FIA The Fibreoptic Industry Association www.fibreoptic.org.uk TECHNICAL SUPPORT GUIDE FIATSD000 TESTING INSTALLED CABLING USING OPTICAL TIME DOMAIN RELFECTOMETER (OTDR) EQUIPMENT This document has been reissued (version.0) to reflect the change in FIA website details and to update the list of FIA Technical Support Documents (both published and in development). A future release (version ) is in preparation which will address the impact of ISO/IEC and BS EN 0 on working practices. If you need advance information about these specific changes please contact the FIA Secretariat or the FIA Technical Director. Price: 0 (free to FIA members) Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk
FIATSD000 DATE: November 00 0 0 0 The Fibreoptic Industry Association An introduction for the new millennium The past decade has been a time in which there has been a vast increase in the use of optical fibre primarily driven by the need to provide a quality, highspeed transmission media for digital trunk telephony services. The specifications for these systems have typically been produced by large national telecommunications service providers. This has resulted in clear standards and specifications exist to which all suppliers to the WAN telecommunications industry must adhere. In parallel there has been a significant growth in optical fibre systems being installed in private data, entertainment and telecommunications networks which are separate from the national telephony and data carrier systems. This part of the industry is characterised by having a large number of relatively small company participants albeit supplying large corporate customers with products and services. The use of optical fibres in private, local area data and sensor networks has increased rapidly throughout the 0 s. In order to support this rate of growth, an organizational focus is required for both suppliers and users in the industry in order to ensure the quality and reliability of network design, installation practice and methods of training. The Fibreoptic Industry Association provides such a focus as a Trade Association to which companies, organizations and individuals involved with, or planning an involvement with, fibre optics can subscribe. In addition, by means of seminars, publications, newsletters, press promotion and similar activities, the Fibreoptic Industry Association is dedicated to raising the profile of the industry and highlighting its many benefits in order to increase its growth and thus provide direct benefits for members. Our overall aims can be summarised as follows: to promote an awareness of the benefits and applications of fibre optic technology as an adjunct to or as a replacement for conventional copper communications technology; to promote an awareness of the existence of a professional fibre optics industry fully capable of meeting the needs of users or, so benefiting both suppliers and their customers; to promote and adopt standards to which professional participants within the fibre optic industry should be expected to adhere; to provide a central source for information on wide ranging aspects of the fibre optic industry; to provide a single voice to promote and represent the interests of the industry obtained by consensus and debate amongst FIA members; to develop and promote codes of practice within the industry both operational and ethical to which members will be expected to adhere and thus offer an assurance that the highest quality of service will be provided. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page i
FIATSD000 DATE: November 00 0 0 FIA TECHNICAL SUPPORT GUIDES This document is one a series of FIA Technical Support Guides. During the year 000 all the existing FIA documents were rewritten or republished in the format used throughout this document. More importantly, the way in which these Technical Support Guides is published has also changed. These documents are now free to FIA members via downloads from the FIA website (www.fiaonline.co.uk). Nonmembers are also able to purchase these documents either by contacting the Secretariat (address shown below) or by online purchase. Members and nonmembers unable to benefit from this service may receive the documents in hardcopy or diskette/cd ROM by contacting the FIA Secretariat (contact details are shown at the bottom of each text page in this document). However, the rapidly changing nature of our technology means that webbased documents can be amended and revised easily and it is the responsibility of the reader to ensure that the latest issue of a document is used. The FIA website will indicate the issue status of each document and will have links to previous issues in order that changes made will be clear to readers. The complete list of FIA Technical Support Guides is shown in the Table below. TOPIC FIATSD TITLE DESIGN 000 : LAN APPLICATION SUPPORT GUIDE COMPONENT SELECTION 000 : CABLE SELECTION GUIDE 000 : CONNECTING HARDWARE SELECTION GUIDE OPERATION 000 : ADMINISTRATION: User Guides 000 : ADMINISTRATION: Cords 000 : POLARITY MAINTENANCE INSTALLATION 000 : INSTALLATION PRACTICE: SPLICING 000 : TESTING Installed cabling using LSPM equipment 000 : TESTING Installed cabling using OTDR equipment 000 : Test cords SAFETY 000 OPTICAL POWER: SAFETY LEVELS 000 OPTICAL FIBRE: HANDLING OF PROCESSING CHEMICALS 000 OPTICAL FIBRE: DISPOSAL OF WASTE Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page ii
FIATSD000 DATE: November 00 FOREWORD AND EXECUTIVE SUMMARY. By Mike Phillips, Chairman of the FIA Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page iii
FIATSD000 DATE: November 00 0 0 0 0 0 Table of Contents FIA Technical Support Guides... ii Foreword and Executive Summary... iii INTRODUCTION... SCOPE... REFERENCES.... Testing standards.... Other standards... DEFINITIONS AND ABBREVIATIONS.... Definitions.... Abbreviations... Optical time domain reflectometry.... Theory of operation..... General..... Optical fibre attenuation measurement..... Localized event measurement..... Cabling length measurement...0. OTDR selection and operating parameters..... Effective characterization..... Mode and wavelength of operation..... Range..... Pulse power..... Pulse width..... Integration or sample count.... Restriction of OTDR capability..... Minimum lengths of operation...... Dead zone...... Equilibrium length (MMF)...... Equilibrium length (SMF)...... Attenuation coefficient..... Ghosting... test cords.... General.... Launch cords..... Simplex interfaces (MMF)..... Duplex interfaces (MMF)..... Simplex interfaces (SMF)..... Duplex interfaces (SMF).... Tail cords..... Simplex interfaces (MMF)..... Duplex interfaces (MMF)..... Simplex interfaces (SMF)..... Duplex interfaces (SMF).... Preparation... The use of OTDR equipment within installations.... Singleended measurement..... Measurement technique..... Interpretation of results.... Doubleended measurement..... Interpretation of results...0.. Comparison of OTDR measurement of total link loss with those obtained by LSPM..... The requirement for bidrectional testing... Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0. Refractive index (IOR)... Test Result Management... QUALITY ASSURANCE.... Quality Plan... Table of Figures Figure : OTDR theory... Figure : OTDR characterization of optical fibre... Figure : OTDR characterization of an installed link... Figure : OTDR characterization of an installed link containing a joint... Figure : OTDR characterization of an installed SMF link containing a break...0 Figure : OTDR characterization using different length launch cords... Figure : OTDR characterization showing ghost effects... Figure : OTDR characterization showing complex ghost effects... Figure : Test cord schematic... Figure 0: Doubleended characterization... Figure : Doubleended characterization reference measurement points... Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 INTRODUCTION An OTDR is possibly the most useful analytical tool available to the installer and user of optical fibre cabling. It can be used to perform inspection and testing of optical fibre cabling of all types and at all stages of installation. The soft and hard copy results produced can be included in contract documentation and represent performance baselines against which subsequent measurements can be compared. An OTDR can detect and locate the presence of poor installation practices or modifications to the installed environment. In addition, the OTDR may be used to test completed installations and provides an accurate assessment or measurement (depending upon how it is used) of the position of, and the attenuation levels produced at, the various interfaces and joints throughout the installed cabling. However, OTDR equipment does have limitations and unskilled use can produce meaningless results. The purpose of this document is to ensure that the OTDR characterizations undertaken are made to a common standard enabling sensible interpretation of the information portrayed. SCOPE This document defines the techniques for the characterization of installed cabling links using optical time domain reflectometer (OTDR) equipment. Measurement error is described in detail in order to allow those in charge of test procedures to prepare for the possibility of nonconforming results. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 REFERENCES. Testing standards BS EN 0 (in development) FIACCP/. Other standards BS 0 BS EN 0 BS EN 0 BS EN 0 EN 0 (in development) ISO/IEC 0 Code of Practice for the installation of fibre optic cabling Information Technology Testing of installed cabling Code of Practice for the installation of fibre optic cabling (withdrawn when BS published) Code of Practice for the installation of apparatus intended for connection to certain telecommunications systems Information technology Generic cabling systems Information technology Cabling installation Part : Specification and Quality Assurance Information technology Cabling installation Part : Installation planning and practices inside buildings Information technology Generic cabling systems Part : General requirements and office areas (Ed. of EN 0) Information technology Generic cabling for customer premises Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 DEFINITIONS AND ABBREVIATIONS. Definitions For the purpose of this Technical Support Guide the following definitions apply: Application A system, with its associated transmission method, which is supported by telecommunications cabling (EN 0) Cladding The dielectric material of an optical fibre surrounding the core (BS ). Channel The endtoend transmission path connecting any two pieces of application specific equipment. Equipment and work area cords are included in the channel, but not the connecting hardware into the application specific equipment (EN 0). Channel insertion loss The maximum channel attenuation defined by an application standard e.g. 000BASESX. This is not necessarily the same as optical power budget of the transmission equipment (see FIATSD000). Core The central region of an optical fibre through which most of the optical power is transmitted (BS ). Connection Mated device or combination of devices including terminations used to connect cables or cable elements to other cables, cable elements or application specific equipment (EN 0). Equipment cord Link. Abbreviations For the purpose of this Technical Support Guide the following definitions apply: DWDM LS IOR MMF OLB OPB OTDR PM SMF A cord connecting a link to applicationspecific equipment. Transmission path that excludes work area cords, equipment cords, patch cords and jumpers but includes the connection at each end. It can include a CP link. (EN 0) Dense Wavelength Division Multiplexing Light Source Index of Refraction (refractive index) MultiMode optical Fibre Optical Loss Budget Optical Power Budget Optical Time Domain Reflectometer Power Meter SingleMode optical Fibre Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 OPTICAL TIME DOMAIN REFLECTOMETRY. Theory of operation.. General The OTDR operates by launching a series of very short pulses of LASER light into the optical fibre to be measured. This light is scattered (by Rayleigh scattering) at all points along the fibre and a small fraction is scattered back towards the OTDR. The backscattered light is captured by the OTDR and analysed to produce an attenuation profile of the optical fibre along its length. The amount of light scattered, in effect reflected, back towards the OTDR, is defined by the scattering fraction (k) which is: a) small; b) dependent upon wavelength of transmitted light; c) normally constant within a given batch of optical fibre... Optical fibre attenuation measurement With reference to Figure, two points on the optical fibre under test are considered; Point A at a distance L from the OTDR and Point B at a distance L from the OTDR. Length L Length L OTDR P f = xp 0 P f = x P 0 P r = kxp 0 P r = x P 0 Time Received power (W) Round trip time = Ln/c Round trip time = Ln/c P r = kx P 0 Point A P r = kx P 0 Point B Figure : OTDR theory Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 The power launched into the optical core by the OTDR, P 0 (W), will be attenuated as it passes along the optical fibre until it reaches point A. At point A, the forward power at this point P f = xp 0. The value of x (obviously less than unity) depends upon the optical fibre design and wavelength of the transmitted light. The light scattered back towards the OTDR at point A can be written as P r = kp f = kxp 0. This light is also attenuated as it returns to the OTDR such that the light reaching the OTDR is kx P 0. At point B, the light has travelled twice as far and the forward power P f = x P 0, the scattered power P r = kp f = kx P 0 and the received power at the OTDR = kx P 0. This situation is summarized in Table. Point Distance Power received Time elapsed at OTDR (W) A L kx P 0 xn/c B L kx P 0 xn/c n = refractive index of the optical fibre core c= velocity of light in a vacuum Table : Received power at the OTDR (W) By sampling the received power at predefined time intervals the OTDR is able to measure the reduction in power with increasing distance along the optical core. If the OTDR is provided with the refractive index of the core material then the time sampling can be effectively converted into distance and the results observed will represent loss as a function of distance along the fibre. It is more normal for the OTDR to display the received power in logarithmic form that produces a straight line loss with distance as shown in Table and allows sensible measurements to be made of attenuation using the decibel units used elsewhere within the technology. Point Distance Power received at OTDR (db) A L 0 log 0 (kp 0) 0 log 0 (x) B L 0 log 0 (kp 0) 0 log 0 (x) Difference BA L 0 log 0 (x) Table : Received power at the OTDR (db) A conventional measurement of the attenuation between points A and B would indicate: Attenuation (A to B) (db) = 0 log 0 (kx P 0/kxP 0 ) = 0 log 0 (x) Table shows that measurement using the received powers at the OTDR produces a difference of twice this figure. However, the OTDR converts all measurements to represent a single way path (both in terms of attenuation and distance). The OTDR relies upon the inputting of the correct refractive index (IOR) of the optical fibre (see.) in order to convert time to distance. The measurement produced by an OTDR that directly corresponds to Figure is shown in Figure. This allows the attenuation coefficient (attenuation in dbkm ) of the optical fibre to be measured. The characterization of the optical fibre should be a straight line plot. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 Length L Length L OTDR P f = xp 0 P f = x P 0 P r = kxp 0 P r = x P 0 Length Received power (db) Length L (km) Loss x (db) Attenuation coefficient = x/l dbkm 0 0 Figure : OTDR characterization of optical fibre.. Localized event measurement Figure shows the measurement of attenuation within an optical fibre but the same principle applies to the OTDR measurement of the attenuation of localized events such as connections and joints. To measure an installed link terminated with fixed interfaces it is necessary to use a launch cord. The requirements for the launch cord are detailed in.. Figure shows an OTDR characterization of an installed link. The peak is indicative of the reflection that commonly exists at interfaces due to the small airgap between the optical fibre endfaces (the level of reflection is much lower in SMF than in MMF due to the more stringent return loss specifications). The attenuation of the local interface is shown as the interface loss (db). However, this is: a) not a true measurement since the scattering fraction (k) may be different in the launch cord and the cabling under test. If the scattering fraction of the launch cord is higher than that of the cabling under test then the result will be overstated. If the scattering fraction of the cabling under test is higher than that of the launch cord then the result will be understated (sometimes, but very rarely, producing an apparent amplification). b) subject to the same measurement accuracy as LSPM measurements due to the differences of optical fibre core diameter, cladding diameter, numerical aperture and core eccentricity (within the appropriate optical fire specifications) at the test interface. For this reason a singleended OTDR characterization can only be said to provide an indication of local interface performance. A true measurement of link loss, subject to the measurement accuracy defined in (b) above, can be obtained by the introduction of tail cords (see.) and the use of doubleended measurements (see.). Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 OTDR Launch cord Cabling under test Interface loss (db) Link length (m) Figure : OTDR characterization of an installed link A joint within the cable under test can also be measured with an OTDR. Figure shows an OTDR characterization of an installed link containing a joint. A joint, particularly a fusion splice, should not contain an airgap so reflective peaks are not normally seen. Instead the optical power received simply features a drop at the position of the joint Launch cord Cabling under test OTDR Joint loss (db) 0 Figure : OTDR characterization of an installed link containing a joint Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 The attenuation of the joint is shown as the joint loss (db). However, this is not a true measurement since the scattering fraction (k) of the launch cord may be different than that of the cabling under test. If the scattering fraction of the launch cord is higher than that of the cabling under test then the result will be overstated. If the scattering fraction of the cabling under test is higher than that of the launch cord then the result will be understated (sometime producing an apparent amplification). For this reason a singleended OTDR characterization can only be said to provide an indication of joint performance. A true measurement can be obtained with the use of bidirectional measurements and the calculation of the mean value obtained for the joint in each direction... Cabling length measurement With reference to Figure and Figure it is seen that the end of a terminated link is characterized by a large reflection peak. This peak is caused by Fresnel reflection as the light launched by the OTDR passes from silica to air (the level of reflection is much lower in SMF than in MMF due to the more stringent return loss specifications of SMF connecting hardware). Using the peaks the length of the individual optical fibres within a cable may be measured as shown in Figure. A break in a MMF link would also be characterized by the presence of the reflection peak. However, unlike MMF and as shown in Figure, a break in a SMF may not always be characterized by a reflection peak. In either case, the position of the break can be pinpointed using the OTDR characterization. Launch cord Cabling under test OTDR Figure : OTDR characterization of an installed SMF link containing a break The measurement accuracy is high provided that the correct refractive index value (see.) is input to the OTDR. It should be pointed out that the length measured is that of the optical fibre (which, in general, is longer, rather than the same as, the length of the cable). Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page 0 of
FIATSD000 DATE: November 00 0 0 0 0. OTDR selection and operating parameters.. Effective characterization There are four fundamental parameters that define the operational capability of an OTDR. These are: the wavelength of operation; the LASER pulse peak power; the LASER pulse width; the integration count of the receiver circuit. These parameters are addressed in the following four subclauses. These parameters define the maximum resolution and assessment/measurement capacity of the OTDR and therefore define its capability to effectively characterize installed cabling. The parameters also define the minimum configuration in which tests shall be undertaken (since to exceed the capability of the OTDR will result in poor results and incorrect interpretation). For the purposes of this document effective characterization requires that: link length can be accurately determined; local/remote interface losses can be determined; joint losses can be determined. The ability to distinguish a joint in close proximity to a reflective interface is usually beyond the capability of an OTDR. In such cases the performance of the combined interface and joint is assessed. The minimum configurations that can be effectively characterized will differ for MMF and SMF and from one wavelength to another... Mode and wavelength of operation Optical time domain reflectometers are designed with single or multiple optical sources and detection systems that are specified in terms of their modal characteristics and wavelength of operation. It is important to choose the appropriate equipment for the measurement to be made. That is: a 0 nm MMF OTDR shall be used to check the performance of MMF cabling against a specification at 0 nm; a 00 nm MMF OTDR shall be used to check the performance of MMF cabling against a specification at 00 nm; a 0 nm SMF OTDR shall be used to check the performance of SMF cabling against a specification at 0 nm; a 0 nm SMF OTDR shall be used to check the performance of SMF cabling against a specification at 0 nm. NOTE: the above does not take account of the development of DWDM systems that operate across the wavelength bands between 00 nm and 0 nm The scattering fraction (k), and the associated attenuation coefficient, within an optical fibre reduces dramatically (as the operating wavelength increases. As a result, the amount of reflected light, and the ability of the OTDR to detect and interpret it, will be highest at the shortest wavelength available for a given optical fibre type (i.e. 0 nm for MMF, 0 nm for SMF). However, a number of installationrelated loss events become more significant as operating wavelength increases. For this reason it is common to undertake testing at both wavelengths for a given optical fibre type... Range The OTDR equipment should be selected based upon the cable lengths to be measured. Early OTDR equipment was designed to see as far as possible from one end. This is generally achieved by using high power LASERs and wide LASER pulse widths. The use of such OTDRs for the measurement of the short lengths typically encountered within premises does not tend to produce particularly good results. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 0.. Pulse power With an OTDR, the peak power of the LASER source pulse defines the maximum loss range that can be measured by the equipment. This is often equated to a maximum length of cable but in reality it represents the capability of the OTDR to provide effective characterization over a given optical loss range. The optical loss is in essence a combination of connection loss and cable loss. OTDRs often provide options to select length but in reality they are options to select cabling loss. The use of excessive power in low loss systems blinds the detector at or immediately after reflective events. This prevents the location of loss events in close proximity e.g. the connections at either end of a patch cord. On the other hand, if too little optical power is launched into the optical fibre under test then the ability to observe events at the remote end of a high loss system becomes ineffective due to excessive signal noise. The selection made should be the minimum value that provides an effective characterization. The input optical power is therefore said to define the dead zone of the OTDR. The lower the power, the shorter the dead zone... Pulse width The pulse width of the OTDR defines the resolution of the instrument. This is, in essence, the accuracy with which locations of events may be undertaken. OTDRs often provide options to select resolution but in reality they are options to select LASER pulse width. Long pulse widths increase the overall power launched into the optical fibre and therefore assist in the characterization of high loss systems. The selection made should be the minimum value that provides an effective characterization... Integration or sample count In order to obtain the best characterization trace it is preferable to use a large number of samples (or integrations) of the reflected light. The higher the integration count, the longer it takes to obtain the trace. There is generally a law of diminishing returns. The selection made should be the minimum value that provides an effective characterization.. Restriction of OTDR capability.. Minimum lengths of operation... Dead zone The nonlinear end effects caused by the OTDR launching into the launch cord together with the achievable resolution of the OTDR prevent a sensible assessment of the local interface (or any other event) unless the launch cord is of a minimum length. Figure shows the same link tested using a metre (upper diagram) and a 00 metre (lower diagram). The local interface cannot be seen in the upper diagram. In the lower diagram the loss through the local interface is clearly shown and the loss of the cable in the launch cord is linear. This latter factor suggests that an effective characterization of the local interface loss may be made. The minimum length of launch cord that can be used depends upon the LASER pulse power and LASER pulse width and the lengths will differ for MMF and SMF measurements and for the operating wavelengths used. Additionally the ability to characterize a MMF installed link depends upon obtaining a representative launch condition at the local interface (see...). Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 Figure : OTDR characterization using different length launch cords... Equilibrium length (MMF) Although the installed cabling may be used for both LED and LASER sourced transmission, it is recommended that the MMF cabling be characterized with a light source that approaches a LEDlike state. This requires that the launch cord should deliver as many transmission modes as possible while not containing the high order modes that are rapidly removed within the multimode optical fibre under test. This provides a more accurate assessment of interface/joint performance and attenuation coefficient. Historically, OTDR equipment provided LASERlike launch conditions (small spot size and low N.A.) which would produce under estimates of local interface loss. Launch cords had to be treated to modify these characteristics. One method was to use a long launch lead within which the modes became well distributed. Lengths in excess of 00 metres were typical. An alternative was to use a relatively short cord (longer than the deadzone of the OTDR) but treated to attain the correct launch conditions that required mode scrambing techniques. However, the multimode optical fibres produced over the last few years are remarkably good at retaining spot size. Instead, many newer OTDR equipment now provide LEDlike launch conditions by the use of internal optical management. However, such devices may also launch high order modes that are rapidly removed within optical fibre connected to the OTDR. This can lead to overestimates of the local interface loss. Fortunately, iif long lengths of launch cord (in excess of 0 metres) are used the cord will automaticaly remove high order modes. If shorter cords are used they may be required to provide high order mode stripping (by the use of mandrel wraps as defined in FIATSD000). Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 The OTDR manufacturer should be consulted to determine their recommendations for the design features of the launch cords required to provide the correct launch conditions.... Equilibrium length (SMF) Not applicable.... Attenuation coefficient Attenuation coefficient is calculated by the OTDR by dividing the loss between any two points by the distance between the two points. If the distance is short then the impact of the division process can produce erroneous results particularly of the characterization is slightly noisy. For example, if the loss precision of the trace is 0.0dB and the length measured is 0 metres then the attenuation coefficient may swing by +/ db. For this reason, attenuation coefficient should not be measured below a certain length. The minimum lengths will differ for MMF and SMF measurements and for the operating wavelengths used. The OTDR manufacturer should be consulted to determine their recommendations for the minimum lengths over which attenuation coefficient may be measured reliably. NOTE: another common misuse of equipment occurs where connecting hardware is included in the length over which attenutation coefficient is measured. This produces highly anomalous results and shall be avoided... Ghosting Ghosting, the presence of multiple peaks on the OTDR characterization due to repeated reflections, is a restricting factor when links containing multiple interfaces are measured. The ghost is produced by a second (or third) reflection from a given event. A B Launch cord Cabling under test OTDR C D A C B D Figure : OTDR characterization showing ghost effects Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 A simple example is shown in Figure. The reflective peaks B and D are artifacts produced by multiple reflections. Ghosts appear at predetermined points (i.e. the distance from the datum to B is twice the distance from datum to A, the distance from the datum to D is the distance from the datum to C plus the distance from the datum to A). Ghosts do not add or subtract optical power from the characterization (which allows them to be easily identified on Figure ). However, if the cabling configuration under test becomes too complex the number of ghosts becomes excessive and the traces become impossible to interpret, even by experts. An example of this is shown in Figure. Some OTDR equipment has software analysis tools that remove ghosts but this is not guaranteed to function correctly in all cases. Another way to remove ghosts is to use launch cords that are longer than the links to be tested. However, the simplest method to manage ghost effects is to plan ahead and restrict the allowed test configurations. 0 0 TEST CORDS. General Figure : OTDR characterization showing complex ghost effects The length of optical fibres used to create test cords requires that they be protected in some way. In the case of the MMF launch cords they may also be required to contain treatments to provide core mode scrambling and high order mode stripping. The test cords shall take the form shown in Figure. The cables outside the closure shall be of simplex or duplex ruggedized cable as required. Inside the protective closure the optical fibre may be primary coated spliced to the external cables. This reduces the size, weight and cost of the protective closure. A label applied to either the protective closure or the ruggedized cable shall contain a unique test cord ID of the form xyzzz where: x = statement of optical fibre design (e.g. 0,, SM etc); y = L for launch cord or T for tail cord; zzz = a sequential and unique number. A label applied to the ruggedized cable at the cabling interface shall state TEST INTERFACE. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 ~ metres ~ metres Protective closure 0 0 0. Launch cords.. Simplex interfaces (MMF) Bend and strain relief Figure : Test cord schematic The launch cord shall be constructed from the same design of optical fibre as the link to be tested. The launch cord shall be terminated at one end with a connector suitable for connection to the OTDR and at the other with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The launch cord may also be treated to provide mode scrambling and high order mode stripping... Duplex interfaces (MMF) The launch cord shall contain two optical fibres enabling the testing of both optical fibres in the link by switching the OTDR connections rather than reconnecting leads at the interface under test. The OTDR connections shall be uniquely identified with markers A and B. The launch cord shall be constructed from the same design of optical fibre as the link to be tested. The launch cord shall be terminated at one end with connectors suitable for connection to the OTDR and at the other with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The launch cord may also be treated to provide mode scrambling and stripping... Simplex interfaces (SMF) The launch cord shall be constructed from the same design of optical fibre as the link to be tested. The launch cord shall be terminated at one end with a connector suitable for connection to the OTDR and at the other with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected... Duplex interfaces (SMF) The launch cord shall be constructed from the same design of optical fibre as the link to be tested. The launch cord shall contain two optical fibres enabling the testing of both optical fibres in the link by switching the OTDR connections rather than reconnecting leads at the interface under test. The OTDR connections shall be uniquely identified with markers A and B. The launch cord shall be terminated at one end with connectors suitable for connection to the OTDR and at the other with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 0. Tail cords.. Simplex interfaces (MMF) The tail cord shall be shorter than the corresponding launch lead (but longer than the dead zone of the OTDR) and be constructed from the same design of optical fibre as the link to be tested. This length ensures that the ghost D shown in Figure, if present, occurs beyond the end of the tail cord. The tail cord shall be terminated at one end with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The termination of the other end is optional... Duplex interfaces (MMF) The tail cord shall contain two optical fibres enabling the testing of both optical fibres in the link by switching the OTDR connections rather than reconnecting leads at the interface under test. The two optical fibres shall be shorter than the corresponding launch lead (but longer than the dead zone of the OTDR) but shall be of different lengths. Both optical fibres shall be constructed from the same design of optical fibre as the link to be tested. This length ensures that the ghost D shown in Figure, if present, occurs beyond the end of the tail cord. In addition, it will allow continuity/polarity tests to be made. The launch cord shall be terminated at one end with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The termination of the other end is optional... Simplex interfaces (SMF) The tail cord shall be shorter than the corresponding launch lead (but longer than the dead zone of the OTDR) and be constructed from the same design of optical fibre as the link to be tested. This length ensures that the ghost D shown in Figure, if present, occurs beyond the end of the tail cord. The tail cord shall be terminated at one end with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The termination of the other end is optional... Duplex interfaces (SMF) The tail cord shall contain two optical fibres enabling the testing of both optical fibres in the link by switching the OTDR connections rather than reconnecting leads at the interface under test. The two optical fibres shall be shorter than the corresponding launch lead (but longer than the dead zone of the OTDR) but shall be of different lengths. Both optical fibres shall be constructed from the same design of optical fibre as the link to be tested. This length ensures that the ghost D shown in Figure, if present, occurs beyond the end of the tail cord. In addition, it will allow continuity/polarity tests to be made. The tail cord shall be terminated at one end with a connector of the type and from the same manufacturer as that of the interface to which it is to be connected. The termination of the other end is optional.. Preparation Testing is not a substitute for good inspection practices. Failure to inspect and maintain the qulaity of optical fibre endfaces will impact the accuracy of the measurements made and lead to systematic errors which, in turn, may lead to the need to retest complete sections of cabling. Annex A of FIA document TSD000 contains information relating both to the inspection of terminated optical fibres and installed cabling. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 0 0 Inspection of terminated optical fibre endfaces is mandatory (assuming that it is possible for example, the use of prepolished connections do not allow endface examination of the terminated optical fibre). All connector endfaces on the test cords and the cabling under test shall be cleaned according to the instructions provided by the manufacturer of the interfaces. This should be repeated every time a test cord is connected to cabling under test. NOTE: cleaning methods typically include the use of IPA or IPA/distilled water mixtures and lintfree swabs or wipes. THE USE OF OTDR EQUIPMENT WITHIN INSTALLATIONS. Singleended measurement.. Measurement technique The singleended measurement of a link, i.e. using a launch cord only: produces a characterization of the form shown in Figure ; provides information about the general quality of the local interface and the quality of the installed cable and any embedded joints; does not allow any quantitative measurement of the interfaces unless the scattering characteristics of the optical fibre within the launch cord are the same as those of the optical fibre under test; does not allow any quantitative measurement of embedded joints unless either measurements are taken in both directions or unless the scattering characteristics of the optical fibres on either side of the embedded joint are identical; does not provide any continuity measurement for the link as a whole. The OTDR/optical source shall be selected for the mode/wavelength as defined in.. and the appropriate settings established for: range (LASER pulse power); resolution (LASER pulse width); refractive index (or IOR) (see.). A valid calibration certificate shall be available to support the use of the OTDR and the optical source at the time the tests are undertaken. To undertake the measurement: a launch cord in accordance with. shall be selected and its identifier recorded; the launch cord shall be cleaned and inspected in accordance with.; the operator of the OTDR shall record his/her details together with the launch cord identifier (this is normally possible within the OTDR software); the TEST INTERFACE end of the launch cord shall be connected to the link; if the interface under test is simplex the launch cord shall be connected to the OTDR and the test undertaken; if the interface under test is duplex then the A end of the launch cord shall be connected to the OTDR and the test undertaken then the B end of the launch cord shall be connected to the OTDR and the next test undertaken... Interpretation of results Figure shows the relevant measurement reference points for the calculation of link length and local interface loss (attenuation). It should be remembered that the measured length of the cabling under test is actually the length of the optical fibre (assuming that the correct IOR has been used) rather than the physical length of the cable. The optical fibre will always be equal to, or longer than, the cable itself. The difference between the two lengths is dependent upon the cable construction. The cable manufacturer should be consulted if specific information is required. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 Even if the scattering characteristics of the launch cord and the cabling under test are matched, the inherent variation in optical fibre tolerances render the measured loss the local interface to be valid only for the specific interfaces that are mated. Measurement of the loss (attenuation) of embedded components, i.e. joints within the cabling under test, is generally achieved by determination of the mean value of the results obtained in each direction. This is because the scattering characteristics of the optical fibre on either side of the embedded component may be different. If it can be confirmed that the scattering charateristics of the two optical fibre elements are nominaly identical then bidirectional measurement can be avoided.. Doubleended measurement The doubleended measurement of a link, i.e using a launch lead and a tail lead: provides information about the general quality of both the local and remote interface and the quality of the installed cable and any embedded joints; provides continuity measurement for the link as a whole; produces a characterization of the form shown in Figure ; does not allow any quantitative measurement of the interfaces to the cabling under test unless either measurements are taken in both directions or the scattering characteristics of the optical fibre within the launch and tail cords are the same as those of the optical fibre under test; does not allow any quantitative measurement of embedded joints unless either measurements are taken in both directions or unless the scattering characteristics of the optical fibres on either side of the embedded joint are identical; does not allow any quantitative measurement of the total link attenuation unless either measurements are taken in both directions or the scattering characteristics of the optical fibre within the launch and tail cords are the same. Launch cord Tail cord Cabling under test OTDR 0 Figure 0: Doubleended characterization The OTDR/optical source shall be selected for the mode/wavelength as defined in.. and the appropriate settings established for: range (LASER pulse power); Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 resolution (LASER pulse width); refractive index (or IOR) (see.). A valid calibration certificate shall be available to support the use of the OTDR and the optical source at the time the tests are undertaken. To undertake the measurement: a launch cord in accordance with. shall be selected and its identifier recorded; a tail cord in accordance with. shall be selected and its identifier recorded; the launch and tail cords shall be cleaned and inspected in accordance with.; the operator of the OTDR shall record his/her details together with the launch and tail cord identifiers (this is normally possible within the OTDR software); the TEST INTERFACE end of the launch and tail cords shall be connected to the link; if the interface under test is simplex the launch cord shall be connected to the OTDR and the test undertaken; if the interface under test is duplex then the A end of the launch cord shall be connected to the OTDR and the test undertaken then the B end of the launch cord shall be connected to the OTDR and the next test undertaken... Interpretation of results Figure shows the relevant measurement reference points for the determination of link length, local and remote interface loss and total link loss (attenuation). It should be remembered that the measured length of the cabling under test is actually the length of the optical fibre (assuming that the correct IOR has been used) rather than the physical length of the cable. The optical fibre will always be equal to, or longer than, the cable itself. The difference between the two lengths is dependent upon the cable construction. The cable manufacturer should be consulted if specific information is required. Even if the scattering characteristics of the launch cord, tail cord and the cabling under test are matched, the inherent variation in optical fibre tolerances render the measured loss the interfaces to be valid only for the specific interfaces that are mated. Measurement of the loss (attenuation) of embedded components, i.e. joints within the cabling under test, is generally achieved by calculation of the mean value of the results obtained in each direction. This is because the scattering characteristics of the optical fibre on either side of the embedded component may be different. If it can be confirmed that the scattering charateristics of the two optical fibre elements are nominaly identical then bidirectional measurement can be avoided. Measurement of the total link loss is generally achieved by calculation of the mean value of the results obtained in each direction for the reasons outlined above. However, if the scattering characteristics of the launch and tail cord are the same then there is no requirement for bidirectional testing. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page 0 of
FIATSD000 DATE: November 00 OTDR Launch cord Cabling under test Tail cord Local interface loss (db) Link length (m) 0 0 0 Remote interface loss (db) Figure : Doubleended characterization reference measurement points Total link loss (db).. Comparison of OTDR measurement of total link loss with those obtained by LSPM Provided that the requirements of this document are complied with, the measurements of total link loss obtained by a doubleended OTDR measurement (see..) are expected to be consistent with to those obtained from other types of measurement. However, being consistent does not mean that the results will be identical since the measured loss at any interface is only valid for the specific interfaces that are mated... The requirement for bidrectional testing There are four elements to the loss produced by a link comprising an optical fibre terminated at both ends.. The loss of the cable: this loss comes from the intrinsic loss of the cable itself plus any other losses introduced by the installation process. Obviously the latter should be minimal since induced loss is indicative of macro or microbending which would risk longterm survivability.. The quality of the end faces of the connectors applied to the ends of the cable. Essentially, if the endface inspection meets the requirements of Annex A of FIA document TSD000) then this aspect cannot be improved.. Any microbending loss created by the termination process this is expected to be minimal if manufacturers instructions are followed.. The loss introduced by the adaptors at the connections at the each end of the link. The losses are produced by coretocore offset, angular misalignment and endface separation as well as the inherent tolerance of the optical fibre core diameters and N.A, values. Hopefully, the losses in a simple installation result from the first and last of the four elements. However, OTDR equipment is used to determine the presence of the presence and location of the other loss mechanisms. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of
FIATSD000 DATE: November 00 0 0 0 The losses induced by the first three elements are omnidirectional i.e. independent of the direction of measurement. Losses due to coretocore offset, angular misalignment and endface separation within the adaptors are also omnidirectional. However, the inherent variation in optical fibre tolerances and scattering characteristics render the measured loss of any connection to be valid only for the specific interfaces that are mated. Therefore, even if the scattering characteristics of the test cords and cabling under tests are matched, results obtained when measured in both directions should be consistent although not necessarily, or even generally, identical. When bidirectional measurement of overall loss is undertaken, irrespective of the type of test, two completely separate measurements involving two sets of interfaces are obtained. As a result it is not reasonable to expect to get the same results. Further questions are raised if we do make overall loss measurements in both directions, these include which measured value do we use as the "real" value the lowest, the highest, the mean? Such a question becomes irrelevant since the variation in results in beyond operator control and since the actual channel seen by the transmission equipment will have another value dictated by the interfaces on the ends of the equipment cords used.. Refractive index (IOR) The use of the correct refractive index is important for the accurate measurement of length ONLY. It has no other impact. Where the refractive index is known it shall be used. Where no information is available the values shown in Table shall be used. 0 nm 00 nm 0 nm 0 nm SMF.0.0 MMF (0/).0.0 MMF (./).0.0 TEST RESULT MANAGEMENT Table : Default IOR values The test results shall be identified with the port ID of the local interface. In the case of duplex interfaces, the suffix A and B shall be added to the test result identifier. A database/spreadsheet shall contain a hyperlinked direct reference from the identifier of the port under test to the file containing the OTDR characterization. Each characterization shall have the link length recorded. Web: www.fiaonline.co.uk email: jane@fiasec.demon.co.uk Page of