We propose a high-sensitivity bidirectional torsion sensor using a helical seven-core fiber taper embedded in multimode fiber (MHSTM). Sensors with different taper waists and helical pitches are fabricated, and their transmission spectra are obtained and analyzed. The waist and length of the sandwiched seven-core fiber are finally determined to be 68 µm and 3 mm, respectively. The experimental results show that the clockwise and counterclockwise torsion sensitivities of the proposed sensor are 2.253 nm/(rad/m) and -1.123 nm/(rad/m), respectively. When tapered waist diameter reduces to 48 µm, a superior torsion sensitivity of 5.391 nm/(rad/m) in the range of 0–4.24 nm/(rad/m) is obtained, which is 46 times as large as the traditional helical seven-core fiber structure. In addition, the MHSTM structure is also relatively stable to temperature variations.

We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity. This sensor is configured by a triangular-prism-shaped long-period fiber grating, which is fabricated by the high frequency CO2 laser polished method. The unique design of the triangular-shaped structure breaks the rotational symmetry of the optical fiber and provides high sensitivity for torsion measurement. In preliminary experiments, the torsion response of the sensor achieves a good stability and linearity. The torsion sensitivity is 0.54 nm/(rad/m), which renders the proposed structure a highly sensitive torsion sensor.