Five Minute Expert: ‘Bow Tie’ Polarization Maintaining Fiber

‘Bow Tie’ Polarization Maintaining Fiber

Well, here it is, ‘The Big One’. So what makes it different from standard, telecommunications fibers, and how does it work?

As you can see, it got its name because it was invented in 1982, so long ago that Fabricators wore bow-ties – because an ordinary tie could too easily have become wrapped around the lathe ...…OK, so I made this bit up, but at least you were listening!

The distinctive cross-section of a bow-tie fiber is created by the two segments of boron-doped glass that flank the core (you may hear these segments being called SAPs or stress-applying parts). As the fiber cools during the drawing process, the boron makes the S

APs contract more than the rest of the fiber, placing the core in tension. This tension stretches the glass structure along the axis running parallel to the stress and compresses it along the axis running perpendicular to it and, in doing so, changes its optical properties. Light moves less easily through the compressed, densified structure, causing it to travel more slowly – and conversely, more easily and more quickly through the stretched structure. In this way, the fast and slow axes are created – the fast running perpendicular and the slow, parallel to the SAPs. This is called birefringence.

A lot of people (including some of our customers) think that the elliptical shape of the core is caused by stress – it isn’t. The core shape is formed in the preform, during the collapse phase of the fabrication process, when the glass is molten and cannot support stress – the birefringence can only start to happen after the glass has solidified.

Birefringence can be a useful thing to have in a fiber because it causes light waves to travel at different speeds, depending on their orientation relative to the SAPs – so if your optical source is polarized, i.e. if all of the light waves it generates have the same orientation, and you line them up to the fast or to the slow axis, then the polarization state of the source will be maintained – and you have a polarization maintaining or ‘PM’ fiber.

Fibercore’s product range can be found at http://www.fibercore.com/