Fiber-optic communications is based on the principle that light in a glass medium can carry more information over longer distances than electrical signals can carry in a copper or coaxial medium or radio frequencies through a wireless medium. The purity of today’s glass fiber, combined with improved system electronics, enables fiber to transmit digitized light signals hundreds of kilometers without amplification. With few transmission losses, low interference, and high bandwidth potential, optical fiber is an almost ideal transmission medium.
What is Optical Fiber?
An optical fiber (or optical fibre) is a flexible, transparent fiber made of glass (silica) or plastic, slightly thicker than a human hair. It functions as a wave guide, or “light pipe” to transmit light between the two ends of the fiber.The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics.
In the 1920s, the same basic technology was used to actually transmit full images. In the 1930s that technology was used practically to illuminate the inside of a surgery, allowing for much more precise surgery. Optical fiber continues to be used in surgery, especially to facilitate less invasive internal surgeries. The first true optical fiber appeared in the 1950s, and by the end of the decade experiments were underway with a type of fiber very similar to that used today, with glass fibers coated with a transparent sheath
How Fiber Works
The operation of an optical fiber is based on the principle of total internal reflection. Light reflects (bounces back) or refracts (alters its direction while penetrating a different medium), depending on the angle at which it strikes a surface.This principle is at the heart of how optical fiber works. Controlling the angle at which the light waves are transmitted makes it possible to control how efficiently they reach their destination. Lightwaves are guided through the core of the optical fiber in much the same way that radio frequency (RF) signals are guided through coaxial cable. The lightwaves are guided to the other end of the fiber by being reflected within the core.
- Core: The center of the fiber where the light is transmitted.
- Cladding: The outside optical layer of the fiber that traps the light in the core and guides it along - even through curves.
Multimode & Singlemode Fibers
Multimode & Singlemode fiber are the two types of fiber in common use. Both fibers are 125 microns in outside diameter - a micron is one one-millionth of a meter and 125 microns is 0.005 inches- a bit larger than the typical human hair. Multimode fiber has light traveling in the core in many rays, called modes. It has a bigger core (almost always 62.5 microns, but sometimes 50 microns ) and is used with LED sources at wavelengths of 850 and 1300 nm (see below!) for slower local area networks (LANs) and lasers at 850 and 1310 nm for networks running at gigabits per second or more. Singlemode fiber has a much smaller core, only about 9 microns, so that the light travels in only one ray. It is used for telephony and CATV with laser sources at 1300 and 1550 nm. Plastic Optical Fiber (POF) is large core ( about 1mm) fiber that can only be used for short, low speed networks.
Step index multimode
was the first fiber design but is too slow for most uses, due to the dispersion caused by the different path lengths of the various modes. Step index fiber is rare - only POF uses a step index design today. Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fiber is best suited for transmission over short distances.
Graded index multimode
fiber uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber - up to about 2 gigahertz. Contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Due to the graded index, light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion. This type of fiber is best suited for local-area networks.
Singlemode
fiber shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot!
How do I know what type of fiber I need?
This is based on transmission distance to be covered as well as the overall budget allowed. If the distance is less than a couple of miles, multimode fiber will work well and transmission system costs (transmitter and receiver) will be in the $500 to $800 range. If the distance to be covered is more than 3-5 miles, single mode fiber is the choice. Transmission systems designed for use with this fiber will typically cost more than $1000 (due to the increased cost of the laser diode).Should I install single-mode or multimode fiber?
This depends on the application. Multimode fiber will allow transmission distances of up to about 10 miles and will allow the use of relatively inexpensive fiber optic transmitters and receivers. There will be bandwidth limitations of a few hundred MHz per Km of length. Consequently, a 10 mile link will be limited to about 10 to 30 MHz. For CCTV this will be fine but for high speed data transmission it may not be.
Single-mode fiber on the other hand will be useful for distances well in excess of 10 miles but will require the use of single-mode transmitters (which normally use solid-state laser diodes). The higher cost of these optical emitters mean that single-mode equipment can be anywhere from 2 to 4 times as expensive as multimode equipment.
