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Using dark optical fibre

Continuing the May article on optical fibre.

A fibre-optic microscope, an optical power meter kit, and an optical time domain reflectometer (OTDR) allows complete inspection of the optical path. A fibre-optic microscope is used to inspect an optical connector for debris or damage.

Optical loss measurements must be performed in a very specific way and the results of those measurements must be communicated or interpreted in a very specific way, as well. The objective of optical loss testing is to determine the end-to-end optical path loss in a given optical fibre at a specific wavelength in a specific direction. In other words, the following information must be definitively determined or conveyed:

1. Which fibre is being tested?

Clearly, some method of fibre identification must be used. Typically, this is accomplished by referencing the colour of the buffer, but it can be any reference that allows proper identification.

2. Which wavelength is being used to perform the test?

Optical path loss is dependent on the wavelength used in the testing. The wavelength(s) used should correspond to the wavelength(s) used by the equipment that will eventually use the fibre. Wave-lengths are most commonly described in units of nanometres (nm).

3. In which direction is the fibre being tested?

Because optical path loss is directional, the testing on any given fibre must be done separately in each direction. While the results in both directions should be the same or very close to each other, problems in the optical path could produce acceptable results in one direction and poor results in the other direction.

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).

Determining a dark optical fibre’s bandwidth

Once the optical path has been confirmed another test is necessary to determine the fibre’s bandwidth or capability to transmit large volumes of data. As long as the fibre passes the optical path test, it is a safe assumption it can transmit serial data and a single video channel or up to a 100 mbps Ethernet data stream.

A caveat here is whether you are using multimode or single-mode optical fibre. Single-mode fibre generally has the needed bandwidth to transmit large numbers of video or Gigabit Ethernet data. To confirm the dark fibres 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 fibre. Only a few modes are excited at each step, and their arrival times are recorded. The DMD of the fibre 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 fibre.

The next step

There still is no medium available today that affords the flexibility fibre-optic transmission offers. Fibre is one of the only true mediums that allow you to easily change the type of modem on either end and accommodate a completely different signal type. Whether you are taking advantage of an existing in use fibre or an unused dark fibre, companies such as ComNet offer audio, video and data fibre-optic transmission equipment as well as fibre-optic Ethernet transmission products that allow fibre to be used to its greatest potential. The tests outlined above can give you a competitive advantage in using all the resources available to you. Finding installed optical fibre in place in your next installation can be like discovering buried treasure.

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