Using Dark Optical Fiber: Pros and Cons

March 23, 2011

Optical fiber has been one of the better signal transmission solutions introduced to the security industry. About 20 years ago when the potential of fiber optic transmission was realized, miles and miles were installed everywhere. Fiber optic transmission delivers extremely high bandwidth transmission. During the fiber optic boom, it was everywhere. Even without planned use, it was installed in buildings, campuses, stadiums, factories and more. The thinking was it was cheaper in the long run to put it in during the construction phase than try to install it later.

Years later this optical fiber, having never been utilized, is known as dark fiber. Dark, not in color, but because light has never passed through it or it has not been used. It may be just installed or to the point of the connectors; LC, ST or SC may have even been installed. But just because the fiber has never been used doesn’t mean it’s not good.

The most substantial benefit to using dark fiber is cost savings. The cost to pull new optical fiber is not inexpensive. Typical optical fiber costs range from $0.08 per foot per strand for single mode fiber to $0.12 per foot per strand for multimode. Realistically fiber is pulled in multiple strands jacketed together and the general estimate is that it is usually 10 to 20 times the cost of the medium for installation based on location and specification challenges. So the obvious benefit to using dark fiber is the tremendous cost savings.

There is a downside to optical fiber that has never been utilized. The potential disadvantages include the lack of information about the fiber, fiber path or the fiber history. Some of the questions that arise regarding dark fiber include:

Was the fiber pulled correctly or was excessive stress or strain applied? If too much force is used, damage to the optical fiber can occur. Trying to track a damaged fiber is extremely difficult and in reality should any damage be identified the fiber is usually abandoned.

Does the fiber cable contain the proper jacket for the environment? The type of jacket used dictates where it can be used. There are water-resistant jackets, temperature resistant jackets, etc.

What type of fiber is installed?

If the fiber is multimode, could the fiber be old, relatively useless step-index multimode fiber; graded index 50/125 æm or 62.5/125 æm multimode fiber; or LASER-optimized 50/125 æm multimode fiber?

If the fiber is terminated, was it terminated properly?

What is the nature of the optical path? Are there patch panels and splices along the way? If so, how many, where are they and what is their condition? What is the end-to-end optical path loss? Was the minimum bend radius considered when installing fiber around corners or bends? Is documentation available that would show parameters such as optical attenuation coefficients, bandwidth-distance products or dispersion?

Confirm the optical path of the fiber

If the above conditions are satisfied, the next step is to determine the continuity of the fiber. That is, is the dark fiber suitable to be used for transmission? To determine the suitability of a fiber for transmission, the optical path must be confirmed.

First a warning: Never look into an optical fiber or into an inspection device that is connected to a fiber unless you can guarantee that nothing is connected to the other end of the fiber or to any access points along the optical path such as path panels. Irreversible damage could be done to your eye before you could have the chance to react.

A fiber optic microscope, an optical power meter kit, and an Optical Time Domain Reflectometer (OTDR) would allow complete inspection of the optical path. A fiber optic microscope is used to inspect an optical connector for debris or damage.

An optical power meter kit and an OTDR can be used to determine the status of a fiber path. A power meter kit is more accurate than an OTDR for measuring optical loss but an OTDR is required to determine the location of potential faults in the optical path.

Because optical path loss is directional, the testing on any given fiber must be done separately in each direction. While the results in both directions should be the same or very close to of each other, problems in the optical path could produce acceptable results in one direction and poor results in the other direction. The reason for any difference between measurements made on the same fiber but in different directions is generally related to the quality of the fiber termination. A poorly terminated optical connector will increase the insertion loss or, in a worst-case scenario, simply not allow any optical energy into a fiber. A poorly terminated connector will usually cause more of a problem when connected to an optical source than when connected to an optical detector. Nonetheless, a test performed from good connector to bad connector may not show a problem while the same test, performed in the opposite direction (bad connector to good) will likely show a rather significant increase in optical attenuation. Therefore, all optical loss testing must be performed in both directions on each fiber.

All testing must be performed as end-to-end testing, not the summation of the results of several sectional tests. Sectional tests that have been summed do not properly measure loss through intermediate locations such as splices or connector-to-connector couplings such as those found in patch panels. Results of optical loss measurements are most commonly described in units of decibels (dB).

At a minimum, a proper report should include a statement such as the following example that uses multimode fiber between a remote camera and a control center:

The multimode fiber with the green/white buffer has the following end-to-end optical loss measurements: 0.7 dB @ 850 nm from camera to control center; 0.7 dB @ 850 nm from control center to camera; 0.5 dB @ 1300 nm from camera to control center; and 0.4 dB @ 1300 nm from control center to camera.

This report identifies the fiber, the wavelength used during each test, the direction in which each test was performed, the fact that all testing was end-to-end and the actual measured end-to-end optical loss in units of decibels for each test. Anything less than this is not a complete test report.

The information above lists the items that must be included in any optical loss measurement report. In order to determine whether a given fiber has tested acceptably or not based on this information, additional information is required. The information below shows conservative values that may be used as a starting point:

Attenuation from scattering and absorption: Multimode fiber, 850 nm, 3.5 dB/km; Multimode fiber, 1300 nm, 1.5 dB/km; Single mode fiber, 1310 nm, 0.5 dB/km; Single mode fiber, 1550 nm, 0.4 dB/km.

Attenuation from air gaps and splices: Mated connector pair, 0.75 dB; Mechanical splice, 0.3 dB; Fusion splice, 0.1 dB.

This shows that optical loss can be calculated by knowing the following four items: optical attenuation values (based on the fiber type and wavelength); optical path length (in km); number of mated connector pairs; number and type of splices (mechanical and fusion).

A mated connector pair is simply two optical fibers with connectors that are connected by a splice bushing such as inside of a patch panel. Unlike a mechanical or fusion splice, a mated connector pair contains an air gap between the connector faces. The optical energy from one fiber must exit the connector, cross this air gap, and enter the other fiber’s connector. Such a splice typically introduces between 0.5 and 0.75 dB of optical loss.

A mechanical splice dramatically reduces the loss because modern mechanical splices employ an index-matching gel that fills in the air gap between fiber ends. A fusion splice actually fuses two ends of a fiber together using an expensive piece of equipment. Such a splice completely eliminates any air gap essentially creating one continuous connection between two fiber ends. Generally, losses well below 0.1 dB are not uncommon.

Once this information is known, the end-to-end optical path loss for a given fiber may be independently calculated. As the parameters used are conservative values, the calculated results could be a few dB higher than the measured results. Having this information available should allow measured test results to be judged acceptable or not.

Determining dark optical fiber bandwidth

Once the optical path has been confirmed another test is necessary to determine the fiber’s bandwidth or capability to transmit large volumes of data. As long as the fiber passes the optical path test, it is a safe assumption it can transmit serial data and a single video channel or up to a 100Mbps Ethernet data stream. A caveat here is whether you are using multimode or single-mode optical fiber. Single-mode fiber generally has the needed bandwidth to transmit large numbers of video or Gigabit Ethernet data. To confirm the dark fibers bandwidth a test called differential mode delay can be used for potential high bandwidth application. Differential mode delay or DMD is a direct measurement of the light transmission properties affecting bandwidth.

The broadening of the pulse reduces the bandwidth of the system and can cause detection errors at the receiver. The difference in arrival time between modes within a pulse is known as differential mode delay (DMD). In DMD testing, high-powered laser pulses are transmitted in small steps across the entire core of the fiber. Only a few modes are excited at each step, and their arrival times are recorded. The DMD of the fiber is the difference between the earliest and the latest arrival times of all modes at all steps. The lower the differential mode delay, the higher the bandwidth of the fiber.