Highly sensitive fiber-optic torsion sensor based on an ultra-long-period fiber grating

A high sensitivity fiber-optic torsion sensor, which can measure twist rate and determine twist direction simultaneously based on a novel ultra-long-period fiber grating (ULPFG) with a period of up to several millimeters, is proposed and demonstrated. Such an ULPFG is fabricated by using the high-frequency CO₂ Laser pulses exposure technique. The unique torsion characteristics of the ULPFG are simply analyzed by using the mode coupling theory and the birefringence effect. The experimental results show that the high order resonant wavelengths of the ULPFG have higher torsion sensitivities, which is several times higher than that of the normal LPFG. In addition, an intensity-type demodulation approach used to realize real-time torsion measurement is proposed and demonstrated based on the edge filtering effect of the ULPFG.