Experimental and theoretical study of the in- fiber twist sensor based on quasi-fan Solc structure filter

High-performance vector torsion sensor based on high polarization-dependent in-fiber Mach-Zehnder interferometer

We propose a high-performance vector torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of a straight waveguide inscribed in the core-cladding boundary of the SMF by a femtosecond laser in only one step. The length of the in-fiber MZI is 5 mm, and the whole fabrication time does not exceed 1 min. The asymmetric structure makes the device have high polarization dependence, and the transmission spectrum shows a strong polarization-dependent dip. Since the polarization state of the input light entering the in-fiber MZI varies with the twist of the fiber, torsion sensing can be achieved by monitoring the polarization-dependent dip. Torsion can be demodulated by both the wavelength and intensity of the dip, and vector torsion sensing can be achieved by setting the appropriate polarization state of the incident light. The torsion sensitivity based on intensity modulation can reach 5763.96 dB/(rad/mm). The response of dip intensity to strain and temperature is weak. Furthermore, the in-fiber MZI retains the fiber coating, so it maintains the robustness of the complete fiber structure.

Fiber Ring Laser Directional Torsion Sensor with Ultra-Wide Linear Response

In this paper, a comprehensive passive torsion measurement is performed firstly in a 40-cm-long polarization maintaining fiber-based Sagnac interferometer (PMF-SI), and the non-linear torsion response is found and investigated. Then, a fiber laser torsion sensor (FLTS) with a dual-ring-cavity structure is proposed and experimentally demonstrated, in which the PMF-SI is utilized as the optical filter as well as the sensing unit. In particular, the highly sensitive linear range is adjusted through fine phase modulation, and owing to the flat-top feature of fringes, an ~83.6% sensitivity difference is effectively compressed by the generated lasing. The experimental results show that, without any pre-twisting, the ultra-wide linear response from –175 to 175 rad/m is gained, and the torsion sensitivities are 2.46 and 1.55 nm/rad with high linearity (>0.99) in the clockwise and anti-clockwise directions, respectively. Additionally, a high extinction ratio (>42 dB) and small line-width (~0.14 nm) are obtained in the proposed FLTS, and the corresponding detection limit reaches 0.015 rad/m.

Low-cost temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure

We proposed and experimentally demonstrated a temperature- and strain-insensitive twist sensor based on a hybrid fiber grating structure, in which the hybrid grating structure is constructed with a 45°-tilted fiber grating and a chirped fiber Bragg grating UV-inscribed in a single-mode fiber in series. The sensing performance has been evaluated by experimental and numerical analyses, which are in good consistency. The experimental results show that the hybrid-grating-structure-based twist sensor has a maximum twist sensitivity up to 15.037 dB/rad. Moreover, due to the invariability of the fiber birefringence and the state of polarization of the input light, such sensor has intrinsically low temperature and strain sensitivities of 7.86×10^-3  dB/°C and 6.7×10^-5  dB/με, corresponding to the maximum twist measurement error resulting from temperature and strain of 5.2×10^-4  rad/°C and 4.5×10^-6  rad/με, respectively.

Novel temperature-independent microfiber sensor fabricated with the tapering-twisting-tapering technique

In this paper, a novel twist and refractive index microfiber sensor fabricated with the tapering-twisting-tapering technique has been proposed and demonstrated, for the first time to the best of our knowledge. Experimental results show that the interference intensity of the microfiber increases with the increment of pre-tapered length and the intervening twist number. The sensitivity of the microfiber sensor with respect to twist is found to be 2.817 dB/(rad/m) and the refractive index sensitivity of this microfiber sensor can reach to ∼809  nm/refractive index unit in the refractive index ranging from 1.30 to 1.33. Moreover, considering that its temperature sensitivity is 0.005 dB/°C, it will not suffer from the cross sensitivity to temperature.

Intensity-modulated directional torsion sensor based on in-line optical fiber Mach-Zehnder interferometer

In this Letter, we demonstrated an intensity-modulated directional torsion sensor based on an in-line Mach-Zehnder interferometer in single-mode fiber. A non-circular symmetric perturbation is created to excite non-circular symmetric cladding mode and then interference with the core mode at the second perturbation. An initial rotation angle is designed between two perturbations for the purpose of discriminating the torsion direction. Both experimental and theoretical results enforce that the spectral peak/dip turns to be the dip/peak when the fiber is twisted from the counter-clockwise to the clockwise direction. Benefiting from the reversal between peak and dip, an intensity-modulated directional torsion sensor is realized in the range from -50  rad/m to 50 rad/m with a sensitivity of 45.3%/(rad/cm).