This article was written by Joni Nurmi ,Zhen Shi and Sun Hai

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\[1\] Source of the figure: [http://fiberoptic101.blogspot.fi/2010/11/history-of-fiber-optic.html]

Principle of operation

Fiber optical cables are highly transparent, cylindrical conduits for light. These cables are formed of two materials with different refractive indices. This creates a optimal waveguide for transmitting light. When light enters the fiber made of material with higher refractive index than the cladding surrounding it, it stays inside the material due to total internal reflection and is thus transmitted forward.

Index of refraction

Index of refraction is a measurement of speed of light in material. By definition, vacuum has refractive index of 1(c in vacuum = 1). The higher the index, the slower the light will travel in the material. Typical value for fiber core is 1,62 and for the cladding 1,52.

Total internal refraction:

When light travelling inside material hits a boundary, it is partially refracted and partially reflected. However if the angle is greater than the critical angle of the materials them it is reflected back in its entirety. For this to occur the material in which the light is travelling in must have higher refractive index than the material outside.

Multi-mode fiber

Multi-mode fiber & Single-mode fiber written by Zhen Shi

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In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths.

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Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet 914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.  

Single-mode fiber

In optical fiberIn [optical fiber|http://searchtelecom.techtarget.com/definition/optical-fiber] technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission.

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Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

Comparison of single-mode fiber and muti-mode fiber

Constructions are different

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\[2\] Source of the figure: [http://www.rp-photonics.com/multimode_fibers.html]

Transmissions are different

!Fiber single mode.GIF!
!fiber multimode fiber.GIF!
\[3\] Source of the figure: [http://www.arcelect.com/fibercable.htm]

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Special-purpose fiber

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: Polarization - maintaining fibers

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What is polarization-maintaining fibers? 

Polarization-maintaining fibers is a special purpose single mode fiber, it preserves and transmits the polarization state of light that is launched into it. 

Why do we need the polarization-maintaining fibers?

The reason why we need to have polarization-maintaining fibers is that regular single mode fiber cannot maintain the polarization state of input laser beam. Because in a regular single mode fiber, the input polarized laser beam (consist of two perpendicular polarized modes - horizontal and vertical) have the same propagation constant (propagation constant is a measure of the change undergone by the amplitude of the wave as it propagates in a given direction), in other words they both travel at the same velocity (not good). It would cause optical energy from one of these modes (horizontal or vertical mode) to transfer the other or “cross-couple”.  That is the reason why single mode fiber cannot maintain the polarization state of input laser beam. 

How does polarization-maintaining fibers work?

Then the polarization-maintaining fibers was invented to overcome this problem. The polarization-maintaining fibers are designed in such a way that two perpendicular polarized modes are forced to travel at different propagation constants, which means they travel at different velocity. This difference in velocities makes it very hard for optical energy to transfer between each other. As the result that polarization state of input laser beam is preserved.  This special purpose fiber is commonly used in fiber optic sensing, interferometry and slab dielectric waveguides. 

YouTube link: 

http://www.youtube.com/watch?v=7rrb-_Iin-g

References: 

What Are Polarization Maintaining Fibers?    - http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/

Cross Section of Polarization Maintaining Fibers

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Figure. 4. Cross Section of Polarization Maintaining Fibers \[4\]

The above three pictures illustrated the three different types of polarization maintaining fibers, which are Panda, Elliptical-clad, Bow-tie. There fibers contain a unique feature; they have stress rods in fibers (a feature not seen in other fiber types). The stress rods are two circles in the Panda PM fiber, a elliptical clad in elliptical-clad PM fiber and two bow-ties in the Bow-Tie type PM fiber. The most important property of stress rods is that it creates stress in the core of the fiber such that the transmission of only one polarization plane of light is favored.

YouTube video - A Brief Introduction of the Polarization-maintaining Fibers

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Video.1. A Brief Introduction of the Polarization-maintaining Fibers \[5\]

References: 

Principle of operation - Special purpose fiber  fiber                        - http://en.wikipedia.org/wiki/Optical_fiber

\[2\]Multimode fibers                                                                - [http://www.rp-photonics.com/multimode_fibers.html

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\[3\]The basic of fiber optic cable (single mode and multi mode)  - [

http://www.arcelect.com/fibercable.htm

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\[4\] What Are Polarization Maintaining Fibers?                      - [http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/|http://www.

fiberoptics4sale.com/

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wordpress/what-are-polarization-maintaining-fibers/]

\[5\] What is Polarization Maintaining Fiber?                          \- [http://www.youtube.com/watch?v=7rrb-_Iin-g|http://www.youtube.com/watch?v=7rrb-_Iin-g]