What is Polarization Maintaining PM Fiber? - FO4SALE.COM

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Polarization maintaining fiber is a special type of single mode fiber. PM fiber faithfully preserve and transmit the polarization state of the light that is launched into it. So let's see what is polarization state.

The fundamental TEM00 mode that propagates within a single mode fiber is actually a degenerate combination of two orthogonally polarized modes, as shown in the pictures, one at vertical plane, the other at horizontal plane.

In a regular single mode fiber, these two components have the same propagation constant, which means they travel at the same speed. This makes it very easy for optical energy to transfer, or "cross-couple", from one of these modes to the other, especially under the effect of fiber core geometric irregularities, external temperature and mechanical stress on the fiber.

That is why regular single mode fiber cannot maintain the polarization state of single mode laser beam.

So that is why purpose-designed polarization maintaining fiber was invented. PM fibers are engineered in such a way that the two orthogonally polarized modes are forced to travel at different velocities, which means with different propagation constants. This difference in velocities makes it very difficult for optical energy to cross-couple, with the result that the polarization state of the transmitted light is preserved.

This different is created through the introduction of anisotropy within the core of the fiber, either geometric, by making the core elliptical, or, more typically, through the application of a controlled uniaxial stress.

So let's take a look at these different PM fiber designs.

Most PM fibers used today have one of three stress-birefringent geometries: Bowtie, PANDA, and elliptical jacket.

All three designs function in the same way; the cores are wrapped around by areas of high-expansion glass that shrink-back more than the surrounding silica and freeze the core in tension.

This tension induces birefringence, which means it creates two different indices of refraction: a higher index parallel and a lower index perpendicular to the direction of the applied stress. When polarized light is launched along the "slow axis", it is forced to travel at a lower velocity than if it had been launched along the "fast-axis" and vice-versa.

The cross-coupling of light from one axis to the other becomes very difficult because it would require a perturbation capable of making a significant change in the velocity of the transmitted light. The greater the applied stress, the greater the difference in propagation constant between the two axes and the higher the birefringence.

In this way, the polarization state of the light is preserved at either the fast axis or slow axis. Most applications use the slow axis exclusively.

The second type is called form-birefringent fibers. These fibers have a highly elliptical core. However, the high levels of attenuation and lack of compatibility, made them unsuitable for telecom use. However, they did find some application in sensors.

In telecom applications, the fiber of choice is usually the PANDA design, invented by NTT Japan in the early 1980s, then developed and commercialized in the United States and Europe throughout the 1990s.

PANDA fiber is a telecom fiber, its attenuation and mode-field diameter were well matched to those of single mode telecom fibers.

Bowtie fiber was invented in the early 1980s as a sensor fiber, and to this day, bowtie fibers are still typically used in sensor applications.

So there you have it. Please click on the "like" button if you find this video useful.

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