Using Dark Optical Fiber: Pros and Cons

Integrators want to know: Can we put dark fiber to use

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.