Testing a fiber optic network requires choosing the right
tools for the job. This article provides an overview of
the essential test devices that should be in every fiber
technician’s tool kit.
This article is for individuals who are relatively new to fiber optics. Although the equipment explained may seem a little complicated at first, many test devices incorporate pass/ fail testing modes and automated features that enable even novice technicians to test a fiber optic link. A companion section includes examples of specific test devices and their features.
OTDRs - (Optical Time Domain Reflectometer)
An OTDR is an indispensible tool for the fiber optic
technician and is used for both Outside Plant (OSP)
and Local Area Network (LAN) applications. The device
can measure and verify the loss of individual splices
and other components in a fiber link. An OTDR can also
locate fiber breaks, excessive bends in fiber, and other
problems that create loss of optical power (light) in
fiber optic installations.
How it Works
An OTDR introduces pulsed laser light into one end of a fiber link, and measures the light reflected back to the OTDR from splices, connectors, excessive bends, and so forth. This device is well suited to outside plant installations where fiber links can be many miles in length. An added advantage when using an OTDR is that you don’t need to test the fiber from both ends.
An OTDR has a relatively large screen where graphic “traces” and other data are displayed. The traces provide information about the location and severity of loss events, as well as the length of the fiber link itself. Some OTDRs come with certification software that enables the user to access traces stored in the OTDR and download them to a personal computer. This allows for further analysis and printing on any standard-size office printer.
Whether or not you can use an OTDR for a particular test depends on the features of the OTDR, and the length of the fiber link that you are testing. For example, the resolution of some OTDRs is set for the long cable distances typical of outside plant installations.
Until recently, OTDRs have not been very well suited to measuring some of the shorter fiber links that are typical of indoor installations. However, this is changing and many newer OTDRs offer the ability to measure short fiber links, including those within a LAN.
It’s important to note that OTDRs have a “dead zone” that consists of the first several meters of fiber connected to the OTDR. No measurements can be made within this zone. To overcome this limitation, a “launch cable” must be connected between the OTDR and the fiber you are testing. This enables the first connector to be incorporated into your test.
Many OTDRs can provide electronic or printed documents that certify the performance of the fiber link. Certification involves testing a network or fiber link and comparing the test results to either an industry or customer-defined standard. Documentation may relate to attenuation rate and uniformity, segment length, location and insertion loss of splices and connectors, and more.
For “Tier II” testing, an OTDR is the principal test device. Testing includes measuring the network’s total loss (budget loss testing), length limits of cabling (to mitigate modal dispersion), and polarity verification of a fiber link, using bi-directional testing.
For “Tier I” testing, an Optical Loss Test Set consisting of a Power Meter and Light Source can do the job. For more information about OTDRs see the EXFO article.
TESTER’S TIPSThe following tips have been contributed by field technicians involved in the installation and testing of fiber optic networks.
OTDRs - Use a launch cable that is at least as long as the OTDR’s longest dead zone, which will allow you to test the first connector; start your test with the OTDR set at the shortest pulse width; and set the range at least two times the length of the fiber link that you are testing.
An OTDR can also be used to optimize quicktermination connectors. The process typically involves attaching the connector to a launch cable (i.e. pulse suppressor) via a mating sleeve, then pressing the Scan button on the OTDR. Next, slowly insert the fiber into the connector. The scan of the field fiber will rise up the screen as the optical connection improves. By positioning the A and B cursors at opposite sides of the connector trace, the OTDR’s 2-point loss feature will show the throughput of the connector.
Visual Fault Locators - You can identify a faulty fiber optic cable before the cable is installed. Simply use a VFL to do a “continuity check” while the fiber is still on the spool. Shine the VFL light into one end of the fiber and see if the light is visible at the other end. If so, you know there are no breaks in the fiber.
Connector Inspection - When inspecting connectors, use a microscope with sufficient magnification. 100X is suitable for multimode fiber; 200X or higher is better for singlemode fiber, which has a smaller core diameter.
Also, clean your connector endface prior to viewing with an inspection microscope. This will make any cracks or chips more visible. After inspection, clean the endface again to remove any dust it may have picked up from the air or microscope during the inspection process. Be sure to always clean your connector prior attaching it to a test device, mating sleeve or network equipment.
Finally, be cautious. Infrared light in a fiber optic network is invisible and can damage your eyes. Many microscopes that are designed for fiber optic work provide built-in protection against infrared light. You can also use an inexpensive Infrared Detection Card. Simply hold the card adjacent to the fiber enface to reveal if infrared light is present in the connector you are inspecting.
Power Meter and Light Source Test Sets
Power Meter (PM) and Light Source (LS) testing helps ensure that your network has the right level of “optical power” to do the job. Too much power can overload the receiver, resulting in errors; too little power can confuse the receiver, preventing it from distinguishing between signals and system noise.
Optical Power vs. Optical Loss
A PM can also determine “optical loss.” This is a measurement of attenuation caused by connectors and other components within the link. “Optical loss” is sometimes called “relative power” because it is a relative measurement; in end-to-end testing relative power (optical loss) is the difference between the optical power injected at the transmitting end of the link compared to the amount of power that is measured at the receiving end. Optical power at the receiving end is always less than that at the transmitting end. This is due to attenuation that results from “insertion loss” of installed system components such as connectors, splices as well as loss produced by the fiber itself. On test equipment, optical loss is represented as a dB value, and optical power is represented as dBm.
A rough estimate of optical loss should be calculated
in advance of testing to serve as a benchmark for your
tests. This calculation adds together the known loss
value of each system component, according to industry
standards or as otherwise defined by the user. Then,
if PM tests show that loss readings are significantly
higher than the number in your benchmark, you know
there are defective components in the link, such as bad
connectors, defective splices or excessive cable bends.
You can identify the type and location of these “loss
events” by using an OTDR.
Loss testing can also be done on shorter cable assemblies to determine whether the attached connectors are functioning properly. In cables under 10 meters in length, connectors are the major cause of optical loss.
A PM and LS can be purchased individually or together as a “test set.” For some tests, the link’s transmitter can serve as the light source. An alternative is to use a stand-alone device called a Light Source. A LS may be the only option when testing a new cable installation where active equipment is not yet in place. In this case, the LS should match the type of light source (LED or laser) within the transmitter that will eventually be installed. Also, the LS should be set to match the transmitter’s power and operating wavelength.
Ensuring that test results meet the industry standards might seem like a daunting task, requiring technicians to cross-reference industry tables and so forth. However, good certification equipment will typically employ Pass/Fail criteria, with the relevant standards pre-programmed into the device. Either the system meets the defined standards (Pass) or it does not (Fail). This streamlined approach reduces training time and enables accurate certification testing to be done by expert and novice technicians alike.
The technician can also enter user defined values based on the specs of a particular job.
DID YOU KNOW…
Optical fiber is used in data centers, not only
to meet high-bandwidth requirements, but
also to conserve energy. Data centers that
employ fiber optics consume less energy
than copper wire systems and require less
electricity for cooling.
Other advantages of fiber include immunity to Electromagnetic Interference (EMI), Radio Frequency Interference (RFI), and Alien Crosstalk, all of which negatively impact copper wire systems.
Also, fiber optic networks are easier to test than copper wire networks. Testing a copper network can require measuring over 20 different parameters over many different frequencies. In contrast, the performance of a fiber optic link remains the same over all frequency ranges. In most cases, an optical loss test set is all that’s required to verify the performance of an installed fiber link.
Visual Fault Locators (VFLs)
In theory, an OTDR could pretty much do all of your testing. However, some test routines are so simple that using an ODTR would be overkill and cause more work for you. This is especially true for short fiber links typical of indoor installations. For example, to check for continuity in a fiber link, a simple Visual Fault Locator (VFL) tester can accomplish the job in seconds. Fiber optic networks communicate by using light in the infrared range, which is invisible to the human eye. So, if you wish to visibly check for light loss, you must use a VFL to shine visible red light through the cable. This will enable you to see any light leaking from a defective connector. The VFL can also reveal excessive bends in the fiber that cause light to escape through the cable jacket. Remember to darken the room so that the escaping light can be more easily detected.
One of the main contributors of optical loss in a
fiber optic network are connector endfaces that are
damaged or contaminated with particles from the
A good inspection microscope will magnify the connector endface and reveal any dust, dirt, scratches, cracks and chips that appear in the core of the fiber. Hand held units are avaiable that fit easily in a pocket or tool box. Bench top microscopes are also available that present a very large endface image on a an LED screen.
As mentioned earlier, many of today’s test devices incorporate pass/fail features that enable even novice technicians to test a fiber optic link. We won’t say it’s so easy that a caveman can do it. However, equipped with today’s advanced, easy-to-use tools, it’s amazing what can be accomplished. See our companion article on page 14, which provides examples of specific test equipment and features.