Apparatus and method for testing a fibre optic telecommunications network

Abstract

An apparatus (170) for use in performing diagnostics upon a fibre optic cable (180) in a fibre optic network (100), said apparatus comprising: a reflector (240); a dichroic filter (220) configured to output, from input light transmitted through the fibre optic cable: a reflected component; and a transmitted component; an optical switch (230), arranged to receive the transmitted component and to interface with the reflector, and configured to perform switching between optically connecting and disconnecting the reflector to and from the transmitted component; and a photovoltaic, PV, power supply (250), arranged to receive the reflected component, and electrically connected with the optical switch, thereby to generate power from the reflected component and to power the switching of the optical switch. There is also provided a fibre optic network (100) and a method (300) of operating said network.

Description

[0001] APPARATUS AND METHOD FOR TESTING A FIBRE OPTIC TELECOMMUNICATIONS

[0002] NETWORK

[0003] Field of Invention

[0004] The present invention relates to an apparatus for use in testing a fibre optic telecommunications network.

[0005] Background

[0006] A Passive Optical Network (PON) is a fibre-optic telecommunications technology for delivering broadband network access to end-customers utilising a point-to-multipoint topology is implemented, in which a single optical fibre serves multiple endpoints by using unpowered ( / .e. passive) fibre optic splitters to divide the fibre bandwidth among the endpoints via drop fibres.

[0007] An Optical Time-Domain Reflectometer (OTDR) is an optoelectronic instrument used to characterise an optical fibre network. OTDR works by sending a short pulse of light into the optical fibre network and measuring the amount of light that is reflected back. The time it takes for the light to travel to the reflection point and back is used to calculate the distance to the reflection point.

[0008] Distinguishing individual fibre optic cables beyond the optical splitter towards the customer-side termination is difficult using OTDR since there may be no distinguishing feature to such fibre optic cables. Furthermore, even taking effective OTDR measurements beyond the optical splitter may be difficult since the optical loss as a result of the splitter may be too great to detect drop fibres. Furthermore, because of the passive nature of PONs, selective testing of individual such fibre optic cables is also challenging, and may typically require expensive apparatus, such as fibre Bragg gratings.

[0009] It is an aim of the present invention to at least alleviate some of the aforementioned problems.

[0010] Statements of Invention

[0011] According to a first aspect of the present invention, there is provided: an apparatus for use in performing diagnostics upon a fibre optic cable in a fibre optic network, said apparatus comprising: a reflector; a dichroic filter configured to output, from input light transmitted through the fibre optic cable: a reflected component; and a transmitted component; an optical switch, arranged to receive the transmitted component and to interface with the reflector, and configured to perform switching between optically connecting and disconnecting the reflector to and from the transmitted component; and a photovoltaic, PV, power supply, arranged to receive the reflected component, and electrically connected with the optical switch, thereby to generate power from the reflected component and to power the switching of the optical switch.

[0012] Preferably, the optical switch is biased to disconnect the reflector from the transmitted component when the optical switch is unpowered. Preferably, the PV power supply is therefore configured only to power the switching to connect the reflector to the transmitted component.

[0013] Preferably, the apparatus comprises a port for connecting the dichroic filter to an output of a banddrop filter coupled to the fibre optic cable, wherein the input light is received via the port and from the band-drop filter. Optionally, the apparatus comprises the band-drop filter. Optionally, the band-drop filter is configured to output, to the dichroic filter, light at a service signal wavelength and / or a diagnostic test signal wavelength. Preferably, the service signal wavelength is used to communicate a telecommunications service to a user. Preferably, the service signal operates at a service wavelength, and wherein the service wavelength is between 1200nm and 1650nm. Preferably, the diagnostic test signal wavelength does not carry signals for providing a telecommunications service to a user, and is therefore merely for testing and diagnostics, and is operated at a diagnostic test wavelength. Preferably, the diagnostic test wavelength is within the U-band or C-band, as defined by the ITU Telecommunication Standardization Sector. Preferably, the diagnostic test wavelength is between 1200nm and 1700nm, more preferably between 1500nm and 1650nm, and still more preferably between 1525nm and 1575nm.

[0014] Preferably, the fibre optic network comprises: a plurality of Optical Network Units; a splitter; and a plurality of distribution fibre optic cables, each of said cables extending between the splitter and one of the plurality of Optical Network Units; and wherein the apparatus comprises a connector for connecting (and, may be, connected) to one of the plurality of distribution fibre optic cables and downstream of the splitter. Preferably, the plurality of distribution fibre optic cables comprises a drop cable and / or feeder cable. Preferably, the apparatus is connected upstream of an Optical Network Unit (ONU).

[0015] Preferably, the optical filter and / or further optical filter has / have a reflectivity that is substantially independent of temperature.

[0016] According to another aspect of the invention, there is provided a fibre optic network comprising an apparatus as described above. Optionally, the fibre optic network is a Passive Optical Network (PON). Preferably, the fibre optic network comprises at least two of the apparatuses, and wherein: the at least two apparatuses are connected to different distribution fibre optic cables; and each dichroic filter of each of the at least two apparatus is configured to filter at a different wavelength. Preferably, the at least two apparatus are connected to the same fibre optic cable. Preferably, said different wavelength is, in part or wholly, non-overlapping with each other wavelength. Optionally, each band-drop filter, associated with each distribution fibre optic cable, is configured to filter at a different wavelength.

[0017] According to another aspect of the invention, there is provided a method of operating a fibre optic network, said network comprising: an Optical Time-Domain Reflectometer, OTDR; a plurality of ONUs; and at least one apparatus as described above and / or below, wherein said at least one apparatus is coupled to a fibre optic cable serving at least one of the plurality of ONUs; the method comprising the steps of: transmitting, by the OTDR, a diagnostic signal to the plurality of ONUs, wherein the diagnostic signal is configured to switch the at least one apparatus so as to optically connect the reflector with the transmitted component; and measuring, at the OTDR, a reflection from the reflector.

[0018] Preferably, when the network comprises at least two of the apparatuses, and when said apparatuses are connected to different ONUs and are configured to perform the optical switching at different (and more preferably, non-overlapping) wavelengths of diagnostic signal (or “input light”), operating the OTDR so as to transmit a diagnostic signal configured to perform the optical switching of a subset ( / .e. less than the whole) of the at least two apparatuses, and more preferably only one of said apparatuses. Preferably, the method comprises the further step of (optionally, after measuring a reflection from the subset) transmitting a further diagnostic signal, said signal being configured to perform the optical switching of an apparatus from the at least two apparatuses that does not form part of the subset.

[0019] A computer-readable carrier medium comprising a computer program, which, when the computer program is executed by a computer, causes the computer to carry out the method described above.

[0020] Preferably, the reflector is a retroreflector or bi-directional. Preferably, the reflector is a broadband reflector. Optionally, the dichroic filter, and / or further dichroic filter, is / are: interferometric, holographic, or thin-film. Preferably, the dichroic filter is a channel drop filter. Preferably, the dichroic filter is configured to have a transmit and / or reflect wavelength that is substantially independent of temperature, and more preferably a response of less than 0.2pm / °C over a temperature range of -40 °C to +85 °C. Preferably, the PV power supply is arranged to receive only the reflect component and not the transmitted component. Preferably, the PV power supply is remotely powered, via the reflected component, from the input light. Preferably, the input light is provided by an Optical Time-Domain Reflectometer, Optical Frequency-Domain Reflectometer and / or an Optical Line Terminal. Preferably, the PV power supply comprises, or consists of, a photovoltaic cell. Preferably, the apparatus comprises a further dichroic filter arranged between the dichroic filter and the PV power supply, thereby to intercept the reflected component, and to output a portion of the reflected component to the PV power supply. Preferably, the dichroic filter and / or the further dichroic filter is / are a channel drop filter. Preferably, the dichroic filter is configured to filter light at a first wavelength, and the further dichroic filter is configured to filter light at a second wavelength. Optionally, the first wavelength and the second wavelength are the same or different. Optionally, the first wavelength may comprise, wholly or in part, the second wavelength. Preferably, the dichroic filter and the further dichroic filter are arranged in series, or are “daisy-chained”.

[0021] The invention includes any novel aspects described and / or illustrated herein. The invention also extends to methods and / or apparatus substantially as herein described and / or as illustrated with reference to the accompanying drawings. The invention is also provided as a computer program and / or a computer program product for carrying out any of the methods described herein and / or for embodying any of the apparatus features described herein, and a computer-readable medium storing thereon a program for carrying out any of the methods and / or for embodying any of the apparatus features described herein. Features described as being implemented in hardware may alternatively be implemented in software, and vice versa.

[0022] Any apparatus feature may also be provided as a corresponding step of a method, and vice versa. As used herein, means plus function features may alternatively be expressed in terms of their corresponding structure, for example as a suitably-programmed processor.

[0023] Any feature in one aspect of the invention may be applied, in any appropriate combination, to other aspects of the invention. Any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. Particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently.

[0024] As used throughout, the word 'or' can be interpreted in the exclusive and / or inclusive sense, unless otherwise specified.

[0025] The invention extends to an apparatus, a telecommunications network and a method of operating telecommunications network as described herein and / or substantially as illustrated with reference to the accompanying drawings. The present invention is now described, purely by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows, schematically, an exemplary fibre optic telecommunications network comprising a diagnostics apparatus;

[0026] Figure 2 shows a schematic detail of the diagnostics apparatus;

[0027] Figure 3 shows a process of operating the fibre optic telecommunications network; and

[0028] Figures 4 shows schematic examples of Optical Time-Domain Reflectometer (OTDR) traces.

[0029] Figure 1 is a schematic diagram of a fibre optic telecommunications network 100, such as for providing wide-area fixed-access broadband network services.

[0030] The network 100 comprises a / an: Optical Line Terminal (OLT) 110; Optical Time Domain Reflectometer (OTDR) 120; OTDR switch 130; WDM splitter 140; broadband optical splitter (or power splitter) 150; plurality of Optical Network Units (ONUs) 160, including a first 160-1 , second 160-2, third 160-3 and fourth 160-4 ONU; and plurality of diagnostics apparatus 170, including a first 170-1 , second 170-2, third 170-3 and fourth 170-4 diagnostics apparatus associated with each correspondingly numbered ONU.

[0031] The aforementioned components are interconnected using optical links provided using, at least, fibre optic cables (represented as arrowed lines) so as to provide telecommunications services between, at least, the OLT 1 10 and each of the ONUs 160, and diagnostic test services between, at least, the OTDR 120 and the same ONUs 160 and / or each of the plurality of diagnostics apparatuses 170.

[0032] The OLT 1 10 is located at the head-end of the network 100, for example where a local exchange is sited, and is connected to the plurality of ONUs 160 via the WDM splitter 140 and the power splitter 150 (in which the former is located upstream of - that is, sequentially closer than - the latter). Each ONU is connected to the power splitter 150 via a dedicated corresponding distribution (also referred to as a “drop” or “feeder”) fibre optic cable 180; that is, a first optic cable 180-1 for connecting ONU 160-1 , a second fibre optic cable 180-2 for connecting ONU 160-2, a third fibre optic cable 180-3 for connecting ONU 160-3, and a fourth fibre optic cable 180-4 for connecting ONU 160-4.

[0033] The OLT 1 10 and the OTDR 120 are operatively connected to a higher-layer network management software application (not shown) comprising an Element Management System (not shown) and / or a Network Management System (not shown). For example, via the network management software application, a user or the network 100 may trigger a diagnostic test to be performed by the OTDR 120. The OTDR 120 is an optical reflectometry measuring device for the network 100. The OTDR 120 enables discovery of physical discontinuities ( / .e. breaks in the fibre, faulty connections and splices, excessive fibre bends and other structural deformities) in an optical link.

[0034] The OTDR 120 is operated by sending a diagnostics signal down the network 100, this results in backscattering and reflections due to the presence of discontinuities in the fibre structure. By measuring the time that the signal takes to return to the OTDR (from which, distance may be inferred) and the magnitude of the received signal relative to the transmitted signal, a suitably configured OTDR can help determine the location, nature and extent of physical discontinuities. For example, a drop in backscatter to a noise floor may indicate a full break, whereas a small reflection may indicate a connector with an air gap.

[0035] In the example of Fig. 1 , the OTDR 120 is a standalone device separate to the OLT 110, and the OTDR connects to the plurality of ONUs 160 using, at least, the same spine network as the OLT ( / .e. via the WDM splitter 140 and power splitter 150).

[0036] Each diagnostics apparatus 170 is coupled to one of the distribution fibre optic cables 180.

[0037] Figure 2 exemplarily shows the first diagnostics apparatus 170-1 in detail, which is connected, via a band drop filter 210, to the first distribution fibre optic cable 180-1 that serves the first ONU 160-1.

[0038] The band drop filter 210, which is in the form of a dichroic filter, is arranged in-line with the first fibre optic cable 180-1. The band drop filter 210 allows a specific band of frequencies to pass through (e.g. for delivering service signals) to the associated first ONU 160-1 as part of a pass- through limb, while also reflecting another band e.g. for diagnostic signals) as part of a reflected limb (shown in Figure 2 as a dashed line emanating from the band drop filter 210).

[0039] In turn, the first diagnostics apparatus 170-1 comprises a: channel drop filter 220; optical switch 230; reflector 240; and a photovoltaic (PV) power supply 250.

[0040] The channel drop filter 220 is optically connected to the band drop filter 210 so as to receive, as an input, the reflected limb.

[0041] The channel drop filter 220 is also a dichroic filter. As such, with an input at an appropriate spectral band (as described in more detail below), the channel drop filter outputs a: reflected component (shown as a dotted line in Figure 2); and a transmitted component (shown as a dash-dot line in Figure 2). The channel drop filter 220 is in turn optically connected to the optical switch 230 and the PV power supply 250 so as to output the transmitted component to the optical switch and the reflected component to the PV power supply.

[0042] The PV power supply 250 comprises a photovoltaic cell and a battery (not individually shown for conciseness). The PV power supply 250 is configured to generate and then store electrical energy from the reflected component, as input from the channel drop filter 220. The PV power supply 250 is electrically connected to the optical switch 230, such that switching of the optical switch (as discussed in more detail below) is powered by the PV power supply.

[0043] The optical switch 230 is configured to perform switching, as powered by the PV power supply 250, so as to connect and disconnect an optical path between the channel drop filter 220 and the reflector 240. As such, when the optical path is connected, the transmitted component, as received at the optical switch from the channel drop filter, is transmitted on to the reflector, and when the optical path is disconnected, the transmitted component is not transmitted on to the reflector. The optical switch 230 is available to be in the form of a: membrane switch, Micro- ElectroMechanical System (MEMS) switch or acousto-optic switch, electro-optic switch. When unpowered, the optical switch 230 is biased to disconnect the optical path to the reflector 240. However, when, and whilst, powered, the optical switch connects the optical path to the reflector.

[0044] The reflector 240 is a broadband retroreflector. The optical switch 230, channel drop filter 220 and band drop filter 210 all operate bi-directionally. In this way, when the optical switch connects the optical path to the reflector, light incident upon the reflector ( / .e. the transmitted component) is reflected back through the optical switch, channel drop filter and band drop filter, and then on to the OTDR 120. In this way, the OTDR is available to interact with the reflector 240 when performing diagnostic measurements.

[0045] The aforementioned components of the diagnostics apparatus 170-1 are configured to operate ( / .e. switch the reflector in and out-of-path) using a diagnostic signal from the OTDR 120. In this example, the diagnostic signal has, at least, the following characteristics:

[0046] 1 . comprises a range of wavelengths that are: a. reflected by the band drop filter 210, thereby producing the reflected limb for input into the channel drop filter 220; b. subsequently reflected by the channel drop filter 220, thereby producing the reflected component for input into the PV power supply 250; c. transmitted by the channel drop filter 220, thereby producing the transmitted component for input to the reflector 240 via the optical switch 230; d. within the spectral response of the PV power supply 250 so as to energise said component; and e. reflected by the reflector 240; and

[0047] 2. has sufficient intensity to energise the PV power supply 250 and to be detectable by the OTDR having made a round-trip to the reflector 240.

[0048] The diagnostic signal is further configured not to carry signals for providing a telecommunications service to the ONUs 160, but only for testing and diagnostics; this is in contrast to a service signal, which is used to communicate a telecommunications service to the ONUs 160. The diagnostic signal and the service signal are spectrally non-overlapping.

[0049] By matching the diagnostic signal to the filtering wavelengths of the band drop filter 210, the service signal is wholly, and only, output as the transmitted limb, whereas the diagnostic signal is only output as the reflected limb. Therefore, by connecting one of the diagnostics apparatus 170- 1 to the distribution fibre optic cable 180-1 via the band drop filter 210, the diagnostics apparatus 170-1 is capable of receiving input light ( / .e. the reflected limb), whilst also allowing for the transmission of light on to the associated ONU 160-1 ( / .e. via the transmitted limb).

[0050] For example, the service signal uses wavelengths between 1575nm and 1650nm, and the diagnostic test signal uses wavelengths between 1500nm and up to 1575nm. Where the band drop filter 210 is configured to filter out wavelengths below 1575nm and to transmit those above 1575nm, the service signals and diagnostic signals can be split and used in parallel to activate the diagnostic apparatus and to communicate telecommunications services.

[0051] Accordingly, the reflector 240 is available selectively to be brought in- and out-of-path of the OTDR 120 by appropriately switching the optical switch 230 using the diagnostic signal. Each diagnostic apparatus 170 therefore provides a passive means to help selectively test the fibre optic network 100, and which is remotely powered by light transmitted from the headend.

[0052] Where otherwise stated, the second 170-2, third 170-3 and fourth 170-4 diagnostics apparatuses have a corresponding arrangement and configuration to the first diagnostics apparatus 170-1 .

[0053] In one example, the plurality of diagnostic apparatuses 170 are formed into at least two different groups, wherein each group is configured and / or arranged to have a different response to, and in particular to be activated by, a different spectral range of the diagnostic signal; this is achieved by each group having a diagnostic apparatus with an optical filter 220 and / or a PV power supply 250 with different spectral responses and / or coupled to band drop filters 210 each having different spectral responses, respectively.

[0054] In a specific example, four different groups are provided, in which each group is configured to be activated using a channel drop filter 220 having a different filtering wavelength. All else being equal, the first 170-1 , second 170-2, third 170-3 and fourth 170-4 diagnostic apparatuses are provided with a channel drop filter 220 tuned to output a reflected component, respectively, at only between: 1500nm and 1518nm; 1519nm and 1537nm; 1538nm and 1555nm; and 1556nm and 1574nm.

[0055] Accordingly, a diagnostic signal comprising wavelengths at only 1500nm and 1518nm will cause only the first diagnostic apparatus 170-1 to be activated, thereby bringing the associated reflector 240 into path, and in turn only the first diagnostic apparatus 170-1 (and the fibre optic cables thereto) being detectable by the OTDR (as described in more detail below). In this way, by appropriate tuning of the diagnostic signal, testing of only the first distribution fibre optic cable 180-1 is available to be performed.

[0056] Furthermore, a wider-band diagnostic signal (e.g. at 1500nm to 1518nm and 1556nm to 1574nm) can be used simultaneously to activate at least two of the diagnostic apparatuses 170 e.g. the first 170-1 and fourth 170-4), and therefore simultaneously to test the associated distribution fibre optic cables 180 {e.g. the first 180-1 and fourth 180-4).

[0057] Figure 3 shows a process 300 for operating the telecommunications network 100 so as to identify and test the distribution fibre optic cables 180.

[0058] In a first step 310, a distribution fibre optic cable 180 is selected for testing, and the corresponding diagnostics apparatus 170 is identified. For example, the first distribution fibre optic cable 180-1 is to be tested, which is associated with the first diagnostics apparatus 170-1 . In this example, the diagnostic signal for activating only the first diagnostics apparatus 170-1 is pre-known {e.g. 1500nm to 1518nm).

[0059] At a next step 320, the OTDR 120 is operated to transmit a diagnostic signal capable of operating the identified diagnostics apparatus ( / .e. a diagnostic signal spanning 1500nm to 1518nm).

[0060] At a subsequent step 330, the OTDR 120 is operated to monitor for a response from the reflector 240 of the identified diagnostics apparatus. Having detected a response from the reflector of the identified diagnostics apparatus (as described in more detail in relation to Figures 4 below), it may be determined that the identified diagnostics apparatus has been activated. Where each of the diagnostics apparatus are activated at different and non-overlapping wavelengths, the resulting OTDR trace is a diagnostic test result for only the selected distribution fibre optic cable (e.g. the first distribution fibre optic cable 180-1 ).

[0061] Figures 4 show exemplary schematic OTDR traces 400, which are available to be retrieved using process 300.

[0062] Figure 4a shows a first OTDR trace 400-1 retrieved from an OTDR test that is performed using a signal that does not activate any of the diagnostic apparatuses 170 e.g. a service signal). The first OTDR trace 400-1 therefore comprises reflections 410 from all of the distribution fibre optic cables, and with high loss rates beyond the power splitter 150 causing reflections to attenuate to a measurement noise floor 420.

[0063] Figure 4b shows a second OTDR trace 400-2 that is retrieved from an OTDR test that is performed using a diagnostic signal that activates only one of the diagnostics apparatuses e.g. the first diagnostic apparatus 170-1 ), thereby introducing the reflector 240 into the path of the OTDR test. As a result, a high-intensity reflection 430 is detected, which exceeds the measurement noise floor 420, thereby allowing better observations to be made beyond the power splitter 150, compared to the first OTDR trace 400-1 , and for improved isolation of reflections associated only with the first distribution fibre optic cable 180-1 .

[0064] Alternatives and Modifications

[0065] In an alternative example, each of the band drop filters 210 and the diagnostic apparatuses 170 are identical as to their responsiveness to the same diagnostic signal. In this way, the same diagnostic signal will operate all of the diagnostic apparatuses, and therefore the distribution networks for all of the ONUs are available simultaneously to be tested, all whilst still permitting selective - and remotely powered - introduction of the reflector 240.

[0066] In an alternative, the channel drop filter 220 comprises a plurality of dichroic optical filters connected in series, wherein each of said filters is configured to power the 250, and / or another, PV power supply and transmit a diagnostic signal on to the 240, and / or another, reflector. However, wherein each of said filters is tuned to operate at a different, and a progressively narrower-band, diagnostic signal. In this way, a distinct and different wavelength is available selectively to be tested. In one example, the band drop filter 210 and the channel drop filter 220 are configured to have a low wavelength shift with temperature (~0.1 pm / °C) so that there is only a very small change in spectral response over typical operating temperatures (-40 °C to 85 °C). Furthermore, each of said filters is available to be in the form of a thin-film-type filter, which comprises layers of different refractive index optical materials.

[0067] With reference to step 310, it will be appreciated that the wavelengths that activate the diagnostic apparatus need not be known, and can instead be empirically identified. Each feature disclosed herein, and (where appropriate) as part of the claims and drawings may be provided independently or in any appropriate combination.

[0068] Any reference numerals appearing in the claims are for illustration only and shall not limit the scope of the claims.

Claims

Claims1 . An apparatus for use in performing diagnostics upon a fibre optic cable in a fibre optic network, said apparatus comprising: a reflector; a dichroic filter configured to output, from input light transmitted through the fibre optic cable: a reflected component; and a transmitted component; an optical switch, arranged to receive the transmitted component and to interface with the reflector, and configured to perform switching between optically connecting and disconnecting the reflector to and from the transmitted component; and a photovoltaic, PV, power supply, arranged to receive the reflected component, and electrically connected with the optical switch, thereby to generate power from the reflected component and to power the switching of the optical switch.

2. An apparatus according to Claim 1 , wherein the optical switch is biased to disconnect the reflector from the transmitted component when the optical switch is unpowered.

3. An apparatus according to Claim 1 or 2, comprising a port for connecting the dichroic filter to an output of a band-drop filter coupled to the fibre optic cable, wherein the input light is received via the port and from the band-drop filter.

4. An apparatus according to any preceding claim, wherein fibre optic network comprises: a plurality of Optical Network Units; a splitter; and a plurality of distribution fibre optic cables, each of said cables extending between the splitter and one of the plurality of Optical Network Units; and wherein the apparatus comprises a connector for connecting to one of the plurality of distribution fibre optic cables and downstream of the splitter.

5. An apparatus according to any preceding claim, wherein the optical filter and / or further optical filter has / have a reflectivity that is substantially independent of temperature.

6. A fibre optic network comprising an apparatus according to any preceding claim.

7. A fibre optic network according to Claim 6, comprising at least two of the apparatus of Claim6, and wherein: the at least two apparatuses are connected to different distribution fibre optic cables; and each dichroic filter of each of the at least two apparatus is configured to filter at a different wavelength.

8. A method of operating a fibre optic network, said network comprising: an Optical Time-Domain Reflectometer, OTDR; a plurality of ONUs; and at least one apparatus according to any of Claims 1 to 7, wherein said at least one apparatus is coupled to a fibre optic cable serving at least one of the plurality of ONUs; the method comprising the steps of: transmitting, by the OTDR, a diagnostic signal to the plurality of ONUs, wherein the diagnostic signal is configured to switch the at least one apparatus so as to optically connect the reflector with the transmitted component; and measuring, at the OTDR, a reflection from the reflector.

9. A computer-readable carrier medium comprising a computer program, which, when the computer program is executed by a computer, causes the computer to carry out the steps of preceding Claim 8.

Images