Skinny Fibers – Fat Results: A Stunning Presentation from Fibercore Ltd on ‘Ultra-Low Profile, High Birefringence Gyroscope Fiber’

presented by Dr Andy Gillooly at the 'Inertial Sensors & Systems - Symposium Gyro Technology' in Karlsruhe, Germany, on 21st September 2011.

The future for optical fibers has, for some time, been for diameters smaller than the standard 125µm, fibers like our 80µm reduced-clad PM fibers that we ship over a million metres a month of to FOG manufacturers worldwide. The new HB800G(3.4/60) fiber from Fibercore Ltd is even skinnier with a mere 60µm diameter.

The benefits of these 'ultra-low profile' fibers are that they offer enhanced gyro sensitivity by allowing a longer path-length into a smaller volume, are stronger long-term as the reduced bending stress offers increased lifetime, and they are better for PZT phase-modulators as they offer more stretch for less applied force.

These smaller diameter fibers are more sensitive to microbending and require higher birefringence to overcome microbend induced cross-coupling. The very short 65mm beatlength of the new HB800G(3.4/60) is an indication of how high the birefringence is for this ultra-skinny fiber - the beatlength for our 80µm fibers being around 1.5mm, and less than 2mm for our 125µm fibers.

Andy's presentation discussed the mechanical lifetime of the fiber and the stiffness of the fiber compared to the cladding diameter, as well as the effects of the increasing birefringence on PER. He discussed the PER penalty that has to be paid when the fiber is used in a worse case 'basket weave' fiber wind and showed that the 60µm fiber performs as well as 80µm fiber in small diameter coils. Thermal cycling was also discussed with an illustration of how little the PER performance changes over temperature.

The conclusion: for the smallest diameter, highest birefringent fiber - HB800G(3.4/60) is the answer!

'Bow-Tie' Polarization Maintaining Fiber

... more than 1,000,000 metres shipped by Fibercore Ltd every month.

So what makes it different from standard, telecomunications fibers, and how does it work?

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 we 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 SAPs 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 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 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, ie if all of the lightwaves 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.

It's as simple as that!

For more information, visit our Technical Resource Center and download the Fibercore PM Fiber Handbook, or contact our Sales Team at sales@fibercore.com.