Fiber
Content
Locating Faults
Kingfisher International Application Note A11 Locating problems can be a real challenge for a cabling technician! Repair solutions will depend on: • • • • • • • Is the situation during the initial install, or is the link in service? Is another route available to take traffic while the link is being worked on? Access to the cables: Can you walk along the route and inspect it, is it in ducts, on overhead poles or direct buried in the ground? Is the fault a break interrupting service, or just a known loss point that ought to be investigated and fixed? How long is the route, 100 meters or 100 Km? Cabling type. What sort of fault locator is readily available
You would be very well advised to spend some time experimenting with fault finding techniques for your application. This will avoid having to experiment on a live system, which may cause further damage to both the system, and your reputation. The biggest problems with fault finding are: • • Without disrupting a link, it is virtually impossible to measure a transmission signal. Non-metallic cables can be very hard to actually locate. A standard metallic locating device won't work on all dielectric cables. Unfortunately, there is no such thing as a "fiberoptic" locater, so to overcome this, it is common practice to bury some sort of metallic marker near cables for this purpose. Route lengths can be very long, eg 100 Km. That’s a long way to go looking for a tree root! These systems are quite reliable, so people often have little fault finding experience when it does go wrong. These links are often high capacity, high value, and need restoring now (no kidding), and the last working pair must not be disturbed. Because this is a relatively new technology, much of the gear and work practices related to maintenance are changing, and are poorly understood.
• • • •
Some fault location techniques are: • Optical time domain reflectometers. These are basically radars that send a pulse up the line, and analyse the echo. They excel in examining long links, up to 100 Km or more. They were developed as long range instruments, and often not so useful on short links, due to dead zone effects. Really useful attributes are it’s ability to tell you that there is an problem, and to give a good idea of it’s approximate location. When you get to the fault area, there may not give sufficient detail to actually locate the fault.
•
Kingfisher’s cold clump can be used in conjunction with an optic TDR on jelly filled cables. It works by providing a local reference marker which can be positioned near the fault site. The exact distance from the Cold Clamp to the fault can be measured on the instrument, and then physically measured on the ground. Fault location within 1 m at distances of 30 Km can be regularly achieved in this fashion. A visible fault locator can be used to find problems, particularly when working over short lengths. A clip-on identifier is not strictly a fault locator, but is included here because it is often used during fault location to avoid disturbing working systems.
• •
Visible fault location:
This technique was pioneered by the use of Helium-neon lasers producing red light at 628 nm. This worked well, however the lasers used often had a short life and were very bulky. These were gradually replaced by much more convenient solid state lasers, first at 670 nm, then 650 nm, and now commonly at 635 nm. The gradually evolving wavelength of the solid state lasers is important, because of the nature of the human eye, which responds much better to 635 nm light than 670 nm light. The actual power level is limited by safety considerations to below 0 dBm, so the lack of response at 670 nm can not be compensated by boosting the power available. At the same power level 635 nm devices appear 8 times (9dB) brighter than the older 670 nm types. Companies trying to sell the older 670 and 650 nm lasers emphasize that they are visible further along the link than the newer types, since they are attenuated less rapidly. However they are rarely, if ever, used in this manner, so the argument is spurious. It must be emphasized that use of visible fault locators is dependent on many conditions, and attempts to define them as having a particular distance range are fairly pointless. However, the maximum possible distance over which some light can be seen emerging from a cleaved end, is about 10 Km for 670 nm, and rather less for 635 nm. This is only useful if you are actually looking at ends! A common use of visible fault locators is to locate a problem or break in a patch box or cables within an exchange. The break shows as a bright red light shining through the side of the sheath. Of course the ability to do this depends on the light being able to get through the sheath. Many 3 mm patch lead cables readily allow the light through, however some colors ( particularly purple and black ) seem to be opaque to red light, and may not show anything. It is better to verify expected performance with a visible fault locator before proceeding. It is even better to take this into account when specifying the cables in the first place. A common use of visible fault locators in the LAN environments is to check continuity. Another useful function is the ability to see if light can get to a particular point on a link. To do this, put a sharp bend into the fibre, and visible light may leak out of the side of the sheath. It map be appropriate to shield as much ambient light as possible while doing this: maybe cover yourself with a ground sheet. Visible fault locator are also extremely handy for finding problems with installed splitters and active devices. Without this technique, there is often very little alternative to dismantling a
coupler assembly to find a suspected problem. Using visible fault location, it is often possible to find a fault with minimal disturbance. Visible fault locators can also be used to rescue patch leads that have one faulty connector. The faulty connector will often glow brightly when light is injected into it. CLIP ON IDENTIFIERS A clip-on identifier is clamped onto a patch lead to determine if there is a tone present, or traffic or nothing. This requires access to the fibers or patch cables, and a bit of slack to allow some bending.
Readings may be adversely affected by colored plastic coatings absorbing the light. Identifiers should be tested for the amount of increased loss they create, since this can drop out live systems. They tend not to give totally reliable results, and are often affected by stray light.
For these reasons, they should be used only to verify the link status before disconnection. They are preferably not used indiscriminately to locate a one of many possible active fibers. They are, however, a lot better than nothing! OTDRs for fault finding Optical time domain reflectometers send a powerful pulse up a link, and analyse the reflections. The reflected signal is very weak, and may require extensive averaging to reduce detection noise. The user has to input some information such as refractive index. From this, it mathematically deduces the power level at each point, and from this, it is possible to determine loss figures, and the location of point losses. In order to work over a range of applications, the pulse length can be varied from a few nsec to a few msec. Short (low energy) pulses give best distance resolution, but a noisier signal, and will only work at modest attenuation levels. Short pulses may require a lot of averaging to get a good signal, which may take some minutes. A long (high energy) pulse gives fast acquisition, a nice smooth output (ideal for commissioning), but very poor distance resolution (bad for fault location ). OTDRs have some theoretical difficulty with point losses, or reflections, in that the mathematics doesn't work very well at that point. The point loss or reflection is actually located by the intersection of the characteristics each side of it, eg by further deduction. There will also be some practical difficulty with point losses or reflections, in that the high gain detector amplifier may saturate or become slew-rate limited, creating a blind spot immediately after the event. This is called the dead zone, and is a genuine limitation. The dead zone is also pulse length dependent. The theoretically calculated dead zone is shown in the table.
Pulse length Dead zone 1 nsec 10 nsec 100 nsec 1 µsec 10 µsec 100 µsec 0.15 m ( theoretically ) 1.5 m ( theoretically ) 15 m 150 m 1.5 Km 15 Km
In practice, some older instruments have a minimum deadzone of 50 meters, and more modern units have a minimum of 2 - 10 metres on the shortest pulse lengths. Also, some modern units automatically change the pulse length as the unit searches further up the link. This is obviously highly desirable. It should also be noted that deadzone is typically specified with a fairly low level reflective impulse, such as from a mated PC polish connector. In multimode systems, the connectors are highly reflective, so longer dead zones are observed than in the instrument data sheet. This is universal in the industry, and is not the fault of one manufacturer. Originally, OTDRs were used for long range applications over many Km on telecom style links. Effectiveness on multimode systems of under a Km in length is questionable, since dead zone effects mean that it is often impossible to differentiate one loss point ( eg connector), from another. It is often impossible to do much fault location in this type of situation. This problem is often not understood by system designers, who insist on Optical TDR certification on a 100 metre run. The problem ends up as this: you need the highest performance instruments, in a situation where it is of the least possible value. The mathematical deduction process can lead to some peculiar effects: some splices and connectors appear to have optical gain. This happens when joined sections have slightly different characteristics, and the second section has a higher level of intrinsic back scatter than the first. However, if the same joint is measured from the opposite direction, the loss will appear abnormally high. This anomaly is solved by performing the measurement in both directions, and then averaging the result. From all this, it should be apparent that for fault finding, the user must be careful to optimize both distance and amplitude resolution for a particular situation, and that the job will be slower than certification. The noise reduction achievable by signal averaging is limited by the square root of the sample time. Therefore each time signal averaging is extended by a factor of 4, a 3 dB increase in range is obtained. This creates a practical limit, for example extending a 10 minute average ( fairly boring ) to 1 hour ( really boring ), only yields a 4 dB increase in range. However increasing from 1 second to 10 minutes, yields a 14 dB improvement!
Limitations of using an optical TDR by itself In fault finding applications, OTDRs look like they can measure exact distance, however the actual physical location of a fault is uncertain. Even under ideal conditions the distance uncertainty is about ± 1%, eg 20 meters per Km. Some typical causes of error are: • • Even under factory conditions, the accuracy of length markers is about ± 0.5%. By the time cables are laid, this is likely to get worse. There is some variation in the ‘take up factor’, eg the fiber / cabled length ratio. Due to this variation, we understand that experts regard length markers as the most accurate length measurement. Variations in ‘take up factor’ directly affect the accuracy of length measurement. The refractive index may vary along a route. It is often measured & specified to only 3 decimal places. Not all data will be totally accurate, not all installers record it accurately, and not all OTDRs can accept multiple values. Variations in refractive index directly affect the accuracy of length measurement. Link length may not match the route distance due to excess being coiled and left in pits, or undocumented detours. The exact route may also not be precisely mapped or followed. Minor discrepancies that would not be noticed during construction & acceptance, can cause havoc during precise fault location.
•
• •
In practice, these uncertainties do matter, and where the cause of a fault is hidden (eg ground movement, tree roots, rocks, rodents etc ), locating the loss point using OTDRs sometimes takes man days of work, and creates a network hazard while 100 meters or more is unearthed. The nitrogen marker system enables the job to be completed in less than half a day, with minimal disturbance. Cold Clamp fault location The Cold Clamp is a unique device developed by Kingfisher which overcomes some of the fundamental limitations of OTDRs. The Cold Clamp works on jelly filled cables as typically used in long distance links. A Cold clamp is attached to the cable close to the estimated fault location, but far enough away so as to avoid deadzone problems. Liquid nitrogen is poured into the Cold Clamp, which creates a temporary optical loss point of approximately 0.2 - 1 dB. This can be used as a localized reference marker which can be picked up on the optical TDR. It’s distance from the fault is measured, and then it’s distance to the fault is measured on the ground. OSP crews who have used the system find uses for it in all manner of situations where they would like to know a position accurately. For example during installation, to mark known danger points on the route, such as rivers, roads, other cables etc.
Fig 11.1: A typical requirement for the use of a Cold Clamp. Case history: A link was partially broken. An ORDR trace showed a break at 4.2779 Km. The route map showed this as close to a river crossing. During a mobile phone conversation, the site crew remembered that there had been problems at the river crossing before, so they were pretty sure they knew that the problem was at the river crossing. However, the engineer in charge decided to check with a Cold Clamp. The line was excavated and a Cold Clamp applied at a convenient point about 40 metres from the river. A trace as per Fig 11.2 was obtained, showing in general terms the break, and Cold Clamp loss point. The picture was zoomed in and the trace in Fig 11.3 obtained. This clearly showed the loss induced by the Cold Clamp at 4.185 Km and the break at 4.2779 Km. Moving the cursors to the start of each event showed a distance of 92.8 meters. This was surprising, since this was in fact 50 meters away from the expected fault site at the river crossing! There was the inevitable discussion between the crew who thought they knew
from past experience where the fault was to be found, and the measurement crew who disagreed with them! In the end, the measurement crew prevailed, the distance was measured out on the ground, and excavation revealed a fracture "within a shovel width" of the predicted location. It turned out that the construction crew had bogged a D9 dozer at the exact point of the fault. Use of the Cold Clamp in this instance saved hours of work trying to find a fault in the wrong place, with all of the extra network hazard that this would have entailed.
Fig 11.2: Trace of the fault & Cold Clamp loss.
Fig 11.3: Detail showing the relationship between the initial break and the temporarily applied Cold Clamp loss point. Particular points about this incident This was an experienced repair crew, with accurate maps, route data and other aids. They had prior knowledge of the route. It was practically the ‘ideal’ situation. Despite all this, the fault was in a different place to that expected. The fault would of course have been located and fixed in time, however use of the cold clump markedly improved the on-site processes, reduced costs & improved service provision.
Visual Fault Finder, Cable Fault Locator, Fiber Identifier & Cable Continuity Tester
KI 7600 Series Optical Power Meter
A fiber visual fault locator is used as an optical fiber fault finder, cable continuity tester, and cable fault locator over moderate distances. Our visual fiber fault locator uses the more effective 635 nm wavelength. Kingfisher fiber optics continuity testers comply with the safety requirements for Class 2 laser devices. A premium handheld optical power meter used to measure power, loss, continuity and fault find on fibre systems. • • • • • • • • • • • Intuitive to use, rugged & reliable PC interface for KITS™ Freeware Memory capacity of 1,900 dual λ results NEW Autotest loss display, with Autotest light source >360 hours battery life Optional built in 635 nm visual fault locator 1 % traceable accuracy over 22 calibrated CWDM / DWDM wavelengths NEW 3 year calibration cycle No warm up or user dark current offset dBm, dB, mW , uW, nW decibel / linear display units Metal Free Interchangeable FC, SC, ST connector adaptors. Other styles available
Contractor KIT Just 2 instruments with:
850/1300 nm LED 1310 / 1550 nm laser 635 nm visible laser Ge Meter
KI6350 Series
Visual fault locators are used during installation and repair to find faults, check continuity, verify a signal path and identify a fiber. Tests all single mode, multimode and plastic fibers. • • • Very simple to use Durable IP67 rated waterproof and drop resistant construction Class 2 laser device with 0.6 mW or -2.2 dBm typical power output Connecto r Batterie Run s Time 40 hrs
Wavelength Pen style KI 6351 Pen style KI 6352 Key ring style KI635 3
Emission
635 nm
Pulsed
2.5 mm
2 x AAA
635 nm
Pulsed
1.25 mm
2 x AAA
40 hrs
650 nm
Continuou s
2.5 mm
3 x LR44
2 hrs
KI 6150 Fiber Identifier
A optical fibre identifier is used to detect traffic or identification tones. It enables positive long distance fiber identification..
A clip on fiber identifier used to identify traffic, or a test tone generated by a source, or as a route tracer. • • • • • • Easy to use with one hand Active display of traffic or tone direction Supplied with heads for ribbon / 250 μ coated fiber, 900 μ fiber, and 3 mm patch lead (2 mm available) Excellent sensitivity for Metro / LAN applications Uses popular 9 volt battery size. Auto turn on / off Durable metal housing and quality construction Typical sensitivity for various conditions 250/125 μ fiber 1300 / 1310 nm 1550 nm -30 dBm -40 dBm 3mm yellow test lead -20 dBm -23 dBm
OTDR Precision Event Locator "Cold Clamp"
A Cold Clamp is used for long range pinpoint location of hidden fiber optic cable faults. This unique Kingfisher product is ideal for finding hidden loss points.
Overcomes the limitations of any mini OTDR or Optical Time Domain Reflectometer when used to accurately find an event • Locate the exact physical position of a hidden underground cable fault or break, within less than 1 meter, and over long distance! Reduces network hazard and days of labour as exploratory trenching is eliminated Minimises community disruption and excavation damage Finds faults and breaks in fibre cables where there is no visible evidence Minimises service disruption, since it can be used on a live system Field Proven. Telcordia (Bellcore) report available email Unique patented optical cable fault locators Use your existing training & any low specification OTDR
•
• • • • •
•
When to use an OTDR or Loss Test Set
Kingfisher International Application Note A15 There is often some confusion about when to use which type of instrument, since there is often some functional overlap when used by installers. This training note explains their relative merits. Deciding factors are usually to do with: • Labour expense. This is commonly classified as an operational expense, however this is only true if there is little skill involved. Otherwise there is a major hidden "capital expense" component due to the cost of training and the requirement to keep specialised technical staff available on call as required, for the operational life of a system. Getting specialised field staff on site may create quite significant delays, and sometimes suitable staff may simply not be available. The message here is simple:
where possible, use instruments that require modest skill levels and a minimal number of personnel. • Asset expense. This is commonly classified as being a capital expense, however is only partly true. The "Total Cost of Ownership" of an instrument includes many factors. For example a meter costs US$ 700 and is used daily. It has a 20 hour battery life, so the batteries are changed weekly at a cost of US$ 4, so that's US$ 208 a year. It needs calibrating yearly at a cost of US$ 200. So over 5 years this $ 700 instrument has actually cost $ 2,740. If re-chargeable batteries are specified, then the cost in labour and management is higher. Administration expense. This is often a hidden factor, but includes, for example: Managing the asset inventory lifecycle, producing customised acceptance reports for clients (which can take longer than acquiring the data), and the general ability to complete jobs on time with minimal project management overhead. Corporate risk management. The company must be able to show appropriate due diligence in situations where customers have suffered, or may suffer severe penalties or loss of customer confidence in the event of a system outage. These financial consequences generally greatly exceed the total t&m budget. So it is merely good business logic to ensure that t&m processes are performed correctly.
•
•
How does an OTDR work? An Optical Time Domain Reflectometer is essentially an optical radar: it sends out a flash of bright light, and measures the intensity of echo or reflections. This weak signal is averaged to reduce detection noise, and computation is used to display a trace and make a number of mathematical deductions. What are OTDRs used for?
This instrument is really good for measuring points loss on installed systems, where it is used to find faults and measure point losses such as caused by splicing. However to do this accurately is more complicated and time consuming than is commonly supposed, since a measurement should be taken from both ends of the system, and then averaged. If this is not done, spurious excess losses and "gainers" may be recorded where different fibers are joined, resulting in wasted splicing effort while non-existent faults are "repaired". This is a particular issue when measuring the fusion splice joints, where the loss is small, and the adjacent sections may have fibers with different intrinsic backscatter characteristics. OTDRs can be used for return loss measurements, although quoted accuracy is not very high. Who is likely to use an OTDR?
This is most commonly used during installation acceptance and maintenance of cables. In this role, it is likely to be used to identify point losses, the length of various cables, and to measure return loss. Optical TDR Limitations: • Interpreting the trace requires too much skill for most field technicians . These people must rely on the built in automation program to compile data tables, and may have
little idea what to do with the trace. Since only highly skilled users can set up the parameters for this automation, in some circumstances most users can get into major difficulty. • Acceptance verification is relatively easy, since standard procedures and automated measurement can generally be used. However using the same instrument for fault finding may require a totally different class of operator, who understands how control the measurement process in great detail, and also interpret the trace accurately. Because of the skill requirements, the majority of organisations end up with a small number of identified "experienced" operators, who train others, and are called out to problem situations. Limited ability to separate multiple point losses that are fairly close together. This problem happens quite regularly in practice, and is due to the "dead zone" effect. Although instruments may advertise an event dead zone of say 5 m, this is only under specific conditions. In practice the dead zone may go up to a km for long distance work. This makes these instruments of less use on short systems. Other tools, such as a visible laser, may be required to precisely identify the fault. This has become a big issue as the fibre count in cables has increased, which has caused an increase in the requirement to avoid disturbing already installed closures and racks. The distance measurement accuracy is only about 1 - 2 % at best. For example a displayed result of 12.1567 Km is actually more realistically 11.91 - 12.39 Km, an uncertainty to field staff of nearly half a Km. The reasons for this are fundamental and are due to variations in cable manufacture and index of refraction. So a measurement of 1 Km, is typically not 1 Km of cable, and certainly not the exact route length. Use of a Cold Clamp can greatly improve distance accuracy. Limited accuracy when determining the end to end loss of a system. It typically makes a poor job of measuring the loss of the end connectors, which are themselves a cause of problems. Limited use on "passive optical network" systems that use couplers or splitters to connect one source to multiple locations. This is because measuring in this configuration only works in one direction, and so this method cannot be reliable. Cannot be used in compliance with new multimode fiberoptics loss measuring standards, which mandate the use on an LED source with defined characteristics. Accidental connection to a receiver can damage the receiver due to the high instantaneous power levels. There can be some optical safety issues associated with the high pulse powers in these instruments, which often exceed +20 dBm. Factors to look for are now typically ease of use, quality of automation program, good local support, and compatibility with previously acquired measurement file types.
•
•
•
•
•
• •
•
How does an OLTS work?
Optical loss test sets incorporate a stable source and a meter. Measurements are made with a two stage process. First the source power is measured (referenced), then light is put through the device to be tested, and a second measurement is made. The difference in the measurements is the device loss. What is an OLTS best used for?
•
A pair of these units can be used to simply and reliably measure end to end loss of installed systems, preferably using a bi-directional or two-way method at multiple wavelengths, with minimum inventory and modest technician skill levels. There are a wide variety of LTS, with wide differences in resulting productivity. The simplest are just a source and meter in one box. The most sophisticated perform automated bi-directional, multi-wavelength loss and return loss measurement in a few seconds. LTS are fairly easy to use: in most organisations, many technical staff could perform a loss measurement. Most LTS can be used to measure the absolute power of a transmitter or receiver, and some can be used as a tone transmitter or detector. If the LTS measures return loss as well, the requirements for optical TDR evaluation may be eliminated in some cases. Some LTS actually provide the simplest possible solution of all types, since their automation makes them less complex to use than a separate source and meter. A single LTS instrument may be cheaper than a separate source and meter, and so may be a cheaper solution in some cases.
•
• • • • •
Who is likely to use an LTS?
These are widely used by almost everyone involved in hands-on work, since it is the simplest way to ensure that connections are up to standard. Used during work on component manufacture, equipment manufacture, cables and transmission systems. In this role, it is used to to formally accept end to end loss specifications, and sometimes to measure return loss. LTS Limitations: • An LTS cannot identify the position of a point fault in a route that otherwise passes the end to end loss specifications. For this reason, both OTDRs and LTS are often used for acceptance verification. In some situations, it is cheaper and easier to use a separate source and meter. Specific instruments may have particular limitations to do with accuracy, warm up periods, battery lifetime and ease of use. An LTS should have some sort of automated wavelength synchronisation for measurement at multiple wavelengths. Not all units have this useful feature.
• • •
What a Source and Meter are really good at doing: These perform the same functions as an LTS, however with greater flexibility, since a single source and meter pair can also be used each end of a a link. Who is likely to use a Source and Meter?
This performs a similar role to an LTS, with the advantage of great flexibility, and the disadvantage of increased inventory and slower operation. Transmission personnel may use a meter on it's own to measure the absolute power of transmitters and receivers.
Source and Meter Limitations: An LTS may cost less to own than a separate Specific instruments may have particular limitations to do with accuracy, warm up periods, battery lifetime and ease of use. Source and meter combinations that don't have some sort of automated wavelength synchronisation will be harder to operate.
How much integration is desirable? Instruments are available with different levels of integration. It is possible to buy OTDRs with built in source, meter, visible fault finder, talk set etc. However is this desirable? The answer to this is "often not", whatever a sales person says. It depends on: • Staff Skill levels. Typically, the higher the integration, the greater the operating complexity. Functionality that isn't used is not only wasted investment, it may be costing more in wasted staff time and frustration. Overall asset cost. If equipping a staff member with a meter means providing OTDRs with a built in meter, that is an expensive way to operate. Gear should be optimised for the most common tasks. This may vary depending on the group concerned. There is probably no one-size-fits-all solution. Outside Plant crew usually have different requirements to transmission crew.
• •
Kingfisher International Application Note A11 Locating problems can be a real challenge for a cabling technician! Repair solutions will depend on: • • • • • • • Is the situation during the initial install, or is the link in service? Is another route available to take traffic while the link is being worked on? Access to the cables: Can you walk along the route and inspect it, is it in ducts, on overhead poles or direct buried in the ground? Is the fault a break interrupting service, or just a known loss point that ought to be investigated and fixed? How long is the route, 100 meters or 100 Km? Cabling type. What sort of fault locator is readily available
You would be very well advised to spend some time experimenting with fault finding techniques for your application. This will avoid having to experiment on a live system, which may cause further damage to both the system, and your reputation. The biggest problems with fault finding are: • • Without disrupting a link, it is virtually impossible to measure a transmission signal. Non-metallic cables can be very hard to actually locate. A standard metallic locating device won't work on all dielectric cables. Unfortunately, there is no such thing as a "fiberoptic" locater, so to overcome this, it is common practice to bury some sort of metallic marker near cables for this purpose. Route lengths can be very long, eg 100 Km. That’s a long way to go looking for a tree root! These systems are quite reliable, so people often have little fault finding experience when it does go wrong. These links are often high capacity, high value, and need restoring now (no kidding), and the last working pair must not be disturbed. Because this is a relatively new technology, much of the gear and work practices related to maintenance are changing, and are poorly understood.
• • • •
Some fault location techniques are: • Optical time domain reflectometers. These are basically radars that send a pulse up the line, and analyse the echo. They excel in examining long links, up to 100 Km or more. They were developed as long range instruments, and often not so useful on short links, due to dead zone effects. Really useful attributes are it’s ability to tell you that there is an problem, and to give a good idea of it’s approximate location. When you get to the fault area, there may not give sufficient detail to actually locate the fault.
•
Kingfisher’s cold clump can be used in conjunction with an optic TDR on jelly filled cables. It works by providing a local reference marker which can be positioned near the fault site. The exact distance from the Cold Clamp to the fault can be measured on the instrument, and then physically measured on the ground. Fault location within 1 m at distances of 30 Km can be regularly achieved in this fashion. A visible fault locator can be used to find problems, particularly when working over short lengths. A clip-on identifier is not strictly a fault locator, but is included here because it is often used during fault location to avoid disturbing working systems.
• •
Visible fault location:
This technique was pioneered by the use of Helium-neon lasers producing red light at 628 nm. This worked well, however the lasers used often had a short life and were very bulky. These were gradually replaced by much more convenient solid state lasers, first at 670 nm, then 650 nm, and now commonly at 635 nm. The gradually evolving wavelength of the solid state lasers is important, because of the nature of the human eye, which responds much better to 635 nm light than 670 nm light. The actual power level is limited by safety considerations to below 0 dBm, so the lack of response at 670 nm can not be compensated by boosting the power available. At the same power level 635 nm devices appear 8 times (9dB) brighter than the older 670 nm types. Companies trying to sell the older 670 and 650 nm lasers emphasize that they are visible further along the link than the newer types, since they are attenuated less rapidly. However they are rarely, if ever, used in this manner, so the argument is spurious. It must be emphasized that use of visible fault locators is dependent on many conditions, and attempts to define them as having a particular distance range are fairly pointless. However, the maximum possible distance over which some light can be seen emerging from a cleaved end, is about 10 Km for 670 nm, and rather less for 635 nm. This is only useful if you are actually looking at ends! A common use of visible fault locators is to locate a problem or break in a patch box or cables within an exchange. The break shows as a bright red light shining through the side of the sheath. Of course the ability to do this depends on the light being able to get through the sheath. Many 3 mm patch lead cables readily allow the light through, however some colors ( particularly purple and black ) seem to be opaque to red light, and may not show anything. It is better to verify expected performance with a visible fault locator before proceeding. It is even better to take this into account when specifying the cables in the first place. A common use of visible fault locators in the LAN environments is to check continuity. Another useful function is the ability to see if light can get to a particular point on a link. To do this, put a sharp bend into the fibre, and visible light may leak out of the side of the sheath. It map be appropriate to shield as much ambient light as possible while doing this: maybe cover yourself with a ground sheet. Visible fault locator are also extremely handy for finding problems with installed splitters and active devices. Without this technique, there is often very little alternative to dismantling a
coupler assembly to find a suspected problem. Using visible fault location, it is often possible to find a fault with minimal disturbance. Visible fault locators can also be used to rescue patch leads that have one faulty connector. The faulty connector will often glow brightly when light is injected into it. CLIP ON IDENTIFIERS A clip-on identifier is clamped onto a patch lead to determine if there is a tone present, or traffic or nothing. This requires access to the fibers or patch cables, and a bit of slack to allow some bending.
Readings may be adversely affected by colored plastic coatings absorbing the light. Identifiers should be tested for the amount of increased loss they create, since this can drop out live systems. They tend not to give totally reliable results, and are often affected by stray light.
For these reasons, they should be used only to verify the link status before disconnection. They are preferably not used indiscriminately to locate a one of many possible active fibers. They are, however, a lot better than nothing! OTDRs for fault finding Optical time domain reflectometers send a powerful pulse up a link, and analyse the reflections. The reflected signal is very weak, and may require extensive averaging to reduce detection noise. The user has to input some information such as refractive index. From this, it mathematically deduces the power level at each point, and from this, it is possible to determine loss figures, and the location of point losses. In order to work over a range of applications, the pulse length can be varied from a few nsec to a few msec. Short (low energy) pulses give best distance resolution, but a noisier signal, and will only work at modest attenuation levels. Short pulses may require a lot of averaging to get a good signal, which may take some minutes. A long (high energy) pulse gives fast acquisition, a nice smooth output (ideal for commissioning), but very poor distance resolution (bad for fault location ). OTDRs have some theoretical difficulty with point losses, or reflections, in that the mathematics doesn't work very well at that point. The point loss or reflection is actually located by the intersection of the characteristics each side of it, eg by further deduction. There will also be some practical difficulty with point losses or reflections, in that the high gain detector amplifier may saturate or become slew-rate limited, creating a blind spot immediately after the event. This is called the dead zone, and is a genuine limitation. The dead zone is also pulse length dependent. The theoretically calculated dead zone is shown in the table.
Pulse length Dead zone 1 nsec 10 nsec 100 nsec 1 µsec 10 µsec 100 µsec 0.15 m ( theoretically ) 1.5 m ( theoretically ) 15 m 150 m 1.5 Km 15 Km
In practice, some older instruments have a minimum deadzone of 50 meters, and more modern units have a minimum of 2 - 10 metres on the shortest pulse lengths. Also, some modern units automatically change the pulse length as the unit searches further up the link. This is obviously highly desirable. It should also be noted that deadzone is typically specified with a fairly low level reflective impulse, such as from a mated PC polish connector. In multimode systems, the connectors are highly reflective, so longer dead zones are observed than in the instrument data sheet. This is universal in the industry, and is not the fault of one manufacturer. Originally, OTDRs were used for long range applications over many Km on telecom style links. Effectiveness on multimode systems of under a Km in length is questionable, since dead zone effects mean that it is often impossible to differentiate one loss point ( eg connector), from another. It is often impossible to do much fault location in this type of situation. This problem is often not understood by system designers, who insist on Optical TDR certification on a 100 metre run. The problem ends up as this: you need the highest performance instruments, in a situation where it is of the least possible value. The mathematical deduction process can lead to some peculiar effects: some splices and connectors appear to have optical gain. This happens when joined sections have slightly different characteristics, and the second section has a higher level of intrinsic back scatter than the first. However, if the same joint is measured from the opposite direction, the loss will appear abnormally high. This anomaly is solved by performing the measurement in both directions, and then averaging the result. From all this, it should be apparent that for fault finding, the user must be careful to optimize both distance and amplitude resolution for a particular situation, and that the job will be slower than certification. The noise reduction achievable by signal averaging is limited by the square root of the sample time. Therefore each time signal averaging is extended by a factor of 4, a 3 dB increase in range is obtained. This creates a practical limit, for example extending a 10 minute average ( fairly boring ) to 1 hour ( really boring ), only yields a 4 dB increase in range. However increasing from 1 second to 10 minutes, yields a 14 dB improvement!
Limitations of using an optical TDR by itself In fault finding applications, OTDRs look like they can measure exact distance, however the actual physical location of a fault is uncertain. Even under ideal conditions the distance uncertainty is about ± 1%, eg 20 meters per Km. Some typical causes of error are: • • Even under factory conditions, the accuracy of length markers is about ± 0.5%. By the time cables are laid, this is likely to get worse. There is some variation in the ‘take up factor’, eg the fiber / cabled length ratio. Due to this variation, we understand that experts regard length markers as the most accurate length measurement. Variations in ‘take up factor’ directly affect the accuracy of length measurement. The refractive index may vary along a route. It is often measured & specified to only 3 decimal places. Not all data will be totally accurate, not all installers record it accurately, and not all OTDRs can accept multiple values. Variations in refractive index directly affect the accuracy of length measurement. Link length may not match the route distance due to excess being coiled and left in pits, or undocumented detours. The exact route may also not be precisely mapped or followed. Minor discrepancies that would not be noticed during construction & acceptance, can cause havoc during precise fault location.
•
• •
In practice, these uncertainties do matter, and where the cause of a fault is hidden (eg ground movement, tree roots, rocks, rodents etc ), locating the loss point using OTDRs sometimes takes man days of work, and creates a network hazard while 100 meters or more is unearthed. The nitrogen marker system enables the job to be completed in less than half a day, with minimal disturbance. Cold Clamp fault location The Cold Clamp is a unique device developed by Kingfisher which overcomes some of the fundamental limitations of OTDRs. The Cold Clamp works on jelly filled cables as typically used in long distance links. A Cold clamp is attached to the cable close to the estimated fault location, but far enough away so as to avoid deadzone problems. Liquid nitrogen is poured into the Cold Clamp, which creates a temporary optical loss point of approximately 0.2 - 1 dB. This can be used as a localized reference marker which can be picked up on the optical TDR. It’s distance from the fault is measured, and then it’s distance to the fault is measured on the ground. OSP crews who have used the system find uses for it in all manner of situations where they would like to know a position accurately. For example during installation, to mark known danger points on the route, such as rivers, roads, other cables etc.
Fig 11.1: A typical requirement for the use of a Cold Clamp. Case history: A link was partially broken. An ORDR trace showed a break at 4.2779 Km. The route map showed this as close to a river crossing. During a mobile phone conversation, the site crew remembered that there had been problems at the river crossing before, so they were pretty sure they knew that the problem was at the river crossing. However, the engineer in charge decided to check with a Cold Clamp. The line was excavated and a Cold Clamp applied at a convenient point about 40 metres from the river. A trace as per Fig 11.2 was obtained, showing in general terms the break, and Cold Clamp loss point. The picture was zoomed in and the trace in Fig 11.3 obtained. This clearly showed the loss induced by the Cold Clamp at 4.185 Km and the break at 4.2779 Km. Moving the cursors to the start of each event showed a distance of 92.8 meters. This was surprising, since this was in fact 50 meters away from the expected fault site at the river crossing! There was the inevitable discussion between the crew who thought they knew
from past experience where the fault was to be found, and the measurement crew who disagreed with them! In the end, the measurement crew prevailed, the distance was measured out on the ground, and excavation revealed a fracture "within a shovel width" of the predicted location. It turned out that the construction crew had bogged a D9 dozer at the exact point of the fault. Use of the Cold Clamp in this instance saved hours of work trying to find a fault in the wrong place, with all of the extra network hazard that this would have entailed.
Fig 11.2: Trace of the fault & Cold Clamp loss.
Fig 11.3: Detail showing the relationship between the initial break and the temporarily applied Cold Clamp loss point. Particular points about this incident This was an experienced repair crew, with accurate maps, route data and other aids. They had prior knowledge of the route. It was practically the ‘ideal’ situation. Despite all this, the fault was in a different place to that expected. The fault would of course have been located and fixed in time, however use of the cold clump markedly improved the on-site processes, reduced costs & improved service provision.
Visual Fault Finder, Cable Fault Locator, Fiber Identifier & Cable Continuity Tester
KI 7600 Series Optical Power Meter
A fiber visual fault locator is used as an optical fiber fault finder, cable continuity tester, and cable fault locator over moderate distances. Our visual fiber fault locator uses the more effective 635 nm wavelength. Kingfisher fiber optics continuity testers comply with the safety requirements for Class 2 laser devices. A premium handheld optical power meter used to measure power, loss, continuity and fault find on fibre systems. • • • • • • • • • • • Intuitive to use, rugged & reliable PC interface for KITS™ Freeware Memory capacity of 1,900 dual λ results NEW Autotest loss display, with Autotest light source >360 hours battery life Optional built in 635 nm visual fault locator 1 % traceable accuracy over 22 calibrated CWDM / DWDM wavelengths NEW 3 year calibration cycle No warm up or user dark current offset dBm, dB, mW , uW, nW decibel / linear display units Metal Free Interchangeable FC, SC, ST connector adaptors. Other styles available
Contractor KIT Just 2 instruments with:
850/1300 nm LED 1310 / 1550 nm laser 635 nm visible laser Ge Meter
KI6350 Series
Visual fault locators are used during installation and repair to find faults, check continuity, verify a signal path and identify a fiber. Tests all single mode, multimode and plastic fibers. • • • Very simple to use Durable IP67 rated waterproof and drop resistant construction Class 2 laser device with 0.6 mW or -2.2 dBm typical power output Connecto r Batterie Run s Time 40 hrs
Wavelength Pen style KI 6351 Pen style KI 6352 Key ring style KI635 3
Emission
635 nm
Pulsed
2.5 mm
2 x AAA
635 nm
Pulsed
1.25 mm
2 x AAA
40 hrs
650 nm
Continuou s
2.5 mm
3 x LR44
2 hrs
KI 6150 Fiber Identifier
A optical fibre identifier is used to detect traffic or identification tones. It enables positive long distance fiber identification..
A clip on fiber identifier used to identify traffic, or a test tone generated by a source, or as a route tracer. • • • • • • Easy to use with one hand Active display of traffic or tone direction Supplied with heads for ribbon / 250 μ coated fiber, 900 μ fiber, and 3 mm patch lead (2 mm available) Excellent sensitivity for Metro / LAN applications Uses popular 9 volt battery size. Auto turn on / off Durable metal housing and quality construction Typical sensitivity for various conditions 250/125 μ fiber 1300 / 1310 nm 1550 nm -30 dBm -40 dBm 3mm yellow test lead -20 dBm -23 dBm
OTDR Precision Event Locator "Cold Clamp"
A Cold Clamp is used for long range pinpoint location of hidden fiber optic cable faults. This unique Kingfisher product is ideal for finding hidden loss points.
Overcomes the limitations of any mini OTDR or Optical Time Domain Reflectometer when used to accurately find an event • Locate the exact physical position of a hidden underground cable fault or break, within less than 1 meter, and over long distance! Reduces network hazard and days of labour as exploratory trenching is eliminated Minimises community disruption and excavation damage Finds faults and breaks in fibre cables where there is no visible evidence Minimises service disruption, since it can be used on a live system Field Proven. Telcordia (Bellcore) report available email Unique patented optical cable fault locators Use your existing training & any low specification OTDR
•
• • • • •
•
When to use an OTDR or Loss Test Set
Kingfisher International Application Note A15 There is often some confusion about when to use which type of instrument, since there is often some functional overlap when used by installers. This training note explains their relative merits. Deciding factors are usually to do with: • Labour expense. This is commonly classified as an operational expense, however this is only true if there is little skill involved. Otherwise there is a major hidden "capital expense" component due to the cost of training and the requirement to keep specialised technical staff available on call as required, for the operational life of a system. Getting specialised field staff on site may create quite significant delays, and sometimes suitable staff may simply not be available. The message here is simple:
where possible, use instruments that require modest skill levels and a minimal number of personnel. • Asset expense. This is commonly classified as being a capital expense, however is only partly true. The "Total Cost of Ownership" of an instrument includes many factors. For example a meter costs US$ 700 and is used daily. It has a 20 hour battery life, so the batteries are changed weekly at a cost of US$ 4, so that's US$ 208 a year. It needs calibrating yearly at a cost of US$ 200. So over 5 years this $ 700 instrument has actually cost $ 2,740. If re-chargeable batteries are specified, then the cost in labour and management is higher. Administration expense. This is often a hidden factor, but includes, for example: Managing the asset inventory lifecycle, producing customised acceptance reports for clients (which can take longer than acquiring the data), and the general ability to complete jobs on time with minimal project management overhead. Corporate risk management. The company must be able to show appropriate due diligence in situations where customers have suffered, or may suffer severe penalties or loss of customer confidence in the event of a system outage. These financial consequences generally greatly exceed the total t&m budget. So it is merely good business logic to ensure that t&m processes are performed correctly.
•
•
How does an OTDR work? An Optical Time Domain Reflectometer is essentially an optical radar: it sends out a flash of bright light, and measures the intensity of echo or reflections. This weak signal is averaged to reduce detection noise, and computation is used to display a trace and make a number of mathematical deductions. What are OTDRs used for?
This instrument is really good for measuring points loss on installed systems, where it is used to find faults and measure point losses such as caused by splicing. However to do this accurately is more complicated and time consuming than is commonly supposed, since a measurement should be taken from both ends of the system, and then averaged. If this is not done, spurious excess losses and "gainers" may be recorded where different fibers are joined, resulting in wasted splicing effort while non-existent faults are "repaired". This is a particular issue when measuring the fusion splice joints, where the loss is small, and the adjacent sections may have fibers with different intrinsic backscatter characteristics. OTDRs can be used for return loss measurements, although quoted accuracy is not very high. Who is likely to use an OTDR?
This is most commonly used during installation acceptance and maintenance of cables. In this role, it is likely to be used to identify point losses, the length of various cables, and to measure return loss. Optical TDR Limitations: • Interpreting the trace requires too much skill for most field technicians . These people must rely on the built in automation program to compile data tables, and may have
little idea what to do with the trace. Since only highly skilled users can set up the parameters for this automation, in some circumstances most users can get into major difficulty. • Acceptance verification is relatively easy, since standard procedures and automated measurement can generally be used. However using the same instrument for fault finding may require a totally different class of operator, who understands how control the measurement process in great detail, and also interpret the trace accurately. Because of the skill requirements, the majority of organisations end up with a small number of identified "experienced" operators, who train others, and are called out to problem situations. Limited ability to separate multiple point losses that are fairly close together. This problem happens quite regularly in practice, and is due to the "dead zone" effect. Although instruments may advertise an event dead zone of say 5 m, this is only under specific conditions. In practice the dead zone may go up to a km for long distance work. This makes these instruments of less use on short systems. Other tools, such as a visible laser, may be required to precisely identify the fault. This has become a big issue as the fibre count in cables has increased, which has caused an increase in the requirement to avoid disturbing already installed closures and racks. The distance measurement accuracy is only about 1 - 2 % at best. For example a displayed result of 12.1567 Km is actually more realistically 11.91 - 12.39 Km, an uncertainty to field staff of nearly half a Km. The reasons for this are fundamental and are due to variations in cable manufacture and index of refraction. So a measurement of 1 Km, is typically not 1 Km of cable, and certainly not the exact route length. Use of a Cold Clamp can greatly improve distance accuracy. Limited accuracy when determining the end to end loss of a system. It typically makes a poor job of measuring the loss of the end connectors, which are themselves a cause of problems. Limited use on "passive optical network" systems that use couplers or splitters to connect one source to multiple locations. This is because measuring in this configuration only works in one direction, and so this method cannot be reliable. Cannot be used in compliance with new multimode fiberoptics loss measuring standards, which mandate the use on an LED source with defined characteristics. Accidental connection to a receiver can damage the receiver due to the high instantaneous power levels. There can be some optical safety issues associated with the high pulse powers in these instruments, which often exceed +20 dBm. Factors to look for are now typically ease of use, quality of automation program, good local support, and compatibility with previously acquired measurement file types.
•
•
•
•
•
• •
•
How does an OLTS work?
Optical loss test sets incorporate a stable source and a meter. Measurements are made with a two stage process. First the source power is measured (referenced), then light is put through the device to be tested, and a second measurement is made. The difference in the measurements is the device loss. What is an OLTS best used for?
•
A pair of these units can be used to simply and reliably measure end to end loss of installed systems, preferably using a bi-directional or two-way method at multiple wavelengths, with minimum inventory and modest technician skill levels. There are a wide variety of LTS, with wide differences in resulting productivity. The simplest are just a source and meter in one box. The most sophisticated perform automated bi-directional, multi-wavelength loss and return loss measurement in a few seconds. LTS are fairly easy to use: in most organisations, many technical staff could perform a loss measurement. Most LTS can be used to measure the absolute power of a transmitter or receiver, and some can be used as a tone transmitter or detector. If the LTS measures return loss as well, the requirements for optical TDR evaluation may be eliminated in some cases. Some LTS actually provide the simplest possible solution of all types, since their automation makes them less complex to use than a separate source and meter. A single LTS instrument may be cheaper than a separate source and meter, and so may be a cheaper solution in some cases.
•
• • • • •
Who is likely to use an LTS?
These are widely used by almost everyone involved in hands-on work, since it is the simplest way to ensure that connections are up to standard. Used during work on component manufacture, equipment manufacture, cables and transmission systems. In this role, it is used to to formally accept end to end loss specifications, and sometimes to measure return loss. LTS Limitations: • An LTS cannot identify the position of a point fault in a route that otherwise passes the end to end loss specifications. For this reason, both OTDRs and LTS are often used for acceptance verification. In some situations, it is cheaper and easier to use a separate source and meter. Specific instruments may have particular limitations to do with accuracy, warm up periods, battery lifetime and ease of use. An LTS should have some sort of automated wavelength synchronisation for measurement at multiple wavelengths. Not all units have this useful feature.
• • •
What a Source and Meter are really good at doing: These perform the same functions as an LTS, however with greater flexibility, since a single source and meter pair can also be used each end of a a link. Who is likely to use a Source and Meter?
This performs a similar role to an LTS, with the advantage of great flexibility, and the disadvantage of increased inventory and slower operation. Transmission personnel may use a meter on it's own to measure the absolute power of transmitters and receivers.
Source and Meter Limitations: An LTS may cost less to own than a separate Specific instruments may have particular limitations to do with accuracy, warm up periods, battery lifetime and ease of use. Source and meter combinations that don't have some sort of automated wavelength synchronisation will be harder to operate.
How much integration is desirable? Instruments are available with different levels of integration. It is possible to buy OTDRs with built in source, meter, visible fault finder, talk set etc. However is this desirable? The answer to this is "often not", whatever a sales person says. It depends on: • Staff Skill levels. Typically, the higher the integration, the greater the operating complexity. Functionality that isn't used is not only wasted investment, it may be costing more in wasted staff time and frustration. Overall asset cost. If equipping a staff member with a meter means providing OTDRs with a built in meter, that is an expensive way to operate. Gear should be optimised for the most common tasks. This may vary depending on the group concerned. There is probably no one-size-fits-all solution. Outside Plant crew usually have different requirements to transmission crew.
• •
