GB2582895A

GB2582895A - Method for testing quality of optical network components

Info

Publication number: GB2582895A
Application number: GB1902331.6A
Authority: GB
Inventors: Yu Jiliang; Geoffrey Hunt Rowland
Original assignee: United Technologists Europe Ltd
Current assignee: United Technologists Europe Ltd
Priority date: 2019-02-20
Filing date: 2019-02-20
Publication date: 2020-10-14
Anticipated expiration: 2039-02-20
Also published as: GB2582895B; GB201902331D0; WO2020169964A1; EP3928441A1

Abstract

A method of testing the quality of a component in an optical network (10), comprises the steps of: (a) introducing a first calibrated reflector (15) at a location on an optical fibre (14) immediately before or in place of the component (16) to be tested; (b) measuring the reflection Ro from the first calibrated reflector (15) on the optical fibre (14); (c) replacing the first calibrated reflector (15) on the optical fibre (14) with the component (16); (d) connecting a second calibrated reflector (19) after the component (16); (e) measuring the reflection R2, from the second calibrated reflector (19); and (f) calculating the loss Lc through the component (16), by comparison of the reflections Ro and R2

Description

Method for Testing Quality of Optical Network Components This invention relates to a method for characterising connections and components in an optical network. In particular, in relates to a method for testing the quality of an individual optical fibre network component after fitting to an optical fibre, and to assessing the loss and reflectance of an individual field-fitted connector.

Telecommunications signals may be transmitted over optical fibres. Optical fibre transmission has been used for over 30 years for long links in the core of telecommunications networks. Increasingly now, fibre is being introduced into the telecommunications access network. The access network connects 1c/ customers' premises (houses, flats and offices) to the telecommunications company's equipment in a local exchange or central office building, also referred to herein as the head end of the network. The use of optical fibre in the access network is driven by the need for higher bandwidth for Internet-based services to customers.

Optical fibre transmission systems consist of a transmitter which turns electrical pulses into pulses of light, then the optical fibre itself which carries the light from end to end, and finally a receiver which turns the pulses of light back into electrical pulses. The transmitter and receiver are called "opto electronic" components because they convert signals from the optical to the electronic domain or vice versa.

A specific type of optical network known as a Passive Optical Network (PON) is almost universally used in optical access networks to reduce cost. In a PON, a single fibre from the telecommunications company's equipment branches out to multiple fibres towards customers' premises. Branching occurs in a special optical component called a splitter.

In contrast to core networks and their equipment, access networks are characterised by much larger numbers of end-points necessitating a larger and less specialised workforce, by the poor environmental conditions in which much of the equipment is housed (up poles and down holes), and by the need to minimise cost because of strong price competition in end-customer services. -2 -

Optical fibres making up a PON cannot be installed in a single length from customer end to central office, so fibres must be connected together so that light can pass from one to the other. Ideally the connection has no loss so that light passes from one fibre to the other as if the fibres were continuous. Traditionally, optical fibres in core networks have been connected by fusion splicing. In fusion splicing, the two fibre ends to be connected are first cleaved to leave a precise perpendicular end face on each fibre. Next, the ends are aligned mechanically to very high precision. Then, the ends are fused together by an electric spark. Finally, the new splice is sealed into a tube for mechanical protection.

The fusion splicing process is now partly automated but still requires highly skilled operators and preferably very clean conditions. Both these requirements are hard to meet in the access network environment.

An alternative means of connecting fibres is by the use of mechanical connectors. The end of each fibre is held in a ferrule inside a rigid plastic or metal housing, and the fibre end is polished flush with the end of the ferrule. The two housings can be pushed together and locked. This holds the two fibre ends in very close physical contact so that light can pass from one fibre to the other with sufficiently low loss. Fibre optic patch cables of various standard lengths (about 0.1 m to about 100 m) with a connector at each end are mass-produced for use in data centres and office buildings.

The connections between the long transmission fibre and the terminating opto-electronic equipment at each end are usually made with mechanical connectors.

Patch cables of standard lengths are often not suitable for installation in access networks where the length required varies widely. Use of an over-length patch cable is wasteful and it may be difficult to hide the excess length. To overcome this problem, specialist manufacturers of optical fibre components have produced multiple designs of Field Fit Connectors (FFCs). A newly cleaved fibre end can be fitted into a Field Fit Connector and locked in place. Internally, the Field Fit Connector holds the cleaved end of the external fibre in very close physical contact with one end of a short internal fibre. The other end of the internal fibre is held in a ferrule similar to the ferrule of a standard mechanical connector. -3 -

The connector provides one of the standard mechanical-connector interfaces, for example, the SC connector of IEC61754 4. Typically, the junction between the new external fibre and the short fibre inside the connector is treated with an optical index-matching gel with refractive index close to that of the glass fibre, to minimise back-reflection and loss.

Although the assembly of a Field Fit Connector to a freshly-cleaved optical fibre is easier than making a fusion splice, the quality of the resulting connection is not always within the limits needed for robust error-free operation of the optical transmission system carrying the customer's traffic. If an FFC is fitted badly and the problem is detected only after the installation engineer has left the customer's premises, an additional visit is required to fix the problem. This increases the overall cost of installation and may damage the service provider's reputation. In optical networks, it is often a requirement to measure the loss of a connection between two optical fibres or of a section of a network. Optical Time 15 Domain Reflectometers (OTDRs) are widely used to measure connection loss in point-to-point networks. The measurement compares the power received due to Rayleigh back-scattering from the region just upstream of the connection with the back-scattered power from the region just downstream of the connection. If the optical fibre structure is the same on both sides of the connection, the reduction in back-scattered power leads directly to the connection loss.

This type of measurement is not usually possible for connections in Passive Optical Networks (PONs) beyond the splitter, for two reasons. Firstly, the Rayleigh scattering level from beyond the splitter is often below the noise floor of the OTDR instrument, for any reasonable averaging time. Secondly, even if the Rayleigh back-scattered level from optical fibres beyond the splitter is detectable, there may be several contributing optical fibres and the number of contributing fibres is often not known. This means that the change in total back-scattered level due to a lossy connection on a single contributing optical fibre is often very small indeed. Even if the change is measurable, it is possible to calculate the loss of the connection only if the number of parallel optical fibres contributing to total back-scattering is known with certainty. -4 -

The present invention seeks to address the above issues by providing a method for measuring loss in an optical network using a comparison between two calibrated reflections measured in successive tests. The method may be used even where no Rayleigh back-scattered signal is detectable by the OTDR. The method is applicable to, but is not restricted to, branches of a PON beyond the splitter.

Further, it is envisaged that the method according to the present invention will be applicable to any of the following scenarios, namely: (i) the measurement of loss in any section of an optical network which is provided with mechanical connectors at each end thereof; (ii) the measurement of loss of a newly-fitted connector, where the process of fitting the connector requires making a perpendicular cleave of the upstream fibre end, or where a step of making a perpendicular cleave can be added to the process; (iii) the measurement of loss of a splice, where the splicing process requires making a perpendicular cleave of the upstream fibre end and the downstream fibre ends in a connector.

The present invention is provided particularly, though not exclusively, to measure the quality (loss and reflectance) of a Field Fit Connector during installation of the connector. This allows the engineer to rework the connection, or install a replacement connector, if necessary, and re-test. In this way the engineer can ensure that the installation is correct before leaving the site.

According to the present invention there is provided a method of testing the quality of a component in an optical network, comprising the steps of: (a) introducing a first calibrated reflector at a location on an optical fibre immediately before or in place of the component to be tested; (b) measuring the reflection Ro from the first calibrated reflector; (c) replacing the first calibrated reflector on the optical fibre with the component; (d) connecting a second calibrated reflector after the component; -5 - (e) measuring the reflection R2 from the second calibrated reflector; and (f) calculating the loss Lc through the component, by comparison of the reflections Ro and R2.

In the case where the calibrated reflectors have an optical return loss equal to that of a perpendicular cleaved face of an optical fibre, the loss Lc is calculated by the expression: Lc=Ro-R2.

In other cases, the value of the reflection would need to be accounted for in the above expression.

Step (e) preferably further comprises measuring the reflection Ri from the component.

The second calibrated reflector preferably has a known optical return loss pmo, and the method may thus preferably comprise further the step of: (g) calculating the optical return loss ORLc of the component, where: ORLc= Preff +2(Ro-R1).

The reflectance of the first calibrated reflector may be equal to, less than, or greater than, the reflectance of the second calibrated reflector.

Step (a) may comprise cleaving the optical fibre at the location, and the first calibrated reflector may therefore comprise a cleaved face of the optical fibre end formed thereby. In such embodiments of the present invention, the first calibrated reflector is preferably a naturally-calibrated reflector due to Fresnel reflection, and comprises a substantially plane cleaved face whose normal lies in substantially the same direction as the axis of the optical fibre.

In such embodiments of the present invention, the step (c) of replacing the first calibrated reflector with the component preferably comprises fusion splicing or mechanically mating the cleaved optical fibre end to the component.

In the case of the scenario described in (i) above, the reflection measured in the first test in step (b) is obtained by fitting, in step (a), a first calibrated reflector terminating the upstream optical fibre, in place of the section of network which constitutes the component whose loss is to be measured. The first calibrated reflector is removed, and replaced in step (c) by the section of network. A second -6 -calibrated reflector is then placed in step (d) at the downstream end of the section of network. The loss of the section of network is found in step (f) by comparison of the two reflection heights, after performance of step (e).

In the case of the scenario described in (ii) above, the reflection measured in the first test in step (b) is obtained as the natural Fresnel reflection from the perpendicular face of the freshly cleaved fibre end which is formed in step (a), and which constitutes the first calibrated reflector. The reflection measured in the second test in step (e) may be obtained by connecting, in step (d), a short patch cable ending in second a calibrated reflector to the newly-fitted connector fitted in step (c). The loss of the connector is found in step (f) by comparison of the two reflection heights.

In the case of the scenario described at (iii) above, the reflection measured in the first test in step (b) is obtained as the natural Fresnel reflection from the perpendicular face of the freshly cleaved fibre end formed in step (a) and constituting the first calibrated reflector. The reflection measured in the second test in step (e) may be obtained by connecting in step (d) a second calibrated reflector to the connector at the end of the downstream fibre connected by splicing in step (c), and correcting for the expected loss due to the length of the downstream fibre. The loss of the splice is found in step (f) by comparison of the two reflection heights.

Because good splices have loss which is typically less than the variation in loss of a mechanical connector, and there is additional uncertainty from the correction for the loss of the downstream fibre, the method of the present invention can be used to detect very poor splices. However, it may not be sufficiently precise to prove that the splice loss is less than the typically very tight

specification applied to splices in core networks.

The component to be tested may comprise one or more components connected by optical fibre. Said component to be tested may preferably have an optical input and an optical output, each provided with mechanical connectors for connection to adjacent optical components. The optical input may preferably be a fibre end suitable for splicing to upstream optical fibre, and the optical output may preferably be provided with a mechanical connector. -7 -

Most preferably, the component to be tested comprises a Field Fit Connector (FFC) suitable for forming an adaptor between a cleaved fibre end and a mechanical connector.

Although the method according to the present invention may be utilised to test the quality of any type of component in an optical network, it has been developed in particular for use in relation to Field Fit Connectors (FFCs) as hereinbefore described. The component in step (c) therefore preferably comprises a Field Fit Connector (FFC).

The present invention is primarily intended to provide a method to characterise the FFC, that is, to measure the loss through the FFC and the reflectance of the FFC. Accordingly, R1 now represents the reflection from the FFC, and step (f) now comprises calculating the loss LFFc through the FFC, by comparison of the reflections Ro and R2.

In the case where the calibrated reflectors have an optical return loss equal to that of a perpendicular cleaved face of an optical fibre, the loss LFFc is calculated by the expression: LFFc=Ro-R2.

Further, step (g) now comprises calculating the optical return loss ORLFFc of the FFC, where: ORLFFc= Pre/rF2(Ro-R/).

Steps (b) and (e) of the method are preferably carried out using an Optical Time Domain Reflectometer (OTDR). Accordingly, step (b) comprises carrying out a first OTDR trace of the optical network, and step (e) comprises carrying out a second OTDR trace of the optical network. The method thus uses two measurements of the reflection from the region of the FFC, taken with an optical time domain reflectometer (OTDR) located at the other end of the optical fibre. Steps (b) and/or (e) may further comprise measuring the level of Rayleigh 30 scattering So from the start of the trunk fibre nearest the OTDR.

The optical fibre preferably has a known Rayleigh scattering coefficient CR, whilst the OTDR preferably is configured to emit optical pulses having a known -8 -pulse width w. The method can thus also be used to yield the network loss from the location of the OTDR to a point immediately beyond the FFC. Accordingly, the method may further comprise the step of: (h) calculating the network loss LN from the start of the trunk fibre nearest the OTDR to the calibrated reflector, where: Liv=So-R2-( CR/2)-5logio(w)-(pref/2) Although the method according to the present invention may be utilised to test the quality of components in any type of optical network, it has been developed in particular for use in relation to Passive Optical Networks (PONS) as hereinbefore described. The optical network therefore preferably is a Passive Optical Network (PON). The OTDR may be located at the head end of the PON, and the PON may further comprises one or more splitters between the OTDR and the component.

In order that the present invention may be more clearly understood, a preferred embodiment thereof will now be described in detail, though only by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a Passive Optical Network (PON) in which a component is to be characterised by a method according to the present invention, and an Optical Time Domain Reflectometer (OTDR) trace for that network, with signals in the trace aligned with the corresponding physical components in the network schematic; Figure 2 is a schematic representation of the PON of Figure 1, now having added thereto a second calibrated reflector and a Field Fit Connector (FFC) to be characterised by the method of the present invention, and the corresponding OTDR trace, with signals in the trace aligned with the corresponding physical components in the network schematic; Figure 3 is a representation of the reflection measurements made in the OTDR traces carried out in the method of the present invention; Figure 4 is a cross-sectional detailed view of the Field Fit Connector (FFC) of Figure 2; Figure 5 is a cross-sectional view of the Field Fit Connector (FFC) of Figure 4, having a patch cable connected thereto; and -9 -Figure 6 is a more detailed schematic representation of the Passive Optical Network (PON) and Optical Time Domain Reflectometer (OTDR) of Figure 1.

Referring first to Figure 1, there is shown a Passive Optical Network (PON), generally indicated 10. The PON 10 comprises an Optical Time Domain Reflectometer (OTDR) 11 connected via a first length of optical fibre 12, also referred to herein as trunk fibre, to a splitter 13, to which is connected a second length of optical fibre 14. The splitter 13 is characterised as a 1xN splitter, where N is the number of optical fibre lines 14 emanating from the splitter 13. For convenience, Figure 1 shows only one such second length of optical fibre 14.

The second length of optical fibre 14 terminates in a cleaved face 15, constituting a first calibrated reflector, which the installation technician has prepared for installation of a Field Fit Connector (FFC), according to step (a) of the method of the present invention.

Figure 1 further shows an OTDR trace, generally indicated 20, measured for this PON 10. The OTDR trace plots the signal to noise ratio (SNR) in dB against distance from the OTDR. Rayleigh scattered light So is observable over the length of the trunk fibre 12 before the splitter 13. After the splitter 13, the Rayleigh scattered light So is not visible in the OTDR trace 20 against the OTDR detection noise, so the bottom of the trace 21 represents the OTDR noise level. However, the large reflection Ro from the first calibrated reflector formed by the cleaved face 15 of the fibre 14, as prepared for fitting a Field Fit Connector, is clearly visible, as is a reflection 22 from the splitter 13. Measurement of the reflection Ro constitutes step (b) of the method of the present invention.

The changes in the level of the OTDR trace 20 before and after the splitter 13 are characteristic of a PON 10. The method according to the present invention is however equally applicable to a point-to-point network.

Referring now to Figure 2, there is shown the PON 10 after fitting a Field Fit Connector (FFC) 16 to the cleaved face 15 of the fibre 14, and so replacing the first calibrated reflector with the component to be tested, according to step (c) of the present invention. A patch cable 17, typically having a length of 10m, is connected at the newly installed FFC 16 via a patch cable connector 18, and -10 -terminates in a second calibrated reflector 19, according to step (d) of the present invention.

In the case that the patch cable 17 has a low-reflectance Angled Physical Contact connector 18 at the end mated to the new FFC 16, the second calibrated reflector 19 might be as simple as a Physical Contact connector (not angled) giving a 4% reflectance (14 dB Optical Return Loss, ORL).

The corresponding OTDR trace 20 in Figure 2 shows a reduced reflection Ri at the position of the FFC 16 where the previous trace 20 in Figure 1 showed a large reflection Ro from the first calibrated reflector formed by the cleaved face 15. There is a new large reflection R2 from the second calibrated reflector 19 at the end of the patch cable 17. Measurement of the reflections R2 and R2 constitute step (e) of the method of the present invention.

Referring now to Figure 3, there is shown a representation of the respective heights of the three reflection measurements, namely: the reflection Ro from the first calibrated reflector formed by the newly-cleaved end face 15 of the fibre 14, measured in step (b) before installation of the FFC 16; the reflection Ri from the FFC 16 mated with the connector 18 of the patch cable 17, as measured in step (e) after installation; and the reflection R2 from the second calibrated reflector 19 at the end of the patch cable 17, as also measured in step (e).

It can be assumed for the purposes of the present illustration that these values Ro, R/ and R2 are measured by an OTDR 11 which conventionally converts measured back-reflected power to dB according to the formula: P(dB) = Stamm -Po Here, p is the measured optical power and Po is a normalisation which is often taken as the standard deviation of the measured baseline noise 21. The factor 5 (rather than the usual 10) is used to arrange that the OTDR 11 reports losses correctly, because its optical pulse traverses the lossy component twice.

If the first calibrated reflector formed by the cleaved end face 15 is well 30 made, it can be assumed that it gives rise to a Fresnel back-reflection with about 4% power return (ORL 14 dB) for conventional fibre 14 with refractive index n=1.5. If it is assumed that the second calibrated reflector 19 at the end of the patch cable 17 also has ORL=14 dB, the loss through the newly-fitted FFC 16 is just: LFFC = RO-R2 and the Optical Return Loss (ORL) of the newly-fitted FFC 16 is: ORLFFC = 14 + 2(Ro -where the factor 2 comes from the use by the OTDR 11 of the factor 5 rather than 10 in its calculation of levels in dB. Calculation of the loss LFFC constitutes step (f) of the method of the present invention.

If the Rayleigh scattering coefficient CR (dB/ns) of the trunk fibre 12 and the OTDR pulse width w are known, and the OTDR measurement trace 20 gives a level So for Rayleigh scattering from the start of the trunk fibre 12 nearest the OTDR 11, network loss LN from the start of the trunk fibre 12 to the calibrated reflector 19. If the transmission power of the OTDR 11 is PT dBm, then: S0 = -2 [PT + CR 10/ogio(w)] + K and 1, R2 = -2 [PT -2LN -p"fi] + K where K is a calibration factor for the OTDR 11, involving the baseline noise 21 of the OTDR detector, which is the same in the two measurements made in steps (b) and (e). The initial factor 1/2 comes from the use by the OTDR 11 of the factor 5, rather than 10, when calculating power in dB. prep is the Optical Return Loss (ORL) of the calibrated reflector 19 giving rise to the reflection R2. w is the optical pulse width emitted by the OTDR 11 in ns. Forming the difference So -R2 and rearranging, gives: LN = So -R2 --Slogio(w) liref I. 2 2 Taking a numerical example, where So=18 dB, R2=22 dB, CR=-83 dB, w=10 ns and proo=14, would give a value of LN = 25.5 dB.

Referring now to Figures 4, 5 and 6, these are included by way of background explanation only, and illustrate examples of Field Fit Connector 16 and Passive Optical Network 10 for which use of the method of the present invention is particularly, but not exclusively, intended.

-12 -Referring first to Figure 4, there is shown a Field Fit Connector 16 installed on the optical fibre 14. The coating 24 of the fibre 14 is in place at the point at which the fibre 14 enters the FFC 16, but as can be seen in Figure 4 has been stripped from the fibre 14 further along its length by the technician prior to installation. The coated fibre 24 is held against the connector body 23 by a coating clamp 25, and the stripped fibre 14 is then held against the connector body 23 further along its length by a fibre clamp 26. The cleaved end 15 of the fibre 14 is held by the fibre clamp 26 in exact alignment with the end 28 of a stub fibre 27, and refractive index matching gel is applied over this boundary 15/28.

/o The remainder of the stub fibre 27 is held in a ferrule connector 29 and terminates in a factory polished angled face 31.

Referring now to Figure 5, there is shown the Field Fit Connector 16 of Figure 4, connected to a patch cable 17 as hereinbefore described. The ferrule connector 29 of the FFC 16 is mated with a complementary ferrule connector 18 of the patch cable 17 so that the respective fibres 27, 17 are in exact alignment.

This connection 29/18 may be an Angled Physical Contact (APC) connection for low reflectance. The length of the patch cable 17 is pre-determined and selected such that reflectance and loss events in the OTDR trace 20 can be resolved and differentiated according to distance. The distal end of the patch cable 17 has a second ferrule connector 18 terminating in the second calibrated reflector 19.

This second calibrated reflector 19 may simply be a perpendicular polished face of the cable 17, or alternatively a separate reflector may be connected to the ferrule 18.

Finally, referring to Figure 6, there is shown an illustration of how a single 25 OTDR 11 can be used to test multiple PONs 10.

The OTDR 11 is housed within a central office or exchange building 32. The OTDR 11 is connected via switches 33 to Wavelength Division Multiplexors (WDMs) 34, which are also connected to the Optical Line Terminal (OLT) 35. Multiple trunk fibres 12 thus emanate from the central office 32, each said trunk fibre 12 thus defining a separate PON 10. Each PON 10 may comprise a 1xN splitter 13, at which the PON 10 is split into multiple (N) customer fibres 14, each having a different length so as to be identifiable in the OTDR trace. Each customer fibre 14 terminates at an Optical Network Terminal (ONT) 36.

Claims

-14 -Claims 1. A method of testing the quality of a component in an optical network, comprising the steps of: (a) introducing a first calibrated reflector at a location on an optical fibre immediately before or in place of the component to be tested; (b) measuring the reflection Ro from the first calibrated reflector; (c) replacing the first calibrated reflector on the optical fibre with the component; (d) connecting a second calibrated reflector after the component; (e) measuring the reflection R2 from the second calibrated reflector; and (f) calculating the loss Lc through the component, by comparison of the reflections Ro and R2.
2. A method as claimed in claim 1, wherein step (e) further comprises measuring the reflection IR, from the component.
3. A method as claimed in claim 2, wherein the second calibrated reflector has a known optical return loss A-ea, and wherein the method further comprises the step of: (g) calculating the optical return loss ORLc of the component, where: ORLc= n rrefl +2(Ro-R,).
4. A method as claimed in any of the preceding claims, wherein the component to be tested comprises one or more components connected by optical fibre.
5. A method as claimed in claim 4, wherein the component has an optical input and an optical output, each provided with mechanical connectors for connection to adjacent optical components.
6. A method as claimed in claim 5, wherein the optical input is a fibre end suitable for splicing to upstream optical fibre, and the optical output is provided with a mechanical connector.
7. A method as claimed in any of the preceding claims, wherein the component comprises a Field Fit Connector (FFC) suitable for adapting a cleaved fibre end to a mechanical connector.
-15 - 8. A method as claimed in claim 7, wherein step (f) comprises calculating the loss LFFC through the FFC, by comparison of the reflections Ro and R2.
9. A method as claimed in claim 7 or claim 8, when claim 7 is directly or indirectly dependent upon claim 3, wherein step (g) comprises calculating the optical return loss ORLFFC of the FFC, where: ORLFRc= pre/F2(Ro-RI).
10. A method as claimed in any of the preceding claims, wherein the second calibrated reflector is connected to the component via a patch cable.
11. A method as claimed in any of the preceding claims, wherein step (a) comprises cleaving the optical fibre at the location, and wherein the first calibrated reflector comprises a cleaved face of the optical fibre end formed thereby
12. A method as claimed in claim 11, wherein the first calibrated reflector is a naturally-calibrated reflector due to Fresnel reflection, and comprises a substantially plane cleaved face whose normal lies in substantially the same /5 direction as the axis of the optical fibre.
13. A method as claimed in claim 11 or claim 12, wherein the step (c) of replacing the first calibrated reflector with the component comprises fusion splicing or mechanically mating the cleaved optical fibre end to the component.
14. A method as claimed in any of the preceding claims, wherein steps (b) and (e) are carried out using an Optical Time Domain Reflectometer (OTDR).
15. A method as clamed in claim 14, wherein step (b) comprises carrying out a first OTDR trace of the optical network, and step (e) comprises carrying out a second OTDR trace of the optical network.
16. A method as claimed in claim 14 or claim 15, wherein steps (b) and/or (e) further comprise measuring the level of Rayleigh scattering So from the start of the optical fibre adjacent the OTDR.
17. A method as claimed in claim 16, wherein the optical fibre has a known Rayleigh scattering coefficient CR, the OTDR is configured to emit optical pulses having a known pulse width w, and wherein the method further comprises the step of: -16 - (h) calculating the network loss LN from the start of the optical fibre adjacent the OTDR to the calibrated reflector, where: LN= S 0-R2-(CR/2)-51og 7 o(w)-(pre fa)
18. A method as claimed in any of the preceding claims, wherein the optical network is a Passive Optical Network (PON).
19. A method as claimed in claim 18 when dependent upon any of claims 14 to 17, wherein the OTDR is located at the head end of the PON.
20. A method as claimed in claim 19, wherein the PON further comprises one or more splitters between the OTDR and the component.
21. A method as claimed in any of the preceding claims, wherein the reflectance of the first calibrated reflector is substantially equal to the reflectance of the second calibrated reflector.
22. A method as claimed in any of claims 1 to 20, wherein the reflectance of the first calibrated reflector is less than the reflectance of the second calibrated 15 reflector.
23. A method as claimed in any of claims 1 to 20, wherein the reflectance of the first calibrated reflector is greater than the reflectance of the second calibrated reflector.