WO2006092601A2

WO2006092601A2 - Acoustooptic modulator

Info

Publication number: WO2006092601A2
Application number: PCT/GB2006/000745
Authority: WO
Inventors: David John Taylor Heatley; Edmund Sergio Robert Sikora
Original assignee: British Telecommunications Public Limited Company
Priority date: 2005-03-04
Filing date: 2006-03-02
Publication date: 2006-09-08
Also published as: WO2006092601A3; GB0504578D0

Abstract

The present invention relates to the modulation of light, in particular to phase modulation in an optical fibre by means of an acoustooptic modulator. There is provided a modulator arrangement (10) for acoustically modulating an optical fibre link (14), the modulator arrangement including: a transducer (118a, 118b) for generating acoustic vibrations, the transducer having a piezoelectric layer region of uniform thickness (28a), the layer region having a semi-circular shape and being sandwiched between two electrodes (28b, 28c), so as to form an elongate recess for releasably receiving the optical link and for radially applying elastic waves to the optical fibre. Furthermore, two such modulator arrangements can be brought together to form an annulus, so as to hold the optical fibre.

Description

Acoustic modulation

The present invention relates to the modulation of light, in particular to acoustic modulation.

It is known to acoustically modulate light along an optical link. However, such known techniques are not suitable for all situations.

According to the present invention, there is provided a modulator arrangement for acoustically modulating an optical link, the modulator arrangement including: a vibration element for generating acoustic vibrations, the vibration element having a layer region of uniform thickness, the layer region being curved in at least one direction so as to form an elongate recess for releasably receiving the optical link.

Further aspects of the present invention are specified in the appended claims. The invention will now be further described, by way of example only, with reference to the following drawings in which:

Figures 1a-1d shows a modulator arrangement according to the present invention; Figure 2 shows a further coupling arrangement;

Figure 3 shows electrical connections in the modulator arrangements of Figure 1 and 2

Figure 7 shows an embodiment of a monitoring system for detecting a disturbance applied using the coupling arrangement of Figure 1 to 6; and, ' Figures 8 to 13 show further embodiments of the invention.

In Figures 1a - d, there is shown a modulator arrangement 10 having a first transducer component 118a and a second transducer component 118b. As shown in Figure 1c, each transducer component 118a, 118b has the form of a semi-circular arch when viewed in cross section along the axis of the fibre 14 (the direction marked X in Figure 1a).

In Figure 1a,. the two portions are shown in a separated state, an optical cable link 14 residing in the region between the two portions. When the two portions are brought together around the cable 14 (in the direction of the arrows in Figure 1a), the two components 118a, 118b form an annulus region around the fibre, as shown in Figure 1b. In order to retain the transducer components 118a, 118b in position around the fibre, a C- shaped resilient clip 162 can be placed around the transducer components 118a, 118b as shown in Figure 1a.

With reference to Figure 1 c, each transducer component has a piezoelectric (or material with similar appropriate properties) semi-circular layer 28a of uniform thickness sandwiched between inner and outer electrode layers 28b, 28c, the electrode layers having been deposited on the layer of piezoelectric material.

With reference with Figure 1d, a contact portion 28d of the inner electrode 28b extends beyond the piezoelectric layer 28a (when viewed in the transverse direction represented by the arrow marked Y in Figure 1a) so that the inner electrode can be contacted electrically.

One advantage of the embodiment described above is that the transducer modulator can be mounted in an arbitrary position along the optical cable 14, without needing to break the fibre cable in order to pass the fibre in the bore. Because the piezoelectric layer is uniform, the electric field applied by the electrode can be more easily made constant over the layer. Furthermore, because the two components together form a circular annulus, the field can be more homogeneously applied in the radial direction, allowing the acoustic vibrations to be directed to the central region of the cable, where an optical waveguiding region can be located. In addition, because the two elements together have circular symmetry, the propagation of acoustic waves will also to a large extent be circularly symmetric. Finally, because the modulator is in contact with all or almost all (for example, more than 90%) of the of cable perimeter (when viewed in the axial direction), there will be a large coupling area, leading to a good acoustic coupling between the modulator and the cable. Although each component could be held in a respective holding block with a groove for holding the element, this could inhibit the effects of the circular symmetry of the system.

The configuration of the modulator is thought to be particularly efficient and useful for imposing a phase modulation on optical signals. The acoustic waves will be elastic waves, preferably bulk elastic waves, with a frequency of 3 or 4 kHz at the least, although this could extend to several MHz as is done in medical ultra-sound scanning heads.

If the fibre cable 14 is not accessible, for example if the fibre is within a duct, an arrangement similar to that of Figure 1 a can be used to apply acoustic vibrations to the duct. This is shown in Figure 2, where the fibre cable 14 will normally be acoustically coupled to the duct 163 by virtue of a mechanical contact with the inner wall of the duct, either directly or indirectly if one or more further cable lies between the cable and the duct wall. In addition, some coupling will also be achieved through any air between the cable and the inner duct wall.

Although in Figure 1 the two components 118a,b are removable from one another (such that they can preferably be moved feely relative to one another), they could be hinged at a hinge axis, or mounted in hinged holding blocks.

The electrical connections to the first and second components is shown in Figure 3. A driver unit 70 is provided comprising an oscillating voltage source 72 and a modulator driver 74 for generating modulation signals, which signals control the amplitude of the voltage source 70. The voltage source is connected on the one hand to the inner electrode 28b of each component 118a, 118b at respective connector portions 28b, and on the other hand to the outer electrodes 28c of the first and second components. In this way, an oscillating voltage can be applied across each of the respective piezoelectric layers of the first and second components, the voltage to one layer having the same frequency. However, instead, a further driver unit may be provided for one of the components (connected in Figure 3 by a dashed line), allowing the two components to be driven at different frequencies.

In another embodiment shown in Figure 4a, a modulator arrangement is shown where the first and second components are upper and lower blocks 118a, 118b each with a semicircular groove 202a,202b which form a bore for receiving a fibre cable 14 when the first and second bock are brought together as shown in Figure 4b. Upper and lower electrode layers (not shown) are provided on the respective upper and lower faces of each block to allow an electric field to be applied to each of the blocks, the blocks being formed from a piezoelectric material or similar suitable material. The electrodes are connected by electrical leads 204 to a releasable electrical connector 205 as shown in Figure 4c, such as a BNC connector mounted in a plaque 208 preferably mounted on a wall or other immovable object or surface. A driver unit 70 of the form shown in Figure 3 with a corresponding releasable electrical connector can then be temporarily connected to the modulator arrangement 10 in order to drive the modulator arrangement with a modulator signal.

A plurality of modulator arrangements 10 each with a releasable electrical connection may be provided at intervals along a fibre path 14 as shown in Figure 5, such at each attachment point, there is provided a modulator 10 connected to a respective electrical connector 205. A person may then releasably electrically connect a portable or hand held driver unit 70 to one attachment point, remove the drive, and subsequently connect it at a different attachment point in order to drive the modulator located at that new attachment point. Thus, a single driver may be used with differently located modulator arrangements. Likewise, a first and a second driver unit (for example carried by different people) can be used to drive the same modulator (at different times).

The transducer components 118a, 118b of Figure 4 need not be rectangular, but could instead have the form of a split ring as shown in Figure 1. In both the embodiments of Figures 1 and 4, the modulator can conveniently be retro fitted to a cable already laid along an existing path.

A monitoring system for use with the modulator above is described below.

Figure 7 shows a communications system in which a monitoring station 12 is configured to receive acoustically modulated signals which have been applied to an optical link 16 using a modulator arrangement 10. The modulator arrangement is preferably of the form shown in Figures 1 to 6, in which a transducer 118 applies an acoustic wave-like disturbance to the link 16, the wave-like disturbance being amplitude modulated with an information signal.

In more detail, the monitoring station 12 includes an optical source 18 with a short coherence time (random phase changes in the output providing an irregular component to the signal). Sensing signals (waveform portions) from the optical source 18 are fed to an interferometer stage 20, here a Mach Zehnder interferometer having a first path 24 and a second path 26. The interferometer 20 includes a first coupling stage 28 for coupling optical radiation between the optical source 18, the first and second paths 24, 26, and a signal processing system 29. For light travelling in a forward direction, that is, away from the source, the first coupling stage 28 acts as a directional power (intensity) splitter, channelling light from the optical source 18 to each of the paths 24, 26, the power to each path being shared in a predetermined manner, here in a 50:50 ratio.

For each signal provided by the optical source 18 in a given time interval, that signal is copied such that there is a first copy and a second copy, the first and second copies being duplicates of one another. One copy travels along the first path 24 whilst the other copy travels along the second path 26. A second coupling stage 130 is provided for coupling light between the first and second paths 24, 26 and an output 135 of the interferometer, which output is connected to the optical link 16. For light travelling in the forward direction, the coupling stage 130 acts as a combiner, combining the light from the first and second paths and channelling this combined light to the interferometer output 135. The first path of the interferometer has a delay stage 134 for increasing the transit time of light travelling therealong between the first and second coupling stages 28, 130, such that the transit time for light travelling between the coupling stages 28, 130 is longer along the first path 24 than it is along the second path 26. For each signal produced by the optical source, the interferometer 20 serves to delay one of the signal copies relative to the other signal copy, the signal copies being transmitted onto the link 16 at different times to one another.

The additional (differential) delay imposed by the delay stage 134 is much greater than the coherence time of the optical source 18. Thus, when light travelling along the first and second paths is recombined by the second coupling stage 130, the interference between light travelling along the two paths averages out, such that on average (over a timescale much greater than the coherence time) the amplitude of light upon recombination at the second coupling stage 130 is of constant amplitude 18.

An outstation 14 is provided at a far end of the fibre. Reflector means, such as a reflecting surface 132 are provided at the outstation 14 for returning signals to the base station 12. For signals travelling in the return direction, that is, for return signals arriving at the interferometer 20 from the outstation 14, the second coupling stage 130 acts as a power splitter, in a similar fashion to the action of the first coupling stage 28 on light in the forward direction from the optical source 18. In this way, return signals are copied at the second coupling stage 130, one copy being channelled along the first path 24, whilst the other copy is channelled along the second path 26. The first coupling stage 28 then serves to combine light from the first and second paths in the return direction, channelling the interference signal (resulting from the combined light) to a signal processing system 29.

For each signal generated by the source 18, there are thus four duplicates of this signal: a non-retarded signal SO which has travelled along the second path 26 of the interferometer 20 in both the forward and reverse directions; a first retarded signal S1 delayed by a delay D in the forward direction (but not the reverse direction); a second retarded signal S2 retarded by the delay D in the reverse direction (but not the forward direction); and, a twice-retarded signal S3 retarded by a delay 2D, signal S3 being retarded in each of the forward and reverse directions.

The first and second retarded signals S1 , S2 which are retarded in one direction only will return to the first coupler stage 28 at the same time. In the absence of any disturbance in the fibre 16, these signals are copies of one another and the signals will interfere or otherwise combine constructively at the first coupler stage 28. However, if one of the pair of signals S1 , S2 is modulated or otherwise modified by a disturbance along the fibre, the interference between the two signals will result in an interference signal having different spectral characteristics to the interference signal which would otherwise be produced in the absence of any disturbance to the fibre 16.

In the embodiments shown above, the transducer of the modulator arrangement generates a wave-like disturbance, which is coupled to an optical fibre. The result of this is that an elastic wave (preferably a bulk elastic wave) is launched into the transmission medium of the fibre. The elastic waves cause a local distortion of the glass structure, which changes the refractive index experienced by light travelling along the fibre. This change in refractive index caused a phase modulation in one or both of the (carrier) signals of a pair travelling along the link. However, the interference signal will be the result of interference between, on the one hand, a signal having been modulated by the disturbance at one time, and on the other hand, a signal modulated by the disturbance at another time, the two times being separated by the differential delay D. Thus, when an acoustic disturbance is applied to the optical link 16, the interference signal from the first coupling stage 28 will be a signal at the frequency of the applied acoustic disturbance. Likewise, any amplitude modulated applied to the acoustic signal will result in an interference signal with a corresponding amplitude modulation.

The frequency of the elastic wave may be a few kHz, but higher frequencies of a few MHz or more would allow for higher data rates.

The signal processing system includes: a photo-receiver 51 coupled to the first coupling stage 28 for converting optical signals into electrical signals; a filter 52 for receiving electrical signals from the photo-receiver 51 and filtering the electrical signals; and, a signal processing unit 54. If the information signal introduced by the modulator is an analogue signal, the processing unit 54 may simply be an amplifier, since the amplitude of the interference signal will vary in accordance with the amplitude of the applied acoustic signal. However, if the acoustic signal is amplitude modulated in a digital manner, the processing unit will be a digital system.

The light source may be a Light Emitting Diode, a Fabry-Perot Laser Diode, or a source of amplified spontaneous emission such as an Erbium-Doped Fibre Amplifier or a Semiconductor Optical Amplifier, but preferably the light source will be a Super Luminescent Diode, since this has a broad and smooth power spectrum, and a short coherence time of about 0.5 pico seconds. The radiation produced by the optical source will preferably be unpolarised, or alternatively a de-polarising unit 43 may be provided between the light source and the interferometer, for depolarising the light before the light is injected into the interferometer (the de-polarising unit may be for example, a Fibre Lyot de-polariser). A depolariser 49 will preferably be provided in one of the paths of the interferometer, here, the first path, so that the polarisation of light from the first path combining in the return direction at the first coupler 28 is at least partially aligned with that of the light from the other path. Typically, the source will operate at a wavelength of between 1 micron and 2 microns, preferably around 1.31 , 1.48 or 1.55 microns, in order to efficiently make use of standard telecommunications optical fibre, such fibre being configured to support single mode transmission at this wavelength. Typically, the fibre will have a single core of a diameter which is around 9 or 10 microns.

The first coupling stage 28, at the source side of the interferometer, will preferably be a 3x3 coupler (with some ports terminated), whereas the second coupling stage 130, at the transmission side, will preferably be a 2x2 coupler, with one port terminated. The 3x3 coupler is used in order to create a relative phase bias of 120° between the optical fields on its output ports. This can be used to improve the sensitivity of the interferometer.

Further details and further examples of embodiments are provided below, where the coupling arrangement is referred to as a "clip-on".

In order to couple vibrations to a fibre (or fibres) within a cable without having to break into that cable, the clip-on can be designed to clamp rigidly to the outer sheath of the cable. This approach lends itself to situations where the point of access to the fibre cable is in a man hole or surface access point, where fibre cables emerge from ducts on one side and enter ducts on the other side, with just a few metres of exposed cable between. Fig 1a illustrates an example design of a "clip-on" for this situation.

The design of the clip-on can take many forms, the common requirement being that it can be readily retro-fitted to the outer sheath of the fibre cable, and couple vibrations efficiently to the embedded fibre. The design of the piezoelectric (or similar) transducer allows vibrations to be focussed down on to the fibre at the core of the annulus. The coupling efficiency will be influenced by the properties of the fibre cable, which will vary from cable- type to cable-type. Nevertheless, the sensitivity of the Mach Zehnder monitoring system described above is helpful if week modulation is to be detected.

Coupling to a cable duct: in situations where the only point of access is the outer face of the duct which contains the fibre cable(s), then rather than breaking into the duct the solution is to attach an clip-on to the duct, as shown in Fig 2. Essentially this approach is a scaled up version of Fig 1 The coupling efficiency to the fibre may suffer from the possible non-central location of the cable relative to the annulus of the transducer, and the intervening air within the duct will also impact on coupling efficiency. Nevertheless, the high sensitivity of the Mach Zehnder monitoring station will be beneficial in detecting the modulation.

With reference to Figure 4, it may be commercially attractive to use a conventional electrical connector. This can be achieved by clamping the active transducer to the fibre, within the wall mounting, in which case the signal applied to the wall mounted connector is electronic rather than acoustic. By the designing the active transducer as two complementary blocks, they can be clamped to the fibre cable retrospectively with the flexibility described earlier for the split-wedge concept. Each transducer could be made from material, for example a piezo electric crystal, similar to that used in medical ultra- sound scanning heads. Each half block could be a transducer to maximise coupling to the fibre, or just one block with the other passive. Either way, electrical connections from the transducer(s) are brought out to a conventional electrical data connector, such as a BNC, SMA, or similar.

Claims

1. A modulator arrangement for acoustically modulating an optical link, the modulator arrangement including: a vibration element for generating acoustic vibrations, the vibration element having a layer region of uniform thickness, the layer region being curved in at least one direction so as to form an elongate recess for releasably receiving the optical link.

2. A modulator arrangement as claimed in claim 1, wherein the vibration element is removable in a radial direction to the optical link

3. A modulator arrangement as claimed in claim 1 or claim 2, wherein the layer region includes a curved layer of piezoelectric material, the curved layer of piezoelectric material having a uniform thickness.

4. A modulator arrangement as claimed in any of the preceding claims, wherein the layer region includes inner and outer electrode layers, each formed from an electrically conductive material, the layer of piezoelectric material residing between the first and second electrode layers.

5. A modulator arrangement as claimed in any of the preceding claims, wherein there is provided a second vibration element, the first and second vibration elements each being for generating acoustic vibrations, and each element having a respective arcuate receiving surface for receiving an optical link, the first and second elements being removable from one another.

6. A modulator arrangement as claimed in claim 5, wherein the receiving surfaces of the first and second elements are arranged such that when the first and second elements are brought together, the receiving surfaces form a bore in which the optical link can be received.

7. A modulator arrangement as claimed in any of the preceding claims, wherein the optical link is formed by an optical fiber cable, or a duct having one or more optical fiber cables therein.

8. A modulator arrangement as claimed in any of the preceding claims, wherein retaining means are provided for retaining the or each vibration element in a coupled state with the optical link, the retaining means having a resilient member.

9. A modulator arrangement as claimed in claim 8, wherein the retaining means is formed by a resilient clamp.

10. A modulator arrangement as claimed in any preceding claim wherein the optical link has a substantially circular cross section.

11. Communications apparatus including an optical link, a monitoring station for receiving sensing signals previously transmitted onto the optical link, and a modulator arrangement according to any preceding claims for modulating data onto the sensor signals, wherein the sensing signals are formed by pairs of signal copies, which copies of a pair have a time delay relative to one another, and wherein the monitoring station is configured to combine the respective signals of a pair so as to extract the modulated data.

12. Communications apparatus as claimed in claim 11 , wherein the monitoring station includes an optical source for transmitting the sensing signals onto the optical link, the sensing signals being returned along the link to the monitoring station.

13. Communications apparatus as claimed in claim 12, wherein reflector means are provided to return the sensing signals.

14. Communications apparatus as claimed in any of claims 11 to 13, wherein the time , delay is greater than the inverse of the frequency of the modulation.

15. Communications apparatus as claimed in any of claims 11 to 13, wherein the delay is at least 30 micro seconds, preferably at least 100 micro seconds.

16. Communications apparatus as claimed in any of claims 11 to 15, wherein the monitoring station includes interferometer means having a path difference associated therewith for introducing a time delay between signal copies of a pair.

17. Communications apparatus as claimed in claim 16, wherein the interferometer means is used to temporally re-align the returned signal copies of a pair.

18._a Communications apparatus as claimed in any of claims 12 to 17, wherein the optical source has a coherence time associated therewith, the coherence time being longer than the time delay between copies of a pair, preferably by at least a factor of 3, yet more preferably by at least a factor of 10.

19. A method of coupling a modulator arrangement to an optical link, the modulator having a first and second modulator portion, each portion having an arcuate receiving surface, including the step of bringing the first and second portions together around the link, so as to form a bore is in which the link is received.

20. A method as claimed in claim 10, wherein the first and second portions are separable from one another/

21. A method as claimed in claim 10 or claim 11 , wherein the first and second portions each include a curved layer of piezoelectric material, each layer being of uniform thickness, the curved layers each following an actuate contour.

22. A communication system including: an optical fibre link having a path associated therewith; a plurality of acoustic modulators located at spaced apart attachment points along the fibre path; wherein each attachment point has an electrical connector located thereat, the electrical connector at each attachment point being connected to a respective modulator element.

23. A communication system as claimed in claim 22, wherein a modulator driver is provided, the modulator driver having a corresponding electrical connector so that the drive can be temporarily electrically connected to a modulator in order to drive that modulator. 22. A method of using a communication arrangement which includes: an optical waveguide; a removable modulator driver for generating driving signals for acoustic vibrations which modulate light travelling along the optical waveguide; a plurality of attachment points along the waveguide at which the modulator driver can be attached, wherein the modulator driver is releasably connectable at the attachment points and is not connectable between access points, the method includes the steps of: connecting the modulator to an access point; causing acoustic vibrations such that the acoustic vibration cause a modulation of light traveling along the waveguide; and, removing the modulator means from the attachment point.

22. A method as claimed in claim 21 wherein the modulator driver is connected at another attachment point after having been removed from one attachment point.