Method for Monitoring Fiber Optic Cable

This invention relates to a technique for monitoring fiber optic cables to detect failure caused by possible microbending.

Providers of telecommunications services now utilize fiber optic cables to carry both short haul and long haul optically-formatted telecommunication traffic. AT&T, for example, presently maintains thousands of miles of fiber optic cables within its network. The fiber optic cables maintained by telecommunication service providers, such as AT&T, traverse many different types of terrain. Within urban areas, fiber optic cables often pass through steel or PVC ducting buffed underground. In other environments, ducting structures carry fiber optic cables across bridges, roads, underpasses, etc. In some instances, fiber optic cables are buffed directly, with no surrounding ducting.

Many of the fiber optic cables deployed in the manner described are exposed to the ambient temperature. When the temperature is low, especially during winter months, any water that collects at low points within ducting carrying one or fiber optic cables tends to freeze, causing ice to build up. The build up of ice within the ducting tends stress individual fibers within one or more of the cables in the ducting causing, microbending of the fibers and possible failure. If a fiber falls, the equipment to which the fibers likewise falls.

Often, only a few of the fibers in a cable may fall due to microbending. Still, the failure of a small number of fibers will trigger equipment failure. Troubleshooting under such circumstances is often difficult because the failure of a few fibers in a cable due to microbending usually appears as equipment failure, even though the trouble actually lies with the cable.

This, there is a need for a technique for monitoring fiber optic cables to detect microbending of fibers in a fiber optic cable.

Briefly, in accordance with the invention, a technique is provided to monitoring microbending of fibers within a fiber optic cable comprising part of a system that includes a plurality of pain of first and second pieces of equipment. Each pair of equipment pieces is linked by a separate fiber. To detect possible fiber microbending, an alarm monitor checks the operability of pair of equipment pieces associated with a particular fiber. If either of the first and second pieces of equipment of the pair is not operational, then the monitor sets an indicator to reflect the inoperability of the particular piece of equipment. Simultaneously, each fiber in each cable is monitored to determine its operability. A determination is then made whether multiple fibers within a cable have failed when there is no indicator that either the first and second pieces of equipment associated with a corresponding one of the multiple failed fibers is in operative. If multiple failed fibers exit, then the alarm monitor generates an alarm condition indicating possible microbending.

FIG. 1 depicts a block schematic diagram of a fiber-optic transmission system that includes a fiber optic cable that may be monitored in accordance with the present invention.

FIG. 1 illustrates a fiber-optic transmission system 10 that comprises at least one fiber optic cable 12 containing a plurality of individual fibers, identified as 14.sub.1 and 14.sub.2, for carrying optically formatted information. While the cable 12 is depicted as containing two individual fibers 14.sub.1 and 14.sub.2, it should be understood that a larger or smaller number of fibers is possible. Each of the fibers 14.sub.1 and 14.sub.2 couples one of a pair of first equipment pieces 16.sub.1 and 16.sub.2, respectively, to one of a pair of second equipment pieces 18.sub.1 and 18.sub.2, respectively. Each of the first and second equipment pieces 16.sub.1 and 16.sub.2 and 18.sub.1 and 18.sub.2, respectively, may comprise one of a variety of different types of electronic equipment capable of transmitting and/or receiving optically formatted information. For example, the first equipment pieces 16.sub.1 and 16.sub.2 may each comprise a subscriber loop carrier system for aggregating telecommunications traffic, whereas each of the second pieces of equipment 18.sub.1 and 18.sub.2 may comprise a switch for routing telecommunications traffic.

In the illustrated embodiment, only one of fibers 14.sub.1 and 14.sub.2 links one of the pairs of first and second equipment pieces 16.sub.1 and 18.sub.1, and 16.sub.2 and 18.sub.2, respectively. In some instances, multiple fibers (not shown) may link each first equipment piece to a second equipment piece. However, for ease of simplicity, only a single fiber link is depicted.

Depending on environmental conditions, the cable 12 containing the fibers 14.sub.1 and 14.sub.2 may run underground in ducting (not shown) traversing the right-of-way between the equipment pieces 16.sub.1 and 16.sub.2 and the equipment pieces 18.sub.1 and 18.sub.2. In some instances, all or part of the cable 12 may run above ground. Depending on the environment, water may collect in low spots within the ducting carrying the cable 12. In colder climates, the water collecting in the duct will freeze, giving rise to a build-up of ice that can stress the fibers 14.sub.1 and 14.sub.2 within the cable 12, causing fiber microbending. Ultimately, such microbending can lead to fiber failure. In many instances, only some of the fibers in the cable 12 may suffer from microbending, making trouble shooting difficult because failure of a few fibers in the cable may appear as an equipment failure.

In accordance with the invention, a technique is provided for detecting possible microbending. To detect microbending, each of fiber monitoring devices 20.sub.1 and 20.sub.2 is situated proximate a separate one of the ends of the fiber 14.sub.1 to monitor the optically formatted information entering and exiting the fiber. Likewise, each of monitoring devices 22.sub.1 and 22.sub.2 is situated proximate a separate one of the ends of the fiber 14.sub.2 to monitor the optically formatted information entering and exiting the fiber. By monitoring the optically formatted information entering and exiting each of the fibers 14.sub.1 and 14.sub.2, the operability of each fiber may be checked. Information from each of the monitoring devices 20.sub.1 and 20.sub.2 and 22.sub.2 and 22.sub.2 passes to a central alarm monitor 24, typically a personal computer or programmable logic controller, as are well known in the art.

To monitor the operability of the fibers 14.sub.1 and 14.sub.2, two monitoring devices are required, one at or near each end of each fiber. This is because the first and second equipment pieces 16.sub.1 and 18.sub.1 and 16.sub.2 and 18.sub.2 associated with the fibers 14.sub.1 and 14.sub.2, respectively, are typically bi-directional. In other words, the first and second equipment pieces 16.sub.1 and 18.sub.1 and 16.sub.2 and 18.sub.2 each receives and transmits optically formatted information. Only a single monitoring device would be needed for each of fibers 14.sub.1 and 14.sub.2 if only one of the first equipment pieces 16.sub.1 and 16.sub.2 originated optically formatted information while each of the second pieces 18.sub.1 and 18.sub.2 received such information or vice versa.

Each of the first and second equipment pieces 16.sub.1 and 16.sub.2, and 18.sub.1 and 18.sub.2 typically includes internal detecting devices (not shown) for observing the operability of the equipment piece. The internal detecting devices within the equipment pieces 16.sub.1 and 16.sub.2, and 18.sub.1 and 18.sub.2 are coupled to the central alarm monitor 24. The fiber monitoring devices 20.sub.1, 20.sub.1, 22.sub.1 and 22.sub.2 could be incorporated within the equipment pieces 16.sub.1, 16.sub.2, 18.sub.1 and 18.sub.2, respectively, either as separate units, or as part of the detecting devices within each piece of equipment for monitoring its operability.

To detect possible microbending, the alarm monitor 24 continuous monitors each of the equipment pieces 16.sub.1 and 16.sub.2, and 18.sub.1 and 18.sub.2 to determine if any piece is malfunctioning. If so, the alarm monitor 24 sets an internal indicator or flag associated with that equipment piece to indicate that it has malfunctioned. At the same time, the alarm monitor 24 monitors the individual fibers 14.sub.1 and 14.sub.2 via the monitoring devices 20.sub.1 and 20.sub.2, and 22.sub.1 and 22.sub.2, respectively. A detected failure of one of the fibers 14.sub.1 and 14.sub.2 may be attributable to an actual fiber malfunction due to microbending. Even though the fiber itself may be operating properly, the failure of one of the equipment pieces 16.sub.1 and 16.sub.2, and 18.sub.1 and 18.sub.2 will result in a loss of information carried by fiber associated with that equipment piece. Thus, the fiber monitoring devices 20.sub.1 and 20.sub.2, and 22.sub.1 and 22.sub.2 will signal a malfunctioning fiber when an equipment piece associated with the monitored fiber has failed even though the fiber has not actually malfunctioned.

To eliminate equipment failure as a possible source of fiber malfunction indication, the alarm monitor 24 determines whether a malfunction indication of one of the fibers 14.sub.1 and 14.sub.2 is accompanied by a malfunction indication of an associated piece of equipment. Thus, if the fiber 14.sub.1 registers a malfunction indication, the alarm monitor 24 determines whether either of the equipment pieces 16 and 18.sub.1 has malfunctioned. If neither of the equipment pieces 16.sub.1 and 18.sub.1 has malfunctioned, then a malfunction indication of the fiber 14.sub.1 is attributable to the fiber itself.

Microbending typically affects more than one fiber, although not necessarily all fibers in the cable 12. Hence, a possible microbending condition is present if there are multiple malfunctioning fibers in the same cable. Therefore, the alarm monitor 24 determines whether there are multiple failed fibers for a given cable under the condition when both pieces of equipment associated with each potentially failed have exhibited no malfunctions. In other words, the alarm monitor 24 determines how many of the fibers 14.sub.1 and 14.sub.2 register a malfunction under the condition when neither of the equipment pieces associated with each failed fiber registers a malfunction. If there are multiple failed fibers, each associated with a pair of functioning equipment pieces, then the alarm monitor 24 generates an alarm condition indicating possible microbending. Upon the occurrence of such an alarm condition, a technician can be dispatched to inspect the failed fibers. Knowing that the alarm condition occurred as a consequence of microbending, the technician can concentrate his or her search in regions where microbending is most likely to occur, thus saving repair time.

In making generating an alarm condition indicating possible microbending, the alarm monitor 24 may take into account the location of the cable 12, as well as time of year. For example, if the cable 12 were situated in a northern climate, multiple failed fibers likely to be the result of microbending is likely to occur during winter months. Conversely, microbending is far less likely during summer months, especially when the cable lies in a warm climate.

The foregoing discloses a technique for monitoring a fiber optic cable 12 to detect possible microbending of fibers 14.sub.1 and 14.sub.2 by determining whether there are multiple failed fibers under the condition when the equipment pieces 16.sub.1 and 16.sub.2, and 18.sub.1 and 18.sub.2 coupled to the fibers are operating properly.

It is to be understood that the above-described embodiments are merely illustrative of the principles of the invention. Various modifications and changes may be made thereto by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.