Transmission Mediums : Fiber Optic Cable

Fiber optic cable is a strand of glass or plastic that is used to transfer optical energy between points. The size of most fibers is from 10 to 200 microns (1/100th to 1/5th of a mm). Optical fibers are typically used in a unidirectional mode (e.g., data moves in only one direction). Because of this, every transmission system requires at least two fibers (one for transmission and one for reception).

For most fiber systems, the transmitting end-node uses a light amplification through stimulated emission of radiation (LASER) device to convert digital information into pulsed light signals (amplitude modulation). The light signals travel down the fiber strand by bouncing (reflecting) off the sides of the fiber (called the cladding) until they reach the end of the fiber. The end of the fiber is connected to a photo-detector that converts these light pulses back into their electrical signal form.

Optical fibers are often characterized by either single mode or multimode transmission. Single mode of fiber transmission only allows a specific narrow wavelength of light to pass through the fiber. Multimode fiber transmission allows a much wider wavelength of light to pass through the fiber by gradually bending different wavelengths back towards the center of the fiber. Single mode fiber strands are very narrow with a fiber diameter of 9-10 microns (1/100 of a mm). Multimode fibers are much wider as they can have a fiber diameter of 50-125 microns).

Figure below shows single mode and multimode fiber lines. This diagram shows that multimode fibers have a relatively wide transmission channel that allows signals with different wavelengths to bend back into the center of the fiber strand as they propagate down the fiber. The diagram also shows that single mode fiber has a much small transmission channel that only allows a specific wavelength to transfer down the fiber strand.

Single and Multimode Fiber Lines

Because of the relatively wide frequency bandwidth (and high data-transmission rate), multimode fibers are predominantly used for high-speed short runs such as those occurring within a building or around a campus.

Typical multimode fiber runs are less than a mile, but can be several miles (2 – 5). Because of this, it is often referred to a “short haul” fiber solution. Single mode is a “long haul” fiber solution (50 – 75 mile runs). Single-mode fiber has been used by telephone companies and long distance carriers for several years to off-load expanding requirements from traditional terrestrial microwave.

Single-mode fiber can transmit much further than multimode fibers. Single-mode applications use a diode laser as the light source, while multimode uses a light emitting diode (LED). Use of a LASER ensures a high energy at a very narrow optical bandwidth as compared to the LED that has optical energy distributed over a wide optical bandwidth. Single mode fibers use a narrow glass filament with a diameter of approximately 10 microns compared to multimode where the diameter ranges from 50 to 125 microns. The narrow channel of the single mode fiber minimizes the bending of the wave and this results in less dispersion (less smearing of the pulses) over distance.

Figure below shows a fiberoptic communication system that is composed of two end-nodes and a fiber optic cable transmission medium. This diagram shows two optical network units (ONUs) that connect data networks together using fiber cable. This diagram shows that two fiber strands are needed: one for transmitting and one for receiving.

Fiber-Optic Cable Transmission System

To increase the capacity of fiber systems, multiple optical signals of different wavelengths are combined on a single strand of fiber. This is called wave division multiplexing (WDM). When 40 (or more) optical signals are combined on a single fiber strand, this is called dense wave division multiplexing (DWDM).

Figure below shows how a wave division multiplexing over fiber operates. This diagram shows that there are several lasers operating at different optical wavelengths (different colors/frequencies). Each laser converts an electrical signal into a pulsed light signal. These optical signals (optical carriers) are combined by an optical multiplexer (lens) for transmission through the optical fiber. At the receiving end, the different optical carriers are separated by an optical demultiplexer (lens) and each optical carrier is sent to a photo-detector. The photo-detector converts the optical signal back into its original electrical form.

Wave Division Multiplexing

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