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FTTX Installation Verification Testing & Maintenance Troubleshooting

FTTX Installation Verification Testing & Maintenance Troubleshooting

FTTH-installation-testing-troubleshootingWith the increasing demands for bandwidth, FTTH (Fiber to the Home) is considered as a next-generation technology for delivering more bandwidth, reliability, flexibility and security to end-users. PON (Passive Optical Network) is the main structure of FTTH network which can provide optical fiber bandwidth advantages at a lower cost than P2P (Point to Point) architecture affords. However, PON presents unique test challenges when installing and maintaining the FTTH network. Furthermore, optical testing is typically performed at various points in a network’s lifetime. Thus, it is very necessary to know which tests should be done and when and where to do it.

In general, optical testing in FTTH can be divided into two part—installation verification testing and maintenance troubleshooting. This is a very general classification according to the state of the network. Installation verification testing occurs as the network is being constructed or after network installation is complete, but before the network is activated. While maintenance troubleshooting is performed when service outages occur, and typically requires rapid response to restore service as quickly as possible.

In installation verification testing, the most complete testing is performed, including insertion and return loss testing as well as OTDR (Optical Time Domain Reflectometer) testing. Pass/fail criteria may be applied to end-to-end length, loss and optical return loss results, as well as to individual event loss and reflectance measurements for splices, connectors, and splitters. Formal reports may be generated, including all of the measurements, OTDR traces, pass/fail criteria and pass/fail results.

Maintenance troubleshooting is done through reroute restoration and/or fault location, repair, and verification before restoring active service. Troubleshooting may also require non-disruptive fiber identification to ensure in-service fibers are not disconnected. Maintenance personnel may require a visual fault locator (VFL) to precisely pinpoint the location of breaks or macro-bends in splice or access enclosures.

Actually, the installation verification testing and maintenance troubleshooting is not as simple as description above. They usually include various test according to different test location and purpose. And for different tests, the performed time and test equipment, as well as the test wavelengths are not fully consistent. To understand this better, here I use a table to summarize.

Click the table to view large version.

FTTx-test

Relationship Between Dynamic Range & Dead Zones

Relationship Between Dynamic Range & Dead Zones

OTDR_testOTDR (Optical Time Domain Reflectometer), as an important test instrument, is widely used in OSP (Outside Plant) and premises cabling networks to characterize an optical fiber. Dynamic range and dead zone are two main specifications of an OTDR. So, is there any relationship between them? And what do they mean to OTDR performance?

Dynamic Range

Dynamic range is the dB difference between the initial power level reflected from the fiber under test and the value equal to the noise floor of the detector. It is specified at the OTDR’s largest pulse width when making a measurement for 3 minutes. Dynamic range is one of the most important characteristics of an OTDR, because it determines the maximum observable length of a fiber and the OTDR suitability for analyzing a particular network. The higher the dynamic range, the higher the SNR (Signal to Noise Ratio), and the better the trace and event detection. The dynamic range is relatively difficult to determine because all manufactures do not use a standard computation method. Additionally, depending on the noise level reference, there are many difinitions of dynamic range, as the following picture shown:

dynamic_range

Dead Zones

The dead zone of an OTDR is the distance (or time) where the OTDR cannot detect or precisely localize any event or artifact on the fiber link. When a strong reflection occurs, the power received by the detector is saturable. It requires time to recover from its saturated condition. During this time, it will not detect the backscattered signal accurately. We call the length of fiber that is not fully characterized during this period as dead zone. In general, dead zone on the OTDR trace can be divided into event dead zone (EDZ) and the attenuation dead zone (ADZ).

OTDR_dead_zone

Relationship Between Dynamic Range and Dead Zones

In simple terms, the relationship between dynamic range and dead zone is directly proportional, i.e., the higher the dynamic range, the longer the dead zone. As we know, dead zones can be reduced by using a lower pulse width which will decrease the dynamic range. Why? For example, if more dynamic range is needed to test a longer fiber, a wider test pulse is required which will result in a longer deadzone. Because an OTDR will not have “high dynamic range” for short fiber lengths. So, how to balance the relationship between the dynamic range and dead zones? Two common situations will be introduced in the following:

For Premises Fiber Testing & Troubleshooting
In this case, short dead zones are much more important than dynamic range since the distances are short enough that you do not need a great deal of dynamic range. However, in order to detect patch cords and measure the loss at each end of a short fiber link, short dead zones are required.

For Long-Haul Fiber Testing & Communication
Here, the long-haul applications refers to the distance that is greater than 20 km. In this case, loss of the fiber itself creates a significant amount of loss, thus dynamic range is an important specification for these long fiber links. Additionally, distance range is also important for long links. However, OTDR suppliers do not always specify dynamic range in a meaningful way.

Summary

Dynamic range and dead zones are both important specification of OTDRs and they have a directly proportional relationship. As dynamic range increases, the deadzone increases. To balance the relationship between the dynamic range and dead zones should depend on the application enviroment. In addition, due to the specification difference between different OTDR manufactures, it is important to read and understand the fine print in the specification sheets before buying.

Fiber Optic Cable Plants Testing with OTDR

Fiber Optic Cable Plants Testing with OTDR

OTDRIf you are a fiber optic cable (FOC) technician or plants installer, you may be very familiar with Optical Time Domain Reflectometer (OTDR). OTDR testing creates a snapshot of a fiber optic cable. This test is commonly used to verify the quality of the installation and troubleshoot problems. OTDR testing requires interpretation of the data acquired, called the trace or signature, by a skilled operator. No matter you are a beginner or a workflow expert, you should master the basic operation skills and considerations of OTDR to ensure the accuracy and correctness of the test result. This paper is a basic guide of fiber optic cable plants testing with OTDR.

Equipment Needed To Perform This Test
  • OTDR with modules appropriate for the cable plant (e.g. multimode: 850 and/or 1300nm, singlemode, 1310, 1550 and/or 1625nm.)
  • Launch and/or receive reference cables of the same fiber type and size as the cable plant and with connectors compatible to those on the cable plant.Notes:
    a. If you are only testing for length, you only need a launch reference cable. The receive cable allows you to measure loss of the final connector on the cable.
    b. Reference cables must be long enough for the OTDR’s initial test pulse to settle down back to the baseline.c. Connectors on the launch and receive cables must be in good condition (low loss) to properly test connectors on the cable under test.
  • Cleaning supplies.
Test Procedure
  • Turn on OTDR and allow time to warm-up.
  • Set parameters on OTDR appropriate for the cable plant being tested (range, wavelength, number of averages, etc.)
  • Clean all connectors and mating adapters.
  • Attach launch reference cable to OTDR and to cable plant under test.
  • Attach optional receive cable to far end of cable under test.
  • Acquire trace and analyze.

OTDR test diagram

Note: Most OTDRs have an “auto test” function, but these functions are not foolproof. Most problems with OTDR tests occur when untrained users use the autotest function without having an understanding of how the instrument works, what a good trace looks like and, most inportantly, what are the characteristics of the cable plant they are testing (length, number and locations of splices and connectors). Refer to the next section on reading OTDR traces. Once you are confident that the autotest function is giving valid results, it is a major timesaver in OTDR testing.

Options For Testing
  • Use of the receive reference cable is optional, it is required if the far end connector loss is to be measured and included in total cable plant loss
  • Testing at more than one wavelength may be required. Longer wavelength testing is often used to find stress related to installation problems. Traces may be compared for analysis.
Documentation

Record the date of the test, operator, test equipment used, cable and fiber identification, test wavelength(s) and all traces for the fiber under test.

OTDR Trance Information

information-of-OTDR-trances

Notes
  • Insertion loss testing of the cable plant is recommended for acceptance testing.
  • Not all cable plants are long enough for OTDR testing. Ensure the OTDR has sufficient resolution for the cables being tested.
  • Always use a launch cable long enough to allow the OTDR to recover from test pulse overload and permit proper testing of the cable plant.
  • Do not use the OTDR automatic cable analysis until a skilled technician analyzes a trace and confirms it is appropriate for the cable plant under test.

Reference:

  • Fiber Optic Testing With Optical Time Domain Reflectometers – OTDRs (FOA)
  • OTDR Testing of Fiber Optic Cable Plants (FOA)
Overview Of OTDRs

Overview Of OTDRs

When you need to measure points loss on installed systems, where it is used to find faults and measure point losses such as caused by splicing, turn to an OTDR.

otdr

What Is OTDR?

Optical time domain reflectometers (OTDRs) are impressive pieces of equipment which 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. OTDR is most commonly used during installation acceptance and maintenance of outside plant cables. In this role, it is likely to be used to identify point losses, the length of various cables, and to measure return loss.

What Is The Difference Between OTDR Testing And Insertion Loss Testing?

An insertion loss test made with a light source and power meter is a simple test that is similar in principle to how a fiber optic link works. A light is placed on one end of the cable and a power meter measures loss at the other end, just like a link transmitter and receiver use the fiber for communications.

An OTDR works like radar that sends a pulse down the fiber and looks for a return signal from fiber backscatter and reflections from joints, creating a display called a “trace” or “signature” from the measurement of the fiber. From this trace, the OTDR calculates fiber length, attenuation and joint loss. So ODTR does not measure loss directly, it implies it from the trace.

OTDR Limitations
We can use the OTDR to pinpoint breaks in the cable, splices, and connectors, as well as to measure light loss in the system. However, they have the following limitations:

  • High skill requirements – Interpreting the trace requires too much skill for most field technicians. OTDR readings must be analyzed and interpreted by trained and experienced people. 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.

  • Limited 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. In addition, 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.

  • Dead Zone – OTDRs have a “dead zone” that may extend a hundred meters from the unit in which accurate readings are unavailable. You can overcome this limitation if you use a launch cable, but you must carefully interpret the signal trace. Although instruments may advertise an event dead zone of say 5 m, this is only under specific conditions. Inpractice 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.

  • Limited Applications – 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. Additionally, it can not be used in compliance with new multimode fiberoptics loss measuring standards, which mandate the use on an LED source with defined characteristics.

  • Cost – If you plan to use an OTDR frequently, it makes sense to buy a good one. If not, you’ll want to rent one when you need it, but make sure you rent a unit that was recently calibrated. Moreover, you make know that misapplication of OTDR testing will cost you much in wasted time and materials.

Operating An OTDR

Operating an OTDR is not especially difficult, but it does require familiarity with the particulars of the make and model you are using. To properly operate an OTDR, you generally have to make the following settings:

  • Fiber type: Singlemode or multimode.

  • Wavelength: Singlemode is set for 1310 nm or 1550 nm, and multimode is set for 850 nm or 1300 nm.

  • Measurement parameters: The typical parameters to be set are distance range, resolution, and pulse width.

  • Event threshold: This determines how much loss or change will be tagged as an event.

  • Index of refraction: This is the speed of light in that fiber. You can obtain this figure from the fiber manufacturer. In most cases you can take it directly from a standard specsheet.

  • Display units: These are usually labeled in feet or meters.

  • Storage memory: This should be cleared so a new figure can be saved and/or stored.

  • Dead zone jumper: You must connect this fiber, which should be sufficiently long, between the OTDR and the fiber under test. Sometimes you may have to connect it at the farend of the cable, as well.

Measurement Problems

At times, you will encounter some obstacles you cannot overcome. The following events will put your troubleshooting skills to the test.

  • Nonreflective break: This occurs when a fiber has been shattered or immersed in liquid. In both cases, very little light reflects back to the OTDR, and it’s difficult to identify the break.

  • Gainer: A gainer is a splice in a fiber that shows up as a gain in power. A passive device like a splice cannot generate light and cannot cause a gain in light. But if there is amismatch in the fibers that are spliced, it may appear to the OTDR as a gain. For example, if the splice goes from a 50-micron fiber to a 62.5-micron fiber, the difference in backscatter coefficients (the 62.5-micron core being larger) appears to the OTDR as a gain in light.

  • Ghosts: Ghosts are repetitions of a trace or portion of a trace. They are caused by a large reflection in a short fiber, causing light to bounce back and forth.

Tips for Selecting OTDR

Since OTDRs are very expensive and have only specific uses, the decision to buy one must be made carefully. When selecting the right OTDR, network engineers should make sure the tool has certain functionality, such as loss-length certification, channel/event map view, power meter capabilities, an easy-to-use interface, and smart-remote options. In addition, the OTDR needs to provide a reliable means to document the results. However, if you are not familiar with their operation or cannot understand the results of OTDR tests, it would be much better to hire a specialist to do the testing for you.

Summary

ODTRs are valuable fiber optic testers when used properly, but improper use can be misleading and, in our experience, lead to expensive mistakes for the contractor. Once you’re familiar with its limitations and how to overcome them, you’ll be prepared to detect and eliminate your optical fiber events. In a word, if you have an OTDR, it is very important for you to understand how to use it correctly and take good care of it.

How To Repair The Accidentally Cut Fiber Optic Cables

How To Repair The Accidentally Cut Fiber Optic Cables

Underground fiber optic cables can be accidentally cut. The most typical factor which could cause this accident may be the utilization of backhoe while digging. If it happens to you, you can simply search for backhoe and obtain the cut cables.

However, if it is brought on by moles, it will likely be difficult for you to troubleshoot it. You will need some equipment to involve. Here are a few steps suggested for you.

The first thing you need to do is to look for the break in your cable. Commonly, the fiber-optic technicians utilize a device which is known as an optical time-domain reflectometer or OTDR. With the ability to work like radar which sends a light pulse right down to the cable. It will be deflected to your device when it encounters break. It helps technician knows the position of the break.

After knowing the location of the break, you should dig up the cable with the break. Then, strip the fiber around 9 feet of the cable using cable rip cord. Peel the jacket gently so the fiber-optic tubes exposed and get rid of the excess jacket. Then, clean that cable gel using cable gel remover and cut any sheath and yarn. Separate the tubes from the fiber. Avoid damaging the strength member as it is required to hold the cable in fiber splice closure.

The next matter you need to do is to expose fiber cladding at 2 inches by using a fiber-coating stripper oral appliance clean the fiber within the tubes. Trim any damage on the fiber ends using high-precision fiber cleaver. If you want to perform a fusion splice, you have to convey a fusion splice protector to the fiber. Hereafter, you have to clean that striped fiber using lint-free wipes that is soaked in alcohol. In addition, if you want to produce a mechanical connection, you need to put quick-connect fiber-optic connectors to the fiber and clean the stripped fiber with alcohol and lint-free wipes. Ensure that the fiber doesn’t touch anything.

If you make a fusion splice, you have to place the fibers which is spliced within the fusion splicer. Then, fire the fusion splicer in line with the manual. After that, you have to move the fusion connector right into a heat shrink oven. Press a button to heat shrink. In some cases, the fusion splice is preferable to mechanical splice because the signal loss is under 0.1 decibels (dB). However, the mechanical splice has signal loss under 0.5 dB. The very last thing would be to see the connection of fiber-optic using the OTDR. Then put back those splices in to the splice enclosure. Close the enclosure after which rebury the cable.