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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
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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
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In [optical fiber|http://searchtelecom.techtarget.com/definition/optical-fiber]
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In 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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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.
Cross Section of Polarization Maintaining Fibers
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Figure. 4. Cross Section of Polarization Maintaining Fibers: [http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/|http://www.fiberoptics4sale.com/wordpress/what-are-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.
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Video.1. *A Brief Introduction of the Polarization-maintaining Fibers*: [http://www.youtube.com/watch?v=7rrb-_Iin-g|http://www.youtube.com/watch?v=7rrb-_Iin-g] \[5\] |
References:
Principle of operation - Special purpose fiber - http://en.wikipedia.org/wiki/Optical_fiber
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\[2\]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/wordpress/what-are-polarization-maintaining-fibers/] |
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