Understanding Fiber Optic Communications

Fiber optic connections are the communication paths between devices. A link is bidirectional, usually with signals transmitted in two directions over two different fibers. The use of two fibers is generally the most economical way, since the same optical fiber is now cheap as the kite cable and the fishing line! (FTTx PON systems use bidirectional fiber so you can use a PON coupler that transmits and receives at a lower cost to the system.) The link connects electronic signals from two devices that need to communicate, such as a copper cable. The link has a transmitter that converts electronic signals from the communication equipment to optics and a receiver that converts the signal to electronic components at the other end.

Fiber optic transmitters use LEDs or semiconductor lasers to convert electronic signals to optical signals. LEDs, similar to those used everywhere for indicators, except for transmission in the infrared region beyond human perception, are used for slower connections, up to 100 million bits per second (Mb / s), for example , LAN Fast Ethernet. The fastest connections use infrared semiconductor lasers because they have a higher bandwidth, up to tens of billions of bits per second (Gb / s). Lasers have more power and can also increase lengths, as in applications of external systems, such as long-distance telecommunications or CATV.

As noted, transmitters use infrared light. Infrared light has less fiber loss, allowing for longer cables. In general, multimode glass fibers use light at 850 nm, called "short wavelength" and single-mode fiber operates at 1310, 1470 or 1550 nm, called "long wavelength".

Since the light transmitted through the optical fiber is beyond the reach of human vision, it is not possible to look at the end of a fiber and know if light is present. In fact, since some connections carry high power, looking at the end of the fiber, especially with a microscope that concentrates all the light in the eye, can be dangerous. Before visually examining a fiber, always check with a power meter to make sure there is no light, unless you know that the far end of the fiber is disconnected and use a microscope equipped with a laser filter.

At the end of the receiver, a photodiode converts light into electric current. The photodiodes must match the type of transmitter, wavelength, power level and bit rate, in addition to the size of the fiber to optimize performance. It is the receiver that determines the performance of the connection, since it needs adequate power to receive data reliably. Receivers have a certain amount of internal noise which can interfere with reception if the signal is low; therefore, the intensity of the optical signal on the receiver must be at a minimum.

The energy in the receiver is determined by the amount of light connected to the fiber by the transmitter, decreased by the loss in the installation of the fiber optic cable. The installer will check the losses of the cable installation after construction, comparing it with a loss calculated from the typical values ​​of the components called "loss budget". The transmitter power can be measured when the network equipment is installed using a connection cable connected to the transmitter.

The networks adapt the generic fiber optic link described above to the needs of a specific network. An Ethernet link will be optimized for the bit rate and protocol of the version of Ethernet to be used, for example Gigabit Ethernet. Video connections can be analog or digital, depending on the camera, and can include camera controls in one direction and video in the other. Industrial connections can be based on RS-232 or RS-422 protocols.

Most computer or telecommunications networks have adopted standards for fiber optic transmission, as well as copper and wireless cables. However, sometimes the user has equipment with copper interfaces, but wants to use fiber. So that they can use fiber optic media converters, which do exactly what the name suggests. Media converters will be converted from one medium to another, usually covering UTP over optical fiber, coax over optical fiber or multimode over single mode fiber. Media converters are like transmitters and receivers, as they need to be specified for specific network applications to ensure proper operation in that application.

As there are many types of connections, it is impossible to generalize the characteristics of fiber optic connections, but on the FOA website there is a table that details most of the standard networks. When designing or installing fiber optic cables, the contractor can design according to the cabling standards, which allows use with any network or communication system designed for these standards or for a specific network, which allows for optimization cabling of the system. If the actual network for the use of fiber optic cables is unknown, the best plan is to design, install and test the cable system based on standardized specifications for fiber optic components, rather than specific network needs.

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