Fabrication of t-PCF

We use two techniques for producing t-PCF. In the first, an untwisted PCF is post-processed under CO2 laser heating, the fibre being mounted between a motorized rotation stage and a rigid support [Shin (2009)]. As the motor rotates, the focused 10 µm laser beam is scanned along the fibre using a steering mirror fixed to a precision motorized translation stage. Once the target twist period and sample length are set, the laser power and the scanning speed are chosen so as to heat the fibre to the glass-softening temperature. The writing process is computer-controlled and is capable of achieving twist periods as short as 300 μm. The second technique involves spinning the glass preform during fibre drawing, using a motor rotating at a few thousand rpm and a rotary joint with multiple inlets for controlling the pressure inside the hollow channels. It has the advantage that long lengths (100s of metres) of helical PCF with twist periods of a few millimetres can be readily fabricated.

Fabrication procedures for t-PCF (a) Thermal post-processing using a scanned carbon dioxide laser beam to heat the fibre to its softening temperature while spinning one fibre end, the other being rigidly fixed. (b) Spinning the glass preform during fibre drawing in the pulling tower.