Fiber Optic Theory

For telecommunications services, fiber optics is the media of choice. Its high speed and long distance capabilities make it the most cost effective media for communications.

Optical fiber is constructed of two dissimilar glass elements applied in concentric circles. A light-carrying core is surrounded by a cladding layer, which reflects the escaping light back into the core. This continual internal reflection propagates the signal down the length of fiber. By adjusting the atomic elements within the glass layers, the fiber can be made to meet highly specialized performance levels.

Transmission Performance

Transmission performance of fiber optics will depend on various factors, some intrinsic (within the glass) and some extrinsic (construction and installation). Some key terms for the understanding of transmission performance are:

Attenuation is the reduction in intensity of the signal, also referred to as transmission loss. Attenuation is measured in decibels (dB) and the larger the dB reading, the higher the attenuation and the greater the loss of signal strength. A -2dB loss equates to a 100-fold loss in signal strength. Attenuation is a result of two factors, absorption and scattering.

• Absorption is caused by the decay of light and its conversion into heat by colliding with molecules within the glass. Primary absorbers are OH+ and dopants used to modify the refractive index of the glass. Some of these substances are unwanted by-products of the manufacturing process while others are introduced into the glass for enhancing the performance of the glass.
• Scattering is the single largest cause of attenuation. Scattering occurs when light collides with individual elements within the glass and is deflected in multiple directions. Some of the light may be refracted back into the core by the cladding layer, but most of it will deflect out and be absorbed by the cladding layer. Scattering is a function of wavelength, proportional to the inverse fourth power of the wavelength. This means as you double the wavelength of the light, you reduce scattering by 16 times. Long haul transmission minimizes the effects of scattering by using light sources with the longest wavelength characteristics possible.

Bandwidth is the transmission capacity of the fiber and it is length dependent and measured as MHz/km. This transmission capacity is influenced by two factors, modal and chromatic dispersion.

• Modal dispersion can be defined as pulse spreading. As light travels down a fiber, the different rays of light travel different pathways within the core. Since each pathway has a different length, the time of arrival of each ray will be a function of the distance traveled. Hence the initial pulse is spread out over time. Modal dispersion is more prevalent in multimode fibers where larger core diameters and shorter wavelengths offer more pathways for the light to travel. Singlemode fiber, with smaller diameters and longer wavelengths, is less susceptible to modal dispersion. Use of graded index fiber in multimode fiber can minimize the affects of modal dispersion by changing the speed at which the various pathways transport the light.
• Chromatic dispersion is the spreading of the pulse as a result of different speeds of individual light frequencies. This can best be described by the prism affect of light. Ambient light is composed of many frequencies of light, each with its own particular wavelength and speed. As light enters the prism, the frequencies are separated and the rainbow spectrum of light can be seen. Light within fiber optic cables is subjected to the same effect. The larger the spectrum of light within a pulse, the greater the chromatic dispersion will be. This is most evident in LEDs (Light Emitting Devices) whose pulse is a wide spectrum of light as opposed to lasers who concentrate most of their light within a narrow spectral range. Again, the use of graded index fiber can decrease the spectral spread of light within the traveling light pulse.

The effective windows for both singlemode (1310nm and 1550nm) and multimode fiber (850nm and 1300nm) is a result of the balancing between the physical spread of the pulse (modal and chromatic dispersion) and the manufacturer's compensation (grade index doping) for those intrinsic values.