Calculating Fiber Optic Transmission Distance

Most fiber optic transmission systems are designed to work with one of two types of fiber optic cable: single-mode or multimode.

The glass core in the center of single-mode fiber is thinner than in multimode fiber, typically 8 to 10 microns (millionths of a meter) in diameter. In fact, it is so thin that only a single "mode" of light can be transmitted along its core. As a rule, single-mode fiber can transmit signals over a farther distance than multimode fiber.

Fibers with wider core diameters are called multimode fiber. These types of fibers allow for multiple modes of light to pass along their core. The two most common sizes of multimode fiber are 62.5/125 and 50/125. The first number represents the "core" diameter size measured in microns and the second number indicates the diameter of the fiber's "cladding" - the outer coating that surrounds the core and keeps light within the fiber.

When calculating the maximum distance a given fiber optic transmission system can support, it is also necessary to consider the frequency at which the fiber optic signal will be transmitted. The higher the frequency, the greater distance the system will be able to support. For multimode systems, commonly used frequencies are 850 and 1,300 nanometers. For single-mode systems, 1,300 and 1,550 nanometers are standard.

Bandwidth of fiber is described in MHz per kilometer. As the length of fiber increases, the bandwidth decreases proportionally. For example, a fiber that can support a 500 MHz bandwidth at a distance of one kilometer will only be able to support 250 MHz at 2 kilometers and 100 MHz at 5 kilometers.

Due to the way in which light passes through it, single-mode fiber has an inherently higher bandwidth than multimode fiber. Typical fiber bandwidth range from hundreds of MHz per km for multimode fibers to thousands of MHz per km for single-mode fibers.

The maximum distance a system can support will also be a function of how many splices and connectors are used across the entire transmission distance. Each splice or connector that the optical signal must pass through causes some signal loss to occur. The exact amount of loss depends on the types of connectors or splices used, as well as how well they are installed. As a general rule, it is safe to calculate a loss budget of .5 to .75 dB for each splice and connector used. However, more exact specifications can be obtained from the manufacturer of the fiber and connectors.

To determine the system's total optical loss, calculate the sum of the cable loss (as a function of desired transmission distance), splice loss, and connector loss - then add 3 dB as a safety factor.

Compare this figure with the allowable optical loss budget of the receiver to be used within the transmission system. If the loss level is acceptable, and if the fiber's bandwidth is adequate to pass the type of signal desired, then you can feel confident that the system can transmit over the distances desired.

Use Table 1 to determine optical loss when using different fiber types, transmission frequencies, and fiber bandwidths over various transmission distances. Remember - the ultimate goal when designing a fiber optic system is to ensure that enough light reaches the receiver. If this does not happen, the entire system will not operate properly.

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