Which of the following is true when using troubleshooting equipment to test the fiber optic plant?

Table of Contents Show

OTDR Working Principle
OTDR Testing Parameters
Bi-Directional Testing with an OTDR
OTDR Trace Analysis
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Network Cabling Contractors and Installers

An Optical Time Domain Reflectometer (OTDR) is a device that tests the integrity of a fiber cable and is used for the building, certifying, maintaining, and troubleshooting fiber optic systems. Hand-held OTDRs build a virtual image of the fiber optic cable to determine the condition and performance capability of the fiber cable. These tools can also test components along the cable path like connection points, bends, or splices to analyze the cable’s capability from start to finish.

OTDR Working Principle

The process of running these tests requires the OTDR tool to input a light pulse into one end of a fiber cable. The results are based on the reflected signal that returns to the same OTDR port. Some of the light transmitted through the cable will scatter and some will be reflected and returned to the OTDR. This returned scatter and reflections are measured to gather useful information about the cable, such as loss and distances to connectors or faults. This is measured by recording the time it takes for signals to return to the OTDR.

OTDR Testing Parameters

With so many different uses for OTDR testing, setting the correct OTDR parameters can ensure the tests you run and measurements you get are accurate. For some tests, using the auto-test function may be enough to get you accurate results, but other may require you to manually set the OTDR testing parameters based on fiber cable length, type of cable, and complexity of your system. These OTDR parameters will adjust the pulse width, averaging time, dead zones, and the distance range for your given fiber run to offer the most accurate results.

Most customers are familiar with Basic Certification - sometimes known as Tier 1 fiber certification – which measures attenuation (insertion loss), length and polarity. This test ensures that the fiber link exhibits less loss than the maximum allowable loss budget for the immediate application. Simple Light Source/ Power Meters or more automated Optical Loss Test Sets can perform this function.

Viewing trace results is simplified with advanced features such as pinch and zoom

Extended or Tier 2 fiber certification supplements Tier 1 testing with the addition of an Optical Time Domain Reflectometer (OTDR) from end to end. An OTDR trace is a graphical signature of a fiber's attenuation along its length which provides insight into the performance of the link components (cable, connectors and splices) and the quality of the installation by examining non-uniformities in the OTDR trace. More advanced units can provide easy to understand Event Maps and loss values for individual components as well as the link. An OTDR trace helps characterize individual events that can often be invisible when conducting only loss/length (tier 1) testing. Only with a complete fiber certification can installers have a complete picture of the fiber installation and network owners have proof of a quality installation. This fiber test certifies that the workmanship and quality of the installation meets the design and warranty specifications for current and future applications.

Bi-Directional Testing with an OTDR

Industry standards and most manufacturers’ warranties require TIer 2 testing to be done bi-directionally, that is, from both ends of the link.; It's also the only way to know the actual overall loss for a link because measuring the loss of fiber connectors and splices, as well as overall link loss, depends on the test direction. Testing a fiber link in one direction can give you different results than testing the same fiber link in the opposite direction. Averaging the results from both directions is required to achieve an accurate measurement.

Because of the significant time and cost involved in testing from both ends, technicians often try to save as much time as possible by testing all links from one end before moving to the other end. Unfortunately, this method does not work. To accurately test a fiber link in both directions, the launch and tail cords must remain in their initial measurement positions (even the standards say so) during both tests. But that is simply not possible if you test all the links from one end before moving to the other.

Bi-directional testing on an OTDR can test fiber cables in both directions with a loop

To solve this dilemma, you can test two fibers at the same time and use a loop to connect the two fibers together. This allows the two fibers of a duplex link to be tested in one shot without moving the OTDR to the far end. OTDRs like Fluke Networks OptiFiber® Pro OTDR Family of tools feature “SmartLoop” Technology that checks for the presence of the launch, loop and tail fiber when testing a duplex fiber link.

With SmartLoop, technicians can deploy multiple loops at the far end and perform a set of bidirectional tests without ever having to leave the near end--cutting test time by at least 50%.

OTDR Trace Analysis

Typical OTDR trace, showing length, a gradual decline in light strength, and events (A) OTDR connector – note the large reflectance makes it impossible to characterize the loss in the first connector. In this case a launch fiber of about 300 ft. is being used. This allows the OTDR to characterize the first connector of the link under test (B). (C) shows two connectors that are too close together for the OTDR to properly characterize the loss in each. (D) is a loss event with no reflectance, likely a bad splice or APC connector. (E) shows a typical UPC connector with reflectance and loss. (F) depicts a connector with reflectance where the signal after the connector is stronger than before, often called a “gainer”. This is indicative of connecting fiber types with different backscatter properties. (G) is the end of the fiber. Note the strong reflection makes it impossible to determine if there is a connector there and its performance.

OTDRs are also used for troubleshooting fiber plant performance. An OTDR maps the cabling and can illustrate termination quality and location of faults that may hinder network performance. An OTDR allows discovery of issues along the length of a channel that may affect long term reliability. OTDRs characterize features such as attenuation uniformity and attenuation rate, segment length, location and insertion loss of connectors and splices, and other events such as sharp bends that may have been incurred during cable installation or afterwards. Newer technologies, such as 100BASE-DR, also set limits on reflectance for each connector in the link, which can be verified only with an OTDR.

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. Features that make the OTDR easy to operate such as automated setup and Event Map are essential for users who aren’t OTDR experts but need to locate problems fast.

OptiFiber Pro® OTDR Offers Testing and Troubleshooting capabilities.

Tools such as the award winning OptiFiber® Pro OTDR provide the ultimate testing and troubleshooting solution to ensure the health of your most critical network cabling. With the OptiFiber Pro OTDR, network engineers have the in-house capability to perform inspection, verification, certification, troubleshooting, and documentation of fiber cabling in a single, easy-to-use OTDR tool.

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This is intended as an overview and installation checklist for all managers, engineers and installers on the overall process of testing and troubleshooting a fiber optic communications system.

1. Once a fiber optic cable plant, network, system or link is installed, it needs to be tested for four reasons:
a. to insure the fiber optic cable installation was properly installed to specified industry standards. b. to insure the equipment intended for use on the cable plant will operate properly on the cabling c. to insure the communications equipment is working to specifications

d. to document the cable plant and network for reference in case of future problems

2. Tools and Test Equipment Needed The following tools are needed to test and troubleshoot the fiber optic cable plant,system or link properly. a. Optical Loss Test Set or power meter and test source with optical ratings matching the specifications of the installed system (fiber type and transmitter wavelength and type) and proper connector adapters. An OLTS that merely tests cable plant loss may not include a calibrated power meter needed for testing transmitter and receiver power, so a calibrated power meter and source are a better choice for link or system testing. b. Reference test cables with proper sized fiber and connectors and compatible mating adapters of known good quality. These do not need to be “reference quality” but only in good condition, generally defined as having connector losses of less than 0.5 dB. c. Visual fiber tracer and/or visual fault locator (VFL)

d. Optional: OTDR with long launch cable (100 m for Multimode, 1 km or more for singlemode)

3. Testing And Troubleshooting The Installed Cable Plant
All fiber cable plants require certain basic tests to insure they were installed correctly and meet expected performance values. These are guidelines for testing and troubleshooting the cable plant itself. The most valuable data one can have for troubleshooting is the installation documentation.

3.1. What Can Go Wrong There are a number of possible problems with fiber optic cable installations that are caused by installation practice. These include: a. Damage to the cable during installation caused by improper pulling techniques (such as not pulling the fiber cable by the strength member,) excess tension, tight bends under tension, kinking or even too many bends. Most of these problems will be seen on all fibers in the cable. b. Damage to the fibers in the cable during cable preparation for splicing or termination. Fibers may be broken or cracked during cable jacket or buffer tube removal or fiber stripping. This may affect all fibers in the cable or buffer tube or just one fiber. c. High loss splices caused by improper splicing procedures, especially poor cleaving on mechanical splices or improper programming of fusion splicers. Most fusion splicers give feedback on most problems if the operator is properly trained. Individual fibers can be damaged when being placed in splice trays or tubes of fibers damaged during placement in splice closures.

d. High loss connectors may be caused by bad processes or damage after termination. Adhesive/polish connectors may have poor end finishes or cracks in the fiber at the end of the ferrule or internally. Prepolished/splice connectors are generally high loss due to poor mechanical splicing processes during termination causing high internal loss.

3.2. Testing And Troubleshooting Steps For Installed Cable Plants

3.2.1. Before installation, it is advisable to test all cable as received on the reel for continuity using a visual tracer or fault locator. Cables showing signs of damage in shipment may need OTDR testing to determine if the cable itself is damaged. Obviously, no cable showing damage should be installed.

3.2.2. Test insertion loss after installation a. After installation, splicing (if applicable) and termination, all cables should be tested for insertion loss using a source and meter or OLTS (optical loss test set) according to standards OFSTP-14 for multimode fiber, OFSTP-7 for singlemode fiber. b. Generally cables are tested individually (connector to connector for each terminated section of cable and then a complete concatenated cable plant is tested “end-to-end”, excluding the patch cords that will be used to connect the communications equipment which are tested separately. c. It is the concatenated cable test that is used to compare to the link power budget and communications equipment power budget to insure proper operation. d. Insertion loss testing should be done at the wavelength of intended operation if known or at two wavelengths with appropriate sources (850/1300 nm with LEDs for multimode fiber, 1310/1550 nm with lasers for singlemode fiber, 1490 for FTTH.) e. Unless standards call for bi-directional testing, double-ended testing with both launch and receive cables (OFSTP-7/14) is adequate. f. Data on insertion loss of each fiber should be kept for future comparisons if problems arise or restoration becomes necessary. Recording data on a label inside the patchpanel or enclosure is common practice.

g. Long cables with splices may be tested with an OTDR to confirm splice quality and detect any problems caused during installation, but insertion loss testing with an OLTS (light source and power meter) is still required to confirm end-to-end loss. Cables with insertion loss near expected values do not also need OTDR testing. Cables tested with an OTDR should have the data kept on file for future needs in restoration.

3.2.3. Troubleshooting a. First determine if the problem is with one or all the fibers in the cable. If all fibers are a problem, there is a likelihood of a severe cable installation problem. If all fibers are broken or have higher than expected loss, an OTDR will show the location of the problem on longer cables but premises cables may be too short and need physical inspection of the cable run. If the problem is caused by kinking or too tight a bend, the cable will have to be repaired or replaced. Generally OSP cables will be spliced as in a restoration and if the cable is a short OSP cable or a premises cable, replaced. b. High loss fibers have several potential causes, but bad splices or terminations are the most likely cause for field terminated cables. In some cases, using improper termination practices will result in high loss for all fibers, just as in kinking or bending losses, not just one fiber. c. Cables with a fiber or fibers showing very high loss or no light transmission at all should be tested for obvious breaks in the pigtail fiber or cable, generally at the splice or connector, with a visual fault locator or OTDR if of sufficient length (>100m) d. Testing for high loss should start with microscope inspection of terminations for proper polish, dirt, scratches or damage. e. If dirt appears to be the problem, clean the connectors and retest. f. If other connector damage is found on visual inspection, retermination will probably be necessary. Sometimes scratches can be polished out with diamond film by an experienced technician. g. Prepolished splice connectors with internal splices will generally look OK when inspected with a microscope unless damaged after installation. The most likely cause of loss with these connectors is high splice loss in the internal splice. They can be tested with a visual fault locator coupled into the fiber at the far end. High light loss will be seen as an illumination of the connector ferrule. Some connectors have translucent backs hells and can be tested with a VFL coupled directly into the connector. h. If the reason for high loss is not obvious and the connectors are adhesive/polish style, the problem may be a fiber break in the back of the connector. A VFL may help in finding fiber breaks, depending on the connector style and the opacity of the cable jacket. i. Splice loss problems can be pinpointed during OTDR testing. Confirmation with a VFL should be done if the length from the end of the cable is short enough (~2-3km) where a VFL is usable. The VFL can find high loss splices or cracks in fibers caused by handling problems in the splice tray.

j. High loss links where the excessive loss is only a few dB can be tested with a FOTP-171 type single-ended test with a source and power meter. When tested in this manner, a high loss connector will show high loss when connected to the launch cable connector but not when connected directly to the power meter detector which picks up all the light from the fiber.

3.2.4. Hints for troubleshooting a. Having access to design specifications and installation documentation and specifications will greatly assist troubleshooting.

b. If possible, interview the installer to help uncover processes that may lead to issues in installation, such as pulling methods, lubrication, intermediate pulls, splicing or termination methods (like improper field termination of singlemode which can lead to high loss and reflection even when connectors look OK in a microscope.)

3.2.5. Testing And Troubleshooting Patch cords
Patch cords are short factory-terminated cables usually with standard heat-cured epoxy/polish connectors on each end. They are used to connect equipment to the cable plant and as reference cables for testing insertion loss.

3.2.5.1. Likely Problems
Most patch cord problems are connector problems, caused by damage due to handling or numerous matings when used as reference cables for testing other cables. Connectors may also be damaged by breaking fibers at the back of the connector due to excess stress during handling or by placing other equipment on top of them in enclosures or patch panels.

3.2.5.2. Testing And Troubleshooting Steps a. All patch cords, especially those used as reference cables for insertion loss testing, should be tested for insertion loss. b. Patch cords should be tested with an optical loss test set (optical power meter and source) using single-ended FOTP-171 methods with one reference cable used as a launch cable. c. This will test the connector mated to the reference cable and the fiber in the patch cord, which is short enough it should have no measurable loss. d. Since the connector connected to the power meter will not be connected to fiber but presented directly to the detector of the power meter, it effectively has no loss. e. After testing in one direction, reverse the patch cord and test the other end. f. In both directions, factory-made patch cords should have a loss of less than 0.5 or whatever performance the user has specified with patch cord vendors. g. High loss connectors should be inspected with a microscope for dirt or damage. h. If other connector damage is found on visual inspection, retermination will probably be necessary but may not be cost effective, so the patch cord should be replaced. Sometimes scratches can be polished out with diamond film by an experienced technician.

i. Some optical loss test sets include fiber interfaces on both source and meter ports, so all testing is done double-ended, even if the cable under test is directly connected to an input port. A test set such as this makes reverse testing less effective since reversing test direction may not have any significant effect. Test ports on an OLTS like this should be kept covered when not in use and cleaned periodically. Damaged fibers inside an OLTS will require factory repair.