sides of a rotatable mirror associated with the input affects the extent of an arc swept by the rotatable mirror; and

Figures 5a, 5b and 5c show apparatus without a stabilisation stage associated with the optical output.

Referring to Figure 1, an optical switching apparatus, shown generally as 1, is part of an asynchronous transfer mode cross-connect for cyclically switching light from each of a group of optical fibre inputs to each of a group of optical fibre outputs. In the embodiment described, and for the sake of simplicity, three mono-mode input optical fibre inputs 2, 3 and 4 are shown together with three corresponding output optical fibres 5, 6 and 7, however, it is envisaged to have thirty-two inputs and outputs. The input optical fibres 2, 3 and 4 each have respective rotating mirrors 8, 9 and 10, which are shaped as regular dodecagons, and are micro-machined in single-crystal silicon. Each mirror 8, 9 and 10 is rotated at the same constant angular velocity, and rotates with an angular speed of 15,000 revolutions per minute.

The output optical fibres 5, 6 and 7 each have respective similar pairs of rotating mirrors 11/12, 13/14 and 15/16 which, as will be described, increase the dwell time of light incident on the respective output. The input optical fibres 2, 3 and 4 each have respective collimating lenses 17, 18 and 19, and the output optical fibres 5, 6 and 7 each have respective focussing lenses 20, 21 and 22. In the optical path between the collimating lenses 17, 18 and 19 and the mirrors 8, 9 and 10, are fixed mirrors 23, 24 and 25. As will be appreciated the fixed mirrors 23, 24 and 25 enable the input and output optical fibres to be arranged in parallel. This is preferred for ease of fabrication of the apparatus.

In operation, light from each of the input optical fibres 2, 3 and 4 is collimated by its respective lens 17, 18 or 19, and directed onto its respective fixed. mirror 23, 24, or 25. Fixed mirrors 23, 24 and 25 each reflect the collimated light onto the respective rotating mirrors 8, 9 and 10. The angle that light is reflected from each of the rotating mirrors 8, 9 and 10 is dependent on the angular orientation of the mirrors at any given time, and the reflected light from each rotating mirror is swept in an arcuate manner, to intercept in turn with each of the second rotating mirrors 11, 13 or 15. Light from the second rotating mirrors 11, 13 and 15 is further reflected onto the mirrors 12, 14 and 16 respectively and coupled into its respective output optical fibre 5, 6, and 7. All the rotating mirrors 8, 9, 10, 11, 12, 13, 14, 15 and 16 rotate at identical angular velocities, and are phase-locked; that is, the angular orientation of the mirrors is set to ensure light is cyclically switched between each of the optical fibre inputs and outputs, as will be described below.

Referring to Figure 1, this shows the apparatus at a first instance of time, in which light is coupled from each input fibre 2, 3, and 4, to the corresponding output fibre 5, 6, and 7 respectively.

Figure 2 shows the apparatus of Figure 1 at a further instance in time, whereby each of the mirrors 8, 9, 10, 11, 12, 13, 14, 15 and 16 have rotated, such that light from the first input optical fibre 2 is now directed to the second output optical fibre 6, light from the second input optical fibre 3 directed to the third output optical fibre 7, and light from the third input optical fibre 4 is directed to the first output optical fibre 5. It will be appreciated that light is being reflected from the same face of the rotating mirror.

Figure 3 shows the apparatus of Figure 1 at another instance in time, whereby the mirrors 8, 9, 10, 1 1, 12, 13, 14, 15 and 16 have rotated through a further angle, such that light reflected from the first input optical fibre 2 is directed to the third output optical fibre 7, light from the second input optical fibre 3 is directed to the first output optical fibre 5, and light from the third input optical fibre 4 is directed to the second output optical fibre 7.

Since the rotating mirrors 8, 9 and 10 each have twelve reflecting surfaces, the above cyclic switching sequence will be repeated for a like number of times for each rotation of the mirrors. It will be appreciated that the position and relative orientation of the rotating mirrors must be carefully selected to ensure the correct switching function of the apparatus. It will be further appreciated that the illustrations of the embodiment in Figures 1, 2 and 3 are purely schematic. In practical implementations, the optical inputs are likely to be positioned proportionally much closer to one another, most likely abutting each other. Likewise the optical outputs will be similarly positioned. Furthermore, the optical inputs and outputs are likely to be positioned much farther apart in practical implementations that the drawings suggest and as a result, the angles of deflection of the light from the rotating mirrors 8, 9 and 10 are very much smaller than those illustrated. Such an arrangement helps • maximise, over a complete switching cycle of the switch, coupling of the deflected light into an output.

Referring to Figure 4, one of the input optical fibres 3 is shown with its associated collimating lens 18. The orientation of fixed mirror 24 is fixed during operation, but is shown here as tiltable for the purposes of this drawing, as indicated by double-headed arrow 26. The rotating 12-sided mirror 9 is shown as rotating in a clockwise direction,

as indicated by single-headed arrow 27. The angular extent 2 of the arc swept out by

rotation of the mirror 12 is indicated by lines 28 and 29, the angle between these lines

being 2. The value of 2 depends on the size of the reflecting faces and number of faces

of the mirror. The direction of the arc defined by lines 28 and 29 is dependent on the angle of the fixed mirror 24 relative to the rotating mirror 9 or input optical fibre 3.

It will be appreciated that to ensure each rotating mirror 8, 9 and 10 associated τ. ;.h each of the input optical fibres directs light to the output optical fibres in the correct sequence, the relative angular orientation of the mirrors is correctly set. A particularly important feature of the optical switching apparatus of the present invention is the inclusion of the pairs of rotating mirrors 11/12, 13/14 and 15/16, associated with each output optical fibre which provide a stabilising function by counteracting the rotational scanning effect of the rotating mirrors 8, 9 and 10 and converting this into an intermittent scanning of light onto the output fibre. This arrangement increases the dwell time of the light on the output fibre thereby maximising the coupling of the light into the output fibre. In operation it can be seen therefore that the deflected light from each input fibre will dwell at each output fibre core before skipping to the next output fibre core and so forth cycling through each output fibre.

To more clearly understand this principle Figures 5a, 5b and 5c illustrates light being continuously scanned across an optical fibre in the absence of such stabilisation means. Referring to Figure 5 a, one of the input optical fibres 3 with collimating lens 18 is shown, and a focussing lens 21 in front of output optical fibre 6 without any stabilisation means, that would otherwise be provided by mirrors 13 and 14. As described, the input optical fibre 3 is typically a mono-mode fibre, having, a typical core diameter of lOμm and cladding layer of 125μm in diameter. As a result the fibre has a very limited acceptance angle. The angle of light beam 30 is such that much of the light intercepts the cladding of the output optical fibre 6 at this instance of time, thus coupling very poorly with the optical fibre.

Figure 5b shows the same arrangement as Figure 5 a, but at a slightly later instance in time, in which the direction of light beam 30 is such that it intercepts the output optical fibre 6 along a central axis of the fibre, thus a coupling is achieved with minimal signal loss. Figure 5c shows the arrangement at a slightly later instance in time to that of Figure 5b, in which the light beam 30 intercepts the output optical fibre 6 at a lower edge, thus coupling poorly with the fibre. Therefore for a large proportion of the time that light is incident on the fibre the light does not couple into the fibre core. For an optical cross connect switch this corresponds to a long dead time between connection slots. Furthermore, during connection slots, the intensity of light coupled into the core will vary.

It will be appreciated that the inclusion of the rotating mirror pairs 11/12, 13/14 and 15/16 is particularly advantageous since these ensure light is directed onto the respective output fibre in substantially a single direction, preferably the axis of the optical fibre core, over a much greater period of the connection slot thereby maximising the coupling of light into the output fibre and minimising the dead time and corresponding guard band time required between timeslots.

Consider the arrangement shown in Figure 1, wherein the light from input fibre 2 is coupled via collimating lens 17 onto fixed mirror 23 where it undergoes a first reflection. Mirror 23 produces a static light beam that is incident upon the facets of the rotating mirror 8 whereat, the static light beam undergoes a second reflection and forms a beam that scans from left to right. Said left to right scanned beam is in part intercepted by the facets on rotating mirror 11 whereat, the left to right scanning beam is reflected a third time by a mirror that would turn a static beam into one that scanned from right to left. Thus the second reflection beam scanned left to right, when reflected a third time, has a scan counteraction applied to it i.e. its rate of scan is slowed. The beam from the third reflection is largely intercepted by the facets on rotating mirror 12 whereat, the third reflection slowed scanned beam is reflected a fourth time by a mirror that would turn a static beam into one that scanned left to right i.e. is a counteracting scan. Thus the third reflection beam undergoes further reduction in its scan rate at the fourth reflection. The considerably scan rate slowed light beam is then coupled via focussing lens 20 into output fibre 5. By this means the dwell time of the light incident on the respective outputs is increased. The rotational rates of all mirrors are the same and. each phase locked to maintain the set up optical relationships. Relative positioning of the rotating mirrors determines the duration for which an input source fibre light is coupled to an output fibre.

It will be appreciated by those skilled in the art that optical switching apparatus of the present invention is not restricted to the specific embodiment described, and that variations can be made which are within the scope of the claimed invention.

For example, whilst stabilisation of the switched light to aid coupling with optical fibres . 5, 6 and 7 is particularly preferred, this is not essential and will depend upon the acceptance angle of the optical output. The mono-mode optical fibre inputs 2, 3 and 4 can instead be lasers or any other light source or means for channelling light, they may be multi-wavelength inputs. The mono-mode optical fibre outputs 5, 6 and 7 may alternatively be multi- wavelength outputs, or sensors. The 12-sided mirrors 8, 9, 10, 11, 12, 13, 14, 15 and 16 can have any number of sides to suit the geometry of a particular switching arrangement, and could alternatively refract light instead of reflecting it. Refraction may be achieved by rotating prisms, or refracting elements. In fact, any form of rotatable deflecting means can be used. The switching apparatus 1 described has three optical inputs and three optical outputs although other numbers of inputs and outputs can be used depending on a particular application. For example, it is envisaged to have a single optical input and a plurality of outputs, and vice versa. The components of the optical switching apparatus 1 can be of macroscopic dimensions. It is envisaged that the collimating lenses 17, 18 and 19 are incorporated into the fixed mirrors, which have a converging function, or other collimating device. The function of the focussing lenses 20, 21 and 22 could be provided by other focussing means, such as a concave mirror, or the rotatable mirrors 12, 14 and 16 could be shaped in order to provide a focussing function. The geometry of the deflecting surfaces of the optical switching apparatus 1 could be arranged such that the optical fibres 2, 3, 4, 5, 6 and 7 are positioned in different planes.

Claims

1. Optical switching apparatus comprising: at least one optical waveguide input; at least one optical waveguide output; optical deflecting means associated with the or each optical waveguide input for deflecting light from the optical waveguide input in an arcuate manner to couple into the or each optical waveguide output, characterised in that the optical deflecting means is capable of being continuously rotated in a given direction thereby continuously scanning light across the, or each, optical waveguide output.

2. Apparatus as claimed in Claim 1 and further comprising means associated with the, or each, optical waveguide output for converting said continuous scanning of light across the output into an intermittent scanning to increase the dwell time of the light on the output.

3. Apparatus as claimed in Claim 2 wherein said means comprises second and third deflecting means which are capable of being continuously rotated in a given direction and which are arranged such that light which is scanned onto the second deflecting means is deflected onto the third deflecting means and therefrom into the associated output and wherein through the mutual rotation of said deflecting means light is deflected from the third deflecting means in a sub:::-: ially single direction.

4. Apparatus as claimed in Claim 3 wherein the second and third deflecting means rotate in the same direction.

5. Apparatus as claimed in Claims 3 or 4 wherein the second and third deflecting means are rotated at a constant angular velocity.

6. Apparatus as claimed in any one of Claims 3, 4 or 5 wherein the second and third deflecting means rotate at the same angular velocity.

7. Apparatus as claimed in any one of Claims 1 to 6 wherein any of said deflecting means are reflectors.

8. Apparatus as claimed in Claim 7 wherein the reflectors are micro-machined reflectors.

9. Apparatus as claimed in Claim 8 wherein the micro-machined reflectors are fabricated from single-crystal silicon.

10. Apparatus as claimed in any of Claims 1 to 6 wherein any of said deflecting means comprise a refracting element.

11. Apparatus as claimed in Claim 10 wherein the refracting element is a micro- machined prism.

12. Apparatus as claimed in any preceding claim wherein the, or each, optical input or the, or each, optical output is an optical fibre.

13. Apparatus as claimed in Claim 12 wherein said optical fibres are mono-mode fibres.

14. Apparatus as claimed in any one of the preceding Claims, and further comprising focussing means associated with the, or each, optical input and arranged to collimate light from the optical input.

15. Apparatus as claimed in Claim 14 wherein the focussing means is a lens.

16. Apparatus as claimed in any one of the preceding Claims and further comprising focussing means associated with the, or each, optical output and arranged to focus light onto, and to couple with, the optical output.

17. Apparatus as claimed in Claim 16 wherein the focussing means is a lens.

18. Apparatus as claimed in Claim 16 wherein the focussing means is provided by the third deflecting means having a concave reflecting surface.

19. Apparatus as claimed in any preceding claim wherein a plurality of optical inputs are arranged in a plane and a plurality of optical outputs are arranged in the same or a parallel plane, and each input is switchable to each output in a cyclic sequence.

20. An asynchronous transfer mode optical cross connect incorporating an optical switching arrangement as claimed in any preceding claim.

21. A time-slot interchange switch incorporating an optical switching arrangement as claimed in any one of Claims 1-19.

2. Apparatus substantially as hereinbefore described with reference to or substantially as illustrated in the accompanying drawings.