Fiber optic links are the pathways between communications devices. A link is bi-directional with signals transmitted in two directions on two different fibers. Using two fibers is the cheapest way, since the optical fiber is about as cheap as kite string and fishing line. The link connects electronic signals from two devices that need to communicate, just like a copper cable, but the fiber optic link has a transmitter that converts electronic signals from communications equipment to optics and a receiver that converts the signal back to electronics at the other end.
Fiber optic transmitters use LEDs or semiconductor lasers to convert electronic signals to optical signals. LEDs are used for slower links, such as fast Ethernet LANs, up to about 100 million bits per second (Mb/s). Faster links use infrared semiconductor lasers because they have more bandwidth, up to tens of billions of bits per second (Gb/s). Lasers have more power, so they can also go longer lengths for outside plant applications such as long distance telecom or Category 5 cables.
As noted, transmitters use infrared light. Infrared light has lower loss in the fiber, allowing longer cable runs. Typically, glass fibers use light at 850 nm wavelength, referred to as “short wavelength,” and 1,300 or 1,550 nm, called “long wavelength.”
Since the light being transmitted through the optical fiber is in the infrared region beyond the range of human vision, you cannot look at the end of a fiber and tell if light is present. In fact, since some links carry high power, looking at the end of the fiber, especially with a microscope, which concentrates all the light into the eye, can be dangerous. Before examining a fiber visually, always check with a power meter to ensure no light is present unless you know the far end of the fiber is disconnected.
At the receiver end, a photodiode converts light into electrical current. Photodiodes must be matched to the transmitter type, wavelength, power level and bit rate as well as the fiber size to optimize performance. It is the receiver that ultimately determines the performance of the link, as it needs adequate power to receive data reliably. Receivers have a certain amount of internal noise that can interfere with reception if the signal is low, so the power of the optical signal at the receiver must be at a minimal level.
At the receiver, the power is determined by the amount of light coupled into the fiber by the transmitter diminished by the loss in the fiber optic cable plant. The installer will test the cable plant for loss after construction, comparing it to a loss calculated from typical component values called the “loss budget.” Transmitter power can be measured when the networking equipment is installed using a patchcord attached to the transmitter and a fiber optic power meter. Receiver power is measured by unplugging the cable at the receiver and measuring the power.
Specific networks adapt the generic fiber optic link described above to that network’s needs. 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 links may be analog or digital, depending on the camera, and may include camera controls in one direction and video in the other. Since so many link types exist, it is impossible to generalize on-link characteristics, so the best plan is to design, install and the test cable plant based on fiber optic component specifications rather than any specific network needs.
Most computer or telecommunications networks have adopted standards for fiber optic transmission as well as copper wiring and wireless. However, sometimes users have equipment with copper interfaces but want to use fiber. They can use fiber optic media converters, which do exactly what the name suggests.
Media converters will convert from one media to another, typically UTP copper to optical fiber, coax to optical fiber or multimode to single-mode fiber. Media converters are like transmitters and receivers in that they must be specified for specific network applications to ensure the proper operation in that application.
When designing or installing fiber optic cabling, the contractor can either design to cabling standards, which allows use with any network or communications system designed for those standards or for a specific network. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.