The Fiber Optic Dilemma: What’s Limiting Optical Transmission Distance?

Fiber Optic CablesAs the demand for faster Internet continue to rise, electrical engineers are scrambling to keep pace by increasing the signal that passes through fiber optic cables, allowing users to send and receive more data faster.

Optical fiber transmission, however, has certain physical limitations. Fiber optic cable can surely support much further distance than its older counterpart, copper network, but the transmission distance it can support are limited by many factors. With the development of the fiber optic network, optical link distances span meters to kilometers—but optical distance becomes weaker over longer distances.

To understand this issue better, let us look at the major factors limiting optical transmission distance:

Frequency of Transmission

Different laser source supports different frequencies. The maximum distance a system can support mostly depends on the frequency the signal will be transmitted. In general, the higher the frequency, the higher the distance your system can support.

This means you need to choose the module with the right frequency, so you can transmit optical signals without having problems with transmission distance. The Cisco SFP-10G-LRM, according to, for instance, uses frequencies of 1300nm and 1550nm, while multimode fiber cable uses frequencies of 850nm and 1300nm.

Type of Fiber Optic Cable

The maximum transmission distance is also limited by dispersion in fiber optic cable.

There are two types of dispersion affecting transmission distance: 1) Chromatic dispersion, which is the dispersion of the signal because of different light ray speeds, and 2) Modal dispersion, which is the spreading of the signal overtime due to the different propagation mode. If you’re using single-mode fiber, use model dispersion, as chromatic dispersion affects the transmission distance.

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Another important factor influencing transmission distance is the bandwidth your cable supports. In most cases, the transmission distance decreases as the bandwidth increases. That means a fiber that can support 500 MHz bandwidth at one kilometer can only support 250 MHz at 2 kilometers. If you don’t want problems with the transmission, go for a cable that supports a higher bandwidth, such as single-mode fibers.

Overall, optical transmission distance is influenced by a variety of factors. When deploying your fiber optic network, it’s important to keep these factors in mind to break the barriers limiting your transmission distance.