Overcoming ‘which-fiber-is-which?’maintenance challenges

New fiber-detection technology lets you safely identify a fiber with certainty, with no danger to the integrity of the transmission.

When network service providers invest in an outside-plant (OSP) infrastructure, they want to ensure that they obtain the utmost value from it. Therefore, once the network is up and running, one of the most crucial parts of its operation and maintenance is a good fiber-management system that lets technicians readily access the network to perform common operations, such as adding new subscribers, modifying services, and troubleshooting.

Uh-oh...

In the day-to-day maintenance of these networks, technicians must conduct a number of optical tests requiring that fiber be disconnected so that a test signal can be sent through the system. One of the inherent difficulties of these tests is that many of them are conducted at two different ends of the network, making it quite difficult to identify which-fiber-is-which. In fact, it is quite common for technicians to disconnect the wrong fiber by mistake or to damage nearby connections when attempting to troubleshoot a link.

With traditional fiber detectors, a mechanical pull or push bends the fiber to a predetermined angle and forces light into the detector. Click here to enlarge image

Human error is indeed a major cause for concern-and not just on the technician’s end. On the administrative end, it is becoming increasingly clear that fiber-identification records are poorly kept (if at all), so when technicians go out in the field to test a connection, there are no plans or charts to help them determine which patch cord to pull, thus causing outages and inconveniencing numerous customers.

Such occurrences and their consequences are even more serious when working on a passive optical network (no backup) or on a small and contained high-fiber-count termination point, where it becomes nearly impossible to find and reach a specific fiber.

Considering these unchangeable, real-world circumstances, it is clear that the risks related to human error could be greatly reduced if the need to access the actual signal was eliminated-or, at least kept to a minimum. Fortunately, new technology is addressing these challenges and, by doing so, is letting service providers increase profitability and reduce maintenance costs while enhancing customer service.

Thumbs up, in most cases

Live fiber detectors (LFDs) are common tools in the outside-plant world and have many applications:

Determining whether a fiber is active prior to maintenance;

Locating a particular dark fiber using a tone recognition of 270 Hz, 1 kHz, or 2 kHz;

Identifying the traffic direction of a known live fiber;

Providing a rough estimate of the power within a fiber (usually, an optional feature).

Tools of this kind typically operate in the same manner: a thumb-activated function bends the fiber at a fixed angle, and the detector reads the power leaking from the jacket.

EXFO's LFD-300 FiberFinder is an example of a fiber-detection unit where a motor, rather than human effort, minimizes loss.

Click here to enlarge imageBecause the angle is fixed and optimized for one wavelength and one fiber type, in most cases and applications, the bending could cause one of several problems: excessive loss, unreliable fiber detection (the unit does not detect that the fiber is active), unreliable tone or traffic detection, and/or possible permanent damage to the fiber. For those reasons, technicians typically have avoided using instruments of this type for high-data-rate links or long-haul routes.Innovating around needIn all type of fibers and at all wavelengths, there is a relationship between the angle of the fiber’s curvature and the insertion loss. The angles differ, but the behavior remains constant. A bend at a fixed angle will generate excessive loss under some conditions, whereas it will fail to make proper identification under other conditions.New fiber-detection technology brings a different approach to this problem. The power loss is monitored as a function of the bending angle because a precision-step motor (rather than human power) increases the angle, producing different angles automatically optimized for all fiber types and all wavelengths.This innovation achieves a number of advantages:

It can guarantee a maximum loss of 1 dB on all singlemode fibers at all wavelengths (although some dark-coated jackets may not transmit enough light to enable a measurement).

It will not damage the fiber, as the force is not user-dependent but rather encoded, so the unit will stop and release if no power is detected.

It can provide extremely high reliability when it comes to traffic detection, direction identification, and tone detection (although again, dark-coated jackets may not transmit enough light to enable a measurement).

Because output is controlled, the detector has a highly repeatable online non-disruptive power measurement.

Traditional units have a fixed angle (A), which causes them to generate fiber- and wavelength-dependent losses. By monitoring the loss, a more advanced unit can stop the bending when sufficient light is ejected, and thereby control the loss (B). Click here to enlarge imageIn short, this new technology lets a technician identify a fiber with certainty and with no danger to the integrity of the transmission or the fiber itself. It can safely be used on long-haul and high-density fibers.Inline power metersWhile it is possible to have control of insertion loss within a fiber, measured in dB, the absolute value is measured in dBm. So, by knowing the loss in dB and the power level of the existing light, a technician can accurately measure the power of the fiber. Of course, there is also the coupling efficiency factor. For example, 3-mm jackets absorb more than 1.6-mm and 900-µm fibers.On the other hand, because loss is measured versus the position of the motor, the detector knows what type of jacket is being tested (e.g., 900-µm, 1.6-mm, 3-mm). So, the detector automatically uses the proper coupling-efficiency parameters and computes the power within the fiber within a repeatability of 1 dB, regardless of the fiber type or wavelength.Applications for the live fiber detector are numerous, including fiber-to-the-home deployments where no protection fiber exists, and live network maintenance and troubleshooting.Using tone detection, the live fiber detector is a useful tool when looking for a dark or active fiber, and for identifying a particular fiber. The tool can also be used to identify a specific live fiber, ensure that the proper fiber is disconnected, and solve record-keeping issues.Often in maintenance situations, the technician knows the transceiver on which the fiber starts, but is challenged to find the termination on the other end. Frequently, the technician’s approach to finding the correct fiber is to pull on the cable under test and try to see which cable is moving on the other end of the circuit. A live fiber detector ends such guesswork.

This TG-300 FiberFinder is a non-intrusive clip-on unit that attaches to a fiber and induces a non-destructive signature on the signal. Click here to enlarge imagePromising low loss, live fiber-detection equipment is available in a non-intrusive clip-on unit. Once attached to a fiber at the transmission site, these units induce a non-destructive 0.1-dB signature on the signal by applying a low-frequency pressure on the fiber. The signature travels along with the transmitter’s signal and, at the other end, another live fiber detector identifies the fiber that carries the signature. The set achieves non-destructive live fiber identification in seconds.New technology built into these troubleshooting tools saves time, avoids costly mistakes, and eliminates unnecessary downtime. Because the human-error factor is eliminated and faults are found more quickly, repairs can also be faster, enhancing overall customer service. FRANCIS AUDET is senior product manager of EXFO Electro-Optical Engineering’s optical business unit (www.exfo.com).