What Are Polarization Maintaining Fibers? Fiber Optics Tutorial Series 6 By Colin Yao At Isnare.com Ezine Articles

:: What is polarization?

Light waves consist of oscillating electric and magnetic fields. These two fields are perpendicular to each other and to the direction of light traveling. We call the electric field plane as the light's polarization.

Sunlight and many other ordinary light is made of many light waves, each with their electric and magnetic fields oriented randomly. If all waves' electric fields were aligned parallel to each other, we call this light linearly polarized.

:: Polarization's effect on optical fibers

Polarization does not really matter in multimode fibers but it can be crucial in single mode fibers especially for long distance and high speed rate fiber communications. Why? Let me explain it below.

Technically speaking, single mode fibers actually have two modes traveling in it. These two modes have orthogonal polarization and perfect single mode fibers cannot differentiate between them. These two modes are functionally identical and light energy can shift easily between these two polarization modes.

However, fiber's geometry is not perfect. As a result, these two modes actually travel along the fiber at slightly different speeds. The effect is called PMD (polarization mode dispersion). The slight speed difference can cause problems in high speed fiber optic links such as 10Gbit/s and 40Gbit/s.

:: Polarization maintaining fibers come to the rescue!

Polarization maintaining fibers are special kinds of single mode fibers, they are also commonly called PM fibers or Panda PM fibers.

PM fibers have built-in asymmetry which is also called birefringence. The refractive index of PM fiber differs for the two polarizations and this effect prevent light energy from coupling between two polarizations.

PM fibers can transmit light in a single polarization if the input light polarization is aligned to one of its two birefringence axes. And that is why they are called polarization maintaining fibers.

:: The applications of polarization maintaining fibers

PM fibers are rarely used for long distance transmission because of their expensive price and higher attenuation than single mode fiber. They are commonly used for telecommunication applications, fiber optic sensing and interferometry.

:: Polarization Measurements

Polarization is measured by passing light through a polarizer, which transmits light only if its electric field is aligned in a particular direction. The fraction of light transmitted by the polarizer indicates the degree of polarization in the direction of the polarizer.

Polarization has taken on increasing importance in high-performance fiber optic systems and in certain fiber optic sensors. Some fibers are sensitive to polarization. In addition, a number of components are sensitive to polarization, including some optical isolators, couplers, waveguides, and some multiplexers.

:: Polarization Dependent Loss

Polarization dependent loss measures the maximum difference in attenuation for light with various degrees of polarization. For example, if loss is 3dB when transferring horizontally polarized light from a fiber to a waveguide but 6dB for vertically polarized light at the same junction, the polarization-dependent loss is 3dB (assuming that those are the minimum and maximum losses). This is measured by adding polarization analyzers to conventional loss measurement instruments.

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