Types of Fiber Optics

There two main type of fiber optics :-

a. Singlemode
b. Multimode

Single mode fiber has a very small core causing light to travel in a straight line and typically has a core size of 8 or 10 microns. It has unlimited bandwidth that can go unrepeated for over 80 km, depending on the type of transmitting equipment. Single mode fiber has enormous information capacity, more than multimode fiber.

Multimode fiber supports multiple paths of light and has a much larger core and has a core size of 50 or 62.5 microns. The light travels down a much larger path in multimode fiber, allowing the light to go down several paths or modes.

Multimode fiber can be manufactured in two ways: step-index or graded index. Step-index fiber has an abrupt change or step between the index of refraction of the core and the index of refraction of the cladding. Multimode step-index fibers have lower bandwidth than other fiber designs.

Graded index fiber was designed to reduce modal dispersion inherent in step index fiber. Modal dispersion occurs as light pulses travel through the core along higher and lower order modes. Graded index fiber is made up of multiple layers with the highest index of refraction at the core. Each succeeding layer has a gradually decreasing index of refraction as the layers move away from the center. High order modes enter the outer layers of the cladding and are reflected back towards the core. Multimode graded index fibers have less attenuation (loss) of the output pulse and have higher bandwidth than multimode step-index fibers.

Table 1: Single Mode and Multimode Characteristics

	Single Mode Fiber	Multimode Fiber
Bandwidth	High	Lower
Signal Quality	High	Lower
Main Source of Attenuation	Chromatic Dispersion	Modal Dispersion
Fiber Designs	Step index, & Dispersion shifted	Step index & Graded index
Application	Long transmission, higher bandwidth	Short transmission, lower bandwidth

Single mode step-index fibers are not affected by modal dispersion because light travels a single path. Single mode step-index fibers experience light pulse stretching and shrinking via chromatic dispersion. Chromatic dispersion happens when a pulse of light contains more than one wavelength. Wavelengths travel at different speeds, causing the pulse to spread. Dispersion can also occur when the optical signal gets out of the core and into the cladding, causing shrinking of the total pulse.

Single mode shifted fiber uses multiple layers of core and cladding to reduce dispersion. Dispersion shifted fibers have low attenuation (loss), longer transmission distances, and higher bandwidth.

In discussing fiber cables you will hear the terms IFC and OSP. IFC refers to an Intrafacility fiber cable. These types of cables are designed for use with in a controlled environment such as a building or inside equipment. Since the cable is used within a building the cable requires less physical protection and more flexibility. Outside plant cable, or OSP, are used in hostile environments, exposed to extreme temperatures, rain, and wind. The cables are more robust and have extra layers of buffering and sheathing to protect the fiber.

Fibers are assembled into either stranded or ribbon cables. Stranded cables are individual fibers that are bundled together. Ribbon cable is constructed by grouping up to 12 fibers and coating them with plastic to form a multi fiber ribbon. Stranded and ribbon fiber bundles can be packaged together into either loose or tight buffering cable.

Table 2: Cable Characteristics

Loose Buffered Cable	Tight Buffered Cable
Each individual fiber bundle moves freely within the inner sheath	Fiber elements are held in place within the cable
Protects from tensile factors	Smaller in diameter with fewer fibers
Less expensive	More flexible for manipulation
More robust	More sensitive to outside forces
Higher fiber counts	Less toxic when burned
Optimized for long runs	Used in intrafacility applications
Used in OSP applications (aerial, buried, or submerged)	Cables are either distribution or breakout designs. All fiber bundles are in a single jacket or each has a separate jacket

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