How they work

An optical fiber consists of three main structures. Light travels in the core, a thin glass center. The cladding surrounds the core and reflects light back into it. The cladding also prevents "cross talk", the jumping of signal between adjacent fibers. The buffer coating prevents damage due to rodents, weather, etc.

Optical fibers come in two types: single mode and multi-mode. Single mode fibers have smaller cores (3.5x10^-4 in/ 9 microns) and transmit laser light with long wavelengths (1,300-1550 nm). Light travels in one ray, increasing bandwidth to almost infinity (practically 100,000 gigahertz). Single mode is used for telephony--trunk lines between cities and oceans.

Multi-mode fibers have larger cores (2.5x10^-3 in/62.5 microns) and transmit infared light of wavelength 850-1300 from LEDs. Light travels within the core in many rays, or "modes". Mulit-mode fibers are for use with slower LANs.

To understand what happens in an optical fiber, imagine you are using a flash light and Morse code to communicate through a pipe. As long as the pipe is straight this works since light travels in a straight line. But, if there is a bend in the pipe, a mirror must be angled to ensure light will reach the other side. If the pipe has many bends, multiple mirrors must be added and the light must be angled to ensure that it will bounce of the sides of the pipe until it arrives.

This is essentially what occurs within a fiber optic cable. Light moves through the core and is reflected by the cladding through a principle known as total internal reflection. To understand this idea, imagine you are in a dark room with a window. You shine a flashlight at 90 degrees with respect to the window and the light passes through. Then you shine the light at a shallow angle and it bounces off the glass back into the room.

In fiber optic relay systems, analog signals are converted to digital ones (0101s). A laser pulses on and off to send each bit. The best networks can transmit billions of bits/second and the most advanced lasers flash on/off several billions of times/second. The latest technology uses different colors to fit multiple signals on the same fiber. An equipment hut every 40-60 miles retransmits the signal.

There is always some degradation due to impurities and the extent of degradation depends on glass purity and wavelenght of the light. For example light with a wavelength of 850nm degrades at a rate of 70%/km while light of wavelength of 1,300nm loses 50-60%/km. The longer the wavelength, the less the degradation.

The critical angle is the angle at which reflected light travels in between two media, typically glass and air [(sin)(crit angle)=n2/n1]. In physics this angle is measured with respect to the normal angle. In fiber optics, however, it is measured with respect to a long axis running parallel to the fiber. The core always has a higher index of refraction than the cladding which forces light to reflect constantly from the cladding and back into the core. If the angle of light is greater than the critical angle, light reflects no matter how the fiber itself is bent.

Total internal reflection occurs if the incident angle is greater than the critical angle. Rays incident at angles greater than theta max will hit the interior wall at a smaller angle than the the critical angle and will be only partially reflected, leaking out of the pipe and reducing efficiency.

In a fiber optics relay system, the transmitter produces and encodes light signals; this is done by LED or laser. The optical fiber, discussed above, carries the light signal. Optical regenerators are necessary due to signal loss. Optical fibers with a special doping are pumped with laser; these molecules act as their own laser and retransmit a regenerated signal down the line. On the other end, an optical receiver decodes the signals and sends them to the user.