When you need to shed a little light on your optical fiber problems,
turn to an OTDR.
Optical time domain reflectometers (OTDRs) are impressive pieces of equipment. They send pulses of light into optical fibers at a wide range of pulse widths, analyze the minuscule amounts of light reflected back to them from faults in the fibers, and use complex computations to determine the size and distance to events encountered in the fiber run. Events are defined as losses or changes in the fiber’s light-carrying capacity.
An OTDR uses a light backscattering technique to analyze fibers. In essence, it takes a snapshot of the fiber’s optical characteristics by sending a high-powered pulse into one end of the fiber and measuring the light scattered back toward the instrument. As a result, you can use the OTDR to pinpoint breaks in the cable, splices, and connectors, as well as to measure light loss in the system. However, as impressive as OTDRs are, they have the following limitations:
Accuracy of loss testing. If you’re only testing one end of a cable with an OTDR, you will lose accuracy. You can, however, test both ends of the fiber and average the readings you get to obtain a fairly accurate measurement.
Cost. A top-of-the-line unit can cost tens of thousands of dollars, so if you plan to use an OTDR frequently, it makes sense to buy one. If not, you’ll want to rent one when you need it, but make sure you rent a unit that was recently calibrated.
“Dead zones.” OTDRs have a “dead zone” (Fig. 1, right) 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 (Fig. 2, below).
Ease of use. OTDR readings must be analyzed and interpreted by trained and experienced people. It’s difficult for a less qualified installer to operate an OTDR and make sense out of it. As a result, using this device can require considerable time and effort.
When do you need an OTDR? You can use an OTDR to locate a break or similar problem in a cable run, or to take a snapshot of fibers before turning an installation over to a customer. This snapshot, which is a paper copy of the ODTR trace, gives you a permanent record of the state of that fiber at any point in time. This can help installers when fibers have been damaged or altered after installation, proving where responsibility for the damage lies. In fact, some customers will demand OTDR testing as a condition for system acceptance.
Although OTDRs are not especially accurate for loss testing, they can be used to conduct loss testing on long, outdoor runs of singlemode fiber where access to both ends of the cable isn’t practical. It can also be helpful for preventive maintenance procedures, such as routine checkups on a facility’s fibers.
OTDR Specifications. To reap the benefits of an OTDR, you must understand the following specifications:
Dynamic range. This is the combination of the total pulse power of the laser source and the sensitivity of the sensor.
Dead zone. As mentioned above, the dead zone is the space on a fiber trace following a fresnel reflection, in which the high return level of the reflection covers up the lower level of backscatter. This space is directly related to the pulse width of the laser source; a short pulse yields a relatively small dead zone, and a long pulse yields a relatively large dead zone.
Resolution. This is the ability of the OTDR to distinguish between the levels of power it receives. It may also refer to spatial resolution, which is how close the individual pieces of data are spaced in time.
Level accuracy and linearity. These are measurements of how closely the electrical current output corresponds to the input optical power. This is expressed as a plus-or-minus (+/-) dB amount or a percentage of the power level.
Distance accuracy. Accuracy is dependent upon clock stability, data point spacing, and the level of uncertainty of the index of refraction.
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 spec sheet.
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 far end of the cable, as well.
Measurement problems. At times you’ll encounter some obstacles you can’t 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 a mismatch 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.
Conclusion. OTDRs are invaluable test instruments that can illuminate problems in your optical fiber before they bring your system to its knees. Once you’re familiar with its limitations and how to overcome them, you’ll be prepared to detect and eliminate your optical fiber events.