WHAT IS POLARIZATION?
Light is a type of electromagnetic wave. It consists of oscillating electrical fields, denoted by E, and magnetic fields, denoted by B. Its properties can be described by studying its electrical field E, although we could just as well describe light and its effects in terms of the magnetic field.
Light waves can vibrate in many directions. Those that are vibrating in one direction – in a single plane such as up and down – are called polarized light. Those that are vibrating in more than one direction – in more than one plane such as both up/down and left/right – are called unpolarized light.
HOW TO ACHIEVE SINGLE POLARIZATION
The most common method of achieving single polarization is using a polarization filter. Polarization filters are made of special materials that are capable of blocking one of the two planes of vibration of an electromagnetic wave.
Polarization filter serves as a device which filters out one-half of the vibrations upon transmission of the light through the filter. When unpolarized light is transmitted through a polarization filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.
WHAT IS POLARIZATION MAINTAINING FIBER?
Polarization maintaining fiber (PM Fiber) is a special type of single mode fiber. Normal single mode fibers are capable of carrying randomly polarized light. However, PM fiber is designed to propagate only one polarization of the input light.
In polarization maintaining fiber, the polarization of linearly-polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes. This polarization maintaining feature is extremely important for some fiber optic components such as external modulators that require a polarized light input.
This characteristic is achieved during the manufacturing process by inducing stresses in the material itself. There are two categories of polarization maintaining fiber (PMF) available, linear polarization maintaining fiber (LPMF) and circular polarization maintaining fiber (CPMF).
CROSS SECTION OF POLARIZATION MAINTAINING FIBERS
These fibers contain a feature not seen in other fiber types. Besides the fiber core, there are stress rods in the fibers. 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.
As their name implies, these stress rods create stress in the core of the fiber such that the transmission of only one polarization plane of light is favored.
When PM fibers are terminated with fiber connectors, it is very important that the stress rods line up with the connector, usually in line with the connector key.
PM fiber also requires a great deal of care when it is spliced. Not only the X,Y and Z alignment have to be perfect when the fiber is melted together, the rotational alignment must also be perfect so that the stress rods align exactly.
Another requirement is that the launch conditions at the optical fiber end face must be consistent with the direction of the transverse major axis of the fiber cross section.
APPLICATIONS OF POLARIZATION MAINTAINING FIBERS
1.PM optical fibers are used in special applications, such as fiber optic sensing, interferometry and slab dielectric waveguides
2.PM fibers are expected to be used in coherent optical transmission systems or long distance bidirectional optical transmission systems
3.They may also be used in transmission applications where the polarization plane of the optical signal is important, such as transmission lines for optical sensors and coupling for optical electrical integrated circuits
4.PM fibers are used in lithium niobate modulators, Raman amplifiers, and other polarization sensitive systems to maintain the polarization of the incoming light and keep cross-coupling between polarization modes at a minimum.