Patent Description:
Optical communication is a known technique by which data signals can be transmitted from a transmitter to a receiver using optical fibre. Typically, an optical transmitter converts an electrical signal into an optical signal, which forms a carrier wave. The carrier wave is modulated with a modulation signal (i.e. the data), and is then transmitted along the optical fibre to a receiver, which converts the optical signal back into an electrical signal and recovers the transmitted data.

A single optical fibre can be used to transmit a large number of individual data streams, or channels, by transmitting radiation having a range of wavelengths (for example broadband radiation) through the optical fibre, and using a wavelength-division multiplexing (WDM) system to divide the radiation into discrete wavelengths or wavebands. A separate data stream can then be transmitted within each waveband.

In order to recover the data transmitted in one data stream (i.e. in one waveband) within an optical fibre, it is necessary to route the data stream out of the optical fibre to a receiver. One known method for achieving this is through the use of an optical add-drop multiplexer (OADM), which routes one data stream out of the optical fibre, and routes another data stream into the optical fibre to make use of the empty waveband.

An example of a system <NUM> using a known optical add-drop multiplexer (OADM) <NUM> is shown schematically in <FIG>. In the exemplary system <NUM>, wavelength-division multiplexing is used to divide the radiation passing through an optical fibre <NUM> into four discrete wavebands λ1, λ2, λ3 and λ4. Each waveband is capable of carrying a single data stream.

An optical fibre grating which, in this case is a fibre Bragg grating (FBG) <NUM>, is formed within the optical fibre <NUM>. A fibre Bragg grating is an optical instrument which can be configured to reflect radiation at a particular wavelength (or in a particular waveband) and to transmit radiation at all other wavelengths. In the example shown in <FIG>, the FBG <NUM> is configured to reflect radiation having wavelength within the waveband λ4, and transmit all other radiation, including the radiation having wavelengths within the wavebands λ1, λ2 and λ3. Therefore, while the data streams being transmitted in the wavebands λ1, λ2 and λ3 are able to travel along the optical fibre <NUM> through the FBG <NUM>, the data stream being transmitted in the waveband λ4 is reflected by the FBG back along the optical fibre in the direction from which it came.

The reflected radiation is fed into a first optical circulator <NUM> which routes the reflected radiation out of the optical fibre <NUM> to a receiver (not shown). The radiation that is routed out of the optical fibre <NUM> by the OADM <NUM> is known as a "dropped" path. Since the data stream that was in the waveband λ4 has been removed from the optical fibre <NUM>, it is possible to add a new data stream to be transmitted in the waveband λ4 (i.e. an "added path"). This is achieved by modulating a carrier wave in the waveband λ4, and feeding it into the optical fibre <NUM> after the FBG <NUM>. In order to provide a carrier wave at the desired wavelength (i.e. in waveband λ4), the known system <NUM> uses a laser (not shown). Radiation at the desired wavelength λ4 from the laser is fed into the optical fibre <NUM>, and a second optical circulator <NUM> routes the radiation along the optical fibre in the original direction of transmission.

A system such as the exemplary system <NUM> might be positioned at each node in an optical fibre communication network. Such a communication network might be installed in an aircraft, where optical fibre networks are preferred to electric cable networks due to their resilience to electromagnetic interference compared to electrical cables, and due to the fact that optical fibres are generally less heavy than shielded electrical cables. However, for each node in the network to be able to transmit data, each node would require a laser source which itself is very power intensive, and may require an active cooling system. Installing a laser source at each node in a network on an aircraft can negate the benefits provided by the lighter cables. <CIT> discloses a condition sensing apparatus/method which includes a sensor-fiber-grating that is subjected to a physical condition or measurand (strain/temperature/pressure) that is to be sensed. A broad-band light beam is directed into the core of the sensor grating, and the constructive-interference light beam that is reflected from this sensor grating is directed into a reference-fiber-grating. The reference grating is subjected to a servo-controlled elongation force or strain, and in this manner the length of the reference grating is varied until the reference grating operates to reflect a maximum intensity of its received light beam, and to transmit a minimum intensity of its received light beam. This maximum/minimum condition of the reference grating is sensed by detecting the minimum intensity of a light beam that is transmitted through the reference grating. The physical strain of the reference grating at which this condition occurs is used as a measure of the measurand to which the sensor grating is subjected. Sensing the mechanical resonant frequency of the reference grating provides an output that indicates the magnitude of the measurand currently at the sensor grating. International patent publication no. <CIT> discloses a simple, fiber-coupled, low-loss, and wavelength-selective semiconductor components that could be cascaded in series with minimal loss and connected in a simple manner. It comprises an optical fiber having a core (<NUM>) and a cladding (<NUM>) that supports cladding mode propagation, a long period grating (<NUM>) for coupling light between the core and the cladding and a semiconductor element (<NUM>) attached at a side of the fiber such that the element has optical coupling with the cladding mode. International patent publication no. <CIT> discloses a controlled fiberoptic filtering system comprising at least one tunable FBG filter engraved into a first optical fibre, the FBG filter comprising at least one grating with a chosen wavelength, inscribed in a chosen length of fibre, the grating providing reflections at least two wavelengths, a filter and a sensor wavelength. The system comprises: actuator means coupled to the filter for providing a change in the reflected wavelengths reflected by the grating, e.g. by providing a strain/compression and/or a temperature change to the grating, thus altering the filter wavelength, a light source coupled to the first optic fibre for providing an optic signal at a range of wavelengths comprising the sensor wavelength of the filter, wavelength readout unit coupled to first optic fibre for measuring the wavelength of optical signals reflected from the grating within the sensor wavelength, and control means coupled to the output of said wavelength readout unit and to said actuator means for positioning the reflected wavelengths or controlling the change in the wavelengths reflected from the grating, as provided by the actuator, and thus simultaneously the position or change in the filter wavelength.

The invention provides a communication system according to independent claim <NUM> and a method of communicating signals via an optical fibre according to independent claim <NUM>. Further embodiments are provided by the dependent claims.

There are numerous advantages of forming a communication system using optical fibre rather than shielded electrical cable. Optical fibre is generally smaller and lighter than shielded electrical cable. Nodes in an optical communication network can, therefore, also be smaller and lighter. Furthermore, a node which is able to transmit a signal without the use of a laser source to generate radiation at a particular wavelength can be produced for a lower cost than a node which includes a laser source. Such a node is also lighter in weight, and less power-intensive than a node requiring a laser source. If a node in an optical network fails, the other nodes in the network can continue to operate, since the signals transmitted to and from each node are carried in different discrete wavebands. Thus, embodiments of the present invention provide advantages over the known art.

Other features of the invention will be become apparent from the following description.

Some embodiments of the present invention are described in the context of optical communication systems on aircraft. It will be appreciated by those skilled in the art, however, that the described communication apparatus, methods and systems may be installed in systems other than aircraft. For example, the described systems may be used in buildings or other vehicles, such as motor vehicles.

Referring to the drawings, <FIG> shows an aircraft <NUM> installed with an optical communication system constructed in accordance with embodiments of the present invention. The aircraft <NUM> has a fuselage <NUM> and wings 204a and 204b. A cockpit <NUM> is located within a front end of the fuselage <NUM>, and a tail <NUM> is located at the rear end of the fuselage. An avionic system (shown schematically as <NUM>) is located within or near to the cockpit <NUM>, and is configured to communicate with and control components in the aircraft <NUM>.

The aircraft <NUM> includes an optical communication system via which the avionic system <NUM> is able to communicate with nodes 212a-e and 214a-c at different locations on the aircraft <NUM> via optical fibres <NUM> and <NUM>. In the example shown in <FIG>, the avionic system <NUM> is in communication with the nodes 212a-e via the optical fibre <NUM>, and is in communication with the nodes 214a-c via the optical fibre <NUM>. In this example, the nodes 212a-e are spaced along the length of the wing 204a, and each node may include one or more sensors to measure parameters such as, for example, temperature, pressure or strain at a leading edge of the wing. Similarly, the nodes 214a-c are, in this example, located near to the rear end of the fuselage <NUM>, and may each include one or more sensors configured to measure various parameters relating to the aircraft including, for example, air temperature, fuel temperature, fuel level, pressure, rotary position, linear position, landing gear load values, and landing gear strain. One or more of the nodes 212a-e and 214a-c could include individual sensors, or be used as communication nodes for transmitting data, as will be discussed below.

It will be appreciated that, while the exemplary communication system shown in <FIG> includes two optical fibres <NUM> and <NUM>, and eight nodes 212a-e and 214a-c, a typical communication system installed in an aircraft is likely to include a greater number of nodes and a greater number of optical fibres. However, it will also be appreciated that such systems may include fewer optical fibres and fewer nodes.

The exemplary communication system shown in <FIG> is configured to communicate data streams via the optical fibres <NUM>, <NUM> by modulating the data streams onto carrier waves, as will be explained below.

Modulation is the encoding of a signal, for example, a pattern, data, data stream or information to a media, for example radiation, in such a way that the encoded signal may be extracted or retrieved at another point in the media.

A source of radiation, such as light source <NUM> is, in this example, located near to the avionic system <NUM> in the aircraft <NUM>. In some embodiments, the light source <NUM> is a broadband light source which supplies broadband light to the nodes 212a-e, 214a-c via both of the optical fibres <NUM> and <NUM>. In some embodiments, the source of radiation <NUM> might include a different type of light source, such as a swept wavelength laser source but, generally, the communication system requires a radiation source capable of generating radiation at multiple wavelengths. It will also be appreciated that while, in this example, the light source <NUM> is located near to the front of the aircraft <NUM> with, near to or forming part of the avionic system <NUM>, the light source may alternatively be located elsewhere on the aircraft.

A single light source <NUM> may be provided to generate light for all of the optical fibres within the communication system. Alternatively, two or more light sources may be provided, and located together on the aircraft <NUM> or separately, at different locations on the aircraft. Advantageously, the communication system described herein is such that the light source <NUM>, or light sources, can be located at a location remote from the nodes <NUM>, <NUM>. Therefore, the light source <NUM> can be located, for example, in a protected area of the aircraft, such as an avionics bay. By locating the light source <NUM> in an area of the aircraft <NUM> where other electronics systems are located, a single cooling system may be used to cool multiple electrical components, rather than installing a cooling system for each electrical component.

Each node <NUM>, <NUM> in the communication system may be configured to function as a primitive passive sensor (arranged to measure, for example, temperature or pressure), as a receiver, as a transmitter, or as a sensor and/or a receiver and/or a transmitter. The transmit and receive functionality possible within the nodes will now be discussed with reference to <FIG> which show, schematically, nodes of a communication system constructed in accordance with various embodiments of the invention.

<FIG> and <FIG> show arrangements in which nodes can function as receivers. <FIG> shows a portion of an optical fibre <NUM> having a core <NUM> and cladding <NUM>. A light source <NUM> is located at an end of the optical fibre <NUM>, and is configured to generate broadband (white) light for transmission along the optical fibre. The light source <NUM> may be the same as the light source <NUM> shown in <FIG>. The broadband light emitted by the light source <NUM> is divided into multiple wavebands using known wavelength division multiplexing (WDM) techniques. In some embodiments, a coarse-WDM technique may be employed to divide the broadband light into around sixteen to twenty wavebands, or channels. In other embodiments, a fine-WDM technique may be employed to divide the broadband light into around thirty wavebands, or channels. Alternatively, different WDM techniques may be used to divide the light into a different number of wavebands or channels. It will be appreciated that, by using different WDM techniques, the broadband light from the light source <NUM> may be divided into more or fewer wavebands, as required. For clarity, the light emitted by the light source <NUM> in this embodiment has been divided into four wavebands. The light within each individual waveband may act as a carrier wave via which a data signal may be transmitted.

A data signal to be transmitted along the optical fibre <NUM> is modulated onto a carrier wave using techniques that will be well known to those skilled in the art. A resulting modulated data stream can then be transmitted along the optical fibre <NUM>. In the embodiments discussed in <FIG> and <FIG>, data streams are transmitted along the optical fibre <NUM> in the two wavebands labelled λ1 and λ2, and the nodes shown in <FIG> and <FIG> are configured to function as receivers.

In the embodiment shown in <FIG>, a node <NUM> is shown in which an optical fibre grating is formed. In this embodiment, the grating is a tilted fibre Bragg grating (FBG) <NUM>, which is formed within the core <NUM> of the optical fibre <NUM>. FBGs can be created in optical fibres using techniques that will be known to those skilled in the art, such as laser etching or inscribing. Generally, FBGs are configured to reflect radiation at a particular wavelength (or in a particular waveband) and to transmit radiation at all other wavelengths. Therefore, when white light travels along an optical fibre in one direction and reaches an FBG, a component of that white light is reflected back along the optical fibre in the opposite direction. The tilted FBG <NUM> reflects a component of the light (in this case, the light in the waveband λ2) out of the core <NUM> and through the cladding <NUM> of the optical fibre <NUM>. A receiver <NUM> adjacent to, near to, or around the optical fibre <NUM> receives the reflected light, and can be used to recover the data from the data stream. The receiver <NUM> may be a photodetector, such as a photodiode, used to detect a signal transmitted, for example, from the avionics bay or cockpit of an aircraft. The transmitted signal may, for example, be an instruction to a particular component, such as to turn a component on or off. Alternatively, the transmitted signal may be used, for example, to activate an in-line switch.

<FIG> shows a node <NUM> constructed according to an alternative embodiment, in which a long-period FBG <NUM> is formed within the core <NUM> of the optical fibre <NUM>. The long-period FBG <NUM> affects only radiation having a wavelength within a particular waveband (in this case, the component of light in the waveband λ1), and transmits radiation at all other wavelengths. Some modes from the core <NUM> of the optical fiber <NUM> (the "core modes") couple with modes from the cladding <NUM> (the "cladding modes"). As a result, the component of light in the waveband λ1 and, therefore, the data stream transmitted in this waveband, is coupled outside the core <NUM> and, therefore, travels into the cladding <NUM> of the optical fibre <NUM>. A receiver <NUM> adjacent to, near to, or around the optical fibre <NUM> receives the reflected light, and can be used to recover the data from the data stream. As with the receiver <NUM>, the receiver <NUM> may be a photodetector, such as a photodiode or phototransistor, used to detect a signal transmitted, for example, from the avionics bay or cockpit of an aircraft. To aid transmission of radiation through the cladding <NUM> of the optical fibre <NUM>, a coating may be applied to at least a portion of the optical fibre. Alternatively, the optical fibre <NUM> may be treated in some other way, or part of the cladding <NUM> may be removed.

The arrangements shown in <FIG> and <FIG> demonstrate how the use of FBGs allows nodes of an optical communication system to be used as receivers. The present invention also provides a mechanism by which a node can function as a transmitter, such that data can be transmitted from the node without the use of a laser source.

The wavelength at which an FBG reflects radiation is determined by the structure of the grating or, more specifically, by the spacing or period of the grating. Changing the structure of an FBG, for example by increasing or decreasing the grating spacing, changes the wavelength at which the FBG reflects radiation. The structure of an FBG can be changed or manipulated in a number of ways. For example, changing the temperature of the FBG can cause the grating spacing to change, with the result that radiation of a different wavelength will be reflected by the FBG. Another way of changing the structure of an FBG is to apply a force or strain to the FBG, for example by using a mechanical device to deform the grating through application of a tensile or a compressive force. In this way, the grating spacing is changed, resulting in a change in the wavelength at which the grating reflects radiation.

The effect of changing the grating spacing in an FBG on the wavelength at which radiation is reflected by the FBG is shown schematically in <FIG>. In <FIG>, an FBG <NUM> is in an unstrained form and, in this form, the FBG reflects radiation having a wavelength λa. <FIG> shows the FBG <NUM> under strain. In this form, the FBG <NUM> reflects radiation having a wavelength λb, different to λa. Thus, by controlling the amount of strain applied to an FBG, it is possible to accurately control the wavelength at which radiation is reflected by the FBG.

<FIG> shows an embodiment of the invention in which a node <NUM> can be used to transmit a signal. In the embodiment shown in <FIG>, an FBG <NUM> is formed within the core <NUM> of the optical fibre <NUM>. The FBG <NUM> is configured to reflect radiation having a wavelength within in the waveband λ3, and transmit all other radiation, including the radiation having wavelengths falling within the wavebands λ1, λ2 and λ4.

A receiver <NUM> is located adjacent to or near to the light source <NUM>, at the end of the optical fibre <NUM>. An advantage of this arrangement, as is discussed above, is that the light source <NUM> and the receiver <NUM> can be located near to each other, for example in the avionics bay of an aircraft, remote from the nodes.

As with the embodiments described above, the light source <NUM> in this embodiment is configured to generate broadband light (e.g. white light). Light having wavelengths falling within the wavebands used to transmit data streams to nodes functioning as receivers may be modulated with data. However, the light having a wavelength falling within the waveband λ3 is not modulated with data. Instead, the white light is transmitted along the optical fibre <NUM> to the FBG <NUM> where the component of the white light within the waveband λ3 is reflected.

A mechanism or instrument <NUM> for manipulating, modifying or modulating the structure of the FBG <NUM> is coupled to the FBG. The instrument <NUM> may be any means suitable for controllably modifying or modulating the grating spacing of the FBG <NUM> including, but not limited to, a mechanical device, an electrical device, a current-inducing coil, a motor, such as a linear solenoid motor, or a magnetic field source, such as a coil, a radio frequency (RF) transmitter or a microwave transmitter, for generating a magnetic field to interact with a magneto-restrictive coating applied to a portion of the optical fibre containing the FBG. An example of a suitable instrument <NUM> is discussed below with reference to <FIG>. In general, however, the instrument <NUM>, and its effect on the FBG <NUM>, may be accurately controlled and, accordingly, it is possible to accurately control the wavelength of radiation that is reflected by the FBG, back along the optical fibre <NUM>, to the receiver <NUM>.

Using the arrangement shown in <FIG>, a signal having a particular wavelength can be transmitted by the node <NUM> to the receiver <NUM>. The receiver <NUM>, or a computer or processor associated with the receiver, may be programmed with threshold values of wavelengths or discrete bands of wavelengths so that, if a signal is received from the node <NUM> above a particular threshold value, or falling within a particular wavelength band, it can be interpreted accordingly. For example, a received signal having a wavelength falling above a threshold value may be interpreted as a "<NUM>" or "ON" response, indicating that a particular component associated with that node is in an "ON" or active state. A received signal having a wavelength falling below the threshold value may be interpreted as a "<NUM>" or "OFF" response, indicating that a particular component associated with that node is in an "OFF" or inactive state.

According to various embodiments of the invention, one or more instruments may be used to varying parameters of the grating, or to affect the grating in different ways. For example, in one embodiment, the instrument is configured to controllably apply pressure to the FBG, thereby modulating the structure of the grating. In another embodiment, the instrument is configured to controllably apply a vibration to the FBG to modulate the grating's structure. In yet another embodiment, the instrument is configured to controllably vary the temperature of the FBG, and/or of the optical fibre which houses the FBG. Varying the temperature of the FBG can change the structure of the grating.

<FIG> shows an embodiment of the invention in which the instrument <NUM> is a mechanical instrument capable of applying a strain to the FBG <NUM>. In this embodiment, the instrument <NUM> includes a first end <NUM> mechanically coupled to the optical fibre at one end of the FBG <NUM> and a second end <NUM> mechanically coupled to the optical fibre at the other end of the FBG. The instrument <NUM> includes a coil <NUM> connected to a data source, for example a current source <NUM>. The arrangement is analogous to a speaker coil arrangement. When a current is applied to the coil, a motive force is induced in the instrument, causing the first and second ends <NUM>, <NUM> of the instrument to move relative to one another. This movement causes a strain to be applied to the FBG <NUM>, thereby modifying the grating spacing.

In another embodiment (not shown), a magneto-restrictive coating is applied to the FBG, or to the portion of the optical fibre that houses the FBG. Magneto-restrictive materials change their shape or dimensions when they are subject to a magnetic field. The instrument includes means for generating a magnetic field, such as a radio frequency (RF) transmitter or a microwave transmitter. A controllable magnetic field is generated and, when the FBG is positioned in the magnetic field, the magneto-restrictive coating changes its shape, causing the structure of the FBG to be modified. By accurately controlling the strength of the magnetic field (for example by controlling a supply voltage or current), it is possible to accurately control the strain induced on the FBG and, therefore, the change in the grating spacing of the FBG.

<FIG> shows an embodiment of the invention in which a light source <NUM> is located at one end of the optical fibre <NUM>, along with a receiver <NUM>. Four FBGs are formed within the core <NUM> of the optical fibre <NUM>. FBGs <NUM> and <NUM> are similar to the FBG <NUM> described above, in that their structures can be controllably manipulated by instruments <NUM> and <NUM> respectively. The FBG <NUM> is configured to reflect light having a wavelength falling within the waveband λ4, and FBG <NUM> is configured to reflect light having a wavelength falling within the waveband λ3. Tilted FBG <NUM> is similar to the tilted FBG <NUM> described above, and is configured to reflect light having a wavelength falling within the waveband λ2 towards a receiver <NUM>. The long-period FBG <NUM> is similar to the long-period FBG <NUM> described above, and is configured to reflect light having a wavelength falling within the waveband λ1 towards a receiver <NUM>. In this embodiment, the FBG <NUM> and the tilted FBG <NUM> are located near to one another on a first node, and the FBG <NUM> and the long-period FBG <NUM> are located near to one another on a second node. Thus, at the first node, a signal may be received by the receiver <NUM> and, in response to the received signal, a response signal may be transmitted to the receiver <NUM> from the FBG <NUM>, using the instrument <NUM> to control its grating spacing. Similarly, at the second node, a signal may be received by the receiver <NUM> and, in response to the received signal, a response signal may be transmitted to the receiver <NUM> from the FBG <NUM>, using the instrument <NUM> to control its grating spacing. Alternatively, the signal received at the receivers <NUM>, <NUM> may be in response to the signal transmitted to the receiver <NUM> from the FBGs <NUM>, <NUM> resulting from the modulation by the instruments <NUM>, <NUM>.

By using the apparatus and system described above, it is possible to transmit both digital and analogue data from a node in an optical fibre, without the use of a laser source or electrically powered components. <FIG> show examples of possible responses from a node functioning as a transmitter, as described above.

<FIG> shows a first set of responses. In this example, a data signal generated by a binary node can represent an "ON" state or an "OFF" state. The receiver interprets a response <NUM> (which has a wavelength falling within a waveband λa) as meaning "ON", and interprets a response <NUM> (which has a wavelength falling within a waveband λb) as meaning "OFF". A response having any other wavelength might be interpreted as an error message, suggesting that something unexpected has happened at the node, such as a short circuit or a power surge.

<FIG> shows a second set of responses. In this example, the receiver scans for a response <NUM> falling within a single waveband λc. A response within the waveband λc would be interpreted as meaning "ON", while a response such as response <NUM>, which has a wavelength falling outside of the waveband λc, or a lack of a response at all, would be interpreted as meaning "OFF".

<FIG> shows a third set of responses. In this example, a wider waveband is scanned and the actual response wavelength is determined in an analogue way between a minimum λd_min and maximum point λd_max. Responses <NUM> and <NUM> therefore represent analogue points on a continuous scale and may represent non-binary data such as a current measurement or samples in an audio signal. Such an audio signal may be the result of analogue passive components and/or energy harvesting techniques, allowing zero-power data communication to be made from the node.

The invention also provides a method for communicating a signal via an optical fibre. <FIG> is a flow diagram detailing the steps of the method <NUM>.

At step <NUM>, an optical fibre is provided. The optical fibre includes an optical fibre grating. In this embodiment, the grating is fibre Bragg grating (FBG) which is formed within the core of the optical fibre. At step <NUM>, a light source may be used to transmit broadband radiation along the optical fibre towards the FBG. At step <NUM>, an instrument that is coupled to the FBG is operated to controllably manipulate the structure of the FBG. At step <NUM>, radiation having a particular wavelength is reflected by the FBG, modulated by the instrument, and transmitted back along the optical fibre to a receiver.

Those skilled in the art of optical communications will appreciate that the apparatus, methods and systems described herein may be used to communicate digital data signals or analogue data signals. It will also be appreciated that, while fibre Bragg gratings (FBGs) have been referred to in some specific embodiments, other types of optical fibre gratings known to those skilled in the art could be used.

Claim 1:
A communication system, comprising:
an optical fibre (<NUM>) along which radiation can be transmitted;
a light source (<NUM>) for generating radiation to be transmitted along the optical fibre, wherein the light source is a swept-wavelength laser or the light source is configured to generate broadband radiation;
a first node comprising:
a first optical fibre grating (<NUM>) formed within the optical fibre, the first optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and
a first instrument (<NUM>) coupled to the first optical fibre grating and configured to receive a data signal and controllably modify the structure of the first optical fibre grating in response to the received data signal, thereby changing the wavelength at which the first optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal;
a second node comprising:
a second optical fibre grating (<NUM>) formed within the optical fibre, the second optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and
a second instrument (<NUM>) coupled to the second optical fibre grating and configured to receive a data signal and controllably modify the structure of the second optical fibre grating in response to the received data signal, thereby changing the wavelength at which the second optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; and
a receiver (<NUM>) configured to receive first radiation reflected by the first optical fibre grating and second radiation reflected by the second optical fibre grating, wherein the light source and the receiver are located at a first end of the optical fibre (<NUM>), and wherein the receiver (<NUM>), or a computer or processor associated with the receiver (<NUM>), is programmed with threshold values of wavelengths or discrete bands of wavelengths so that, if a signal is received from the first or second node above a particular threshold value, or falling within a particular wavelength band, it can be interpreted accordingly.