Intensity modulated torsion sensor based on optical fiber reflective Lyot filter

Highly sensitive torsion sensor based on a coupled optoelectronic oscillator incorporating nonlinear polarization rotation

We propose and demonstrate a new, to the best of our knowledge, technique to implement a high-speed and highly sensitive torsion sensor based on a coupled optoelectronic oscillator (COEO) incorporating nonlinear polarization rotation (NPR). The COEO consists of a mode-locked laser loop and an OEO loop. In the laser loop, the NPR effect effectively induces intensity- and wavelength-dependent loss, which acts as a Lyot birefringent fiber filter. When twisting the polarization-maintaining fiber (PMF), the transmission of the filter varies as well as the laser output wavelength. In the OEO loop, the optical source is provided by the output signal of the mode-locked laser. The variation in the optical carrier wavelength changes the time delay and the oscillation frequency of the OEO loop. The oscillation frequency shift is a linear function of the twist angle. Sensitivities of -60.006 Hz/deg over 360° for a 48 cm PMF and -180.996 Hz/deg over 92° for a 22 cm PMF are achieved.

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.

Dual-parameter demodulated torsion sensor based on the Lyot filter with a twisted polarization-maintaining fiber

We proposed a novel torsion sensor based on the Lyot filter with the twisted polarization-maintaining fiber (PMF) acting as the birefringence medium. Lyot filter is formed by two linear polarizers and a piece of PMF. Based on the high birefringence of the PMF, the output polarization rotates with a rate equal to the twisting rate applied on the PMF, and the sensor realizes a high sensitivity of 90.072 dB/rad. The proposed sensor also demonstrated a low strain sensitivity of 2.32 ×10 ^{- 6} rad/μɛ. On the other hand, based on the phase hits of the polarization interference, the wavelength sensitivity reaches 15.477 nm/rad. The monitoring range of the wavelength demodulation is complementary with the intensity demodulation in one cycle, making the valid sensing range of the proposed sensor expand. The proposed highly sensitive compact torsion sensor, with large sensing range and low crosstalk, has potential applications in many fields such as manufacturing industry, civil engineering, aerospace industry and modern smart structure monitoring.

3D printed long period gratings and their applications as high sensitivity shear-strain and torsion sensors

In this work we demonstrate the capability to measure shear-strain and torsion loads by bonding an optical fiber to a 3D printed periodic grooved plate. The device acts as a long period grating where the resonances show loss tunability ranging from ∼0 up to ∼20 dB, achieving sensitivities values for the dip transmission ratio as function of the load of 0.12 /mε and 0.21/deg, for shear-strain and torsion loads ranging from 0-∼8 mε and 1-∼4 deg, respectively. The low wavelength drift allowed us to operate the sensor through intensity demodulation techniques, showing good tracking performance of external stimuli.

Temperature-Independent Gas Pressure Sensor with High Birefringence Photonic Crystal Fiber-Based Reflective Lyot Filter

A novel temperature-independent gas pressure sensor based on a reflective fiber Lyot filter is presented in this paper. The reflective fiber Lyot filter is simply consist of a fiber polarizer and a segment of hollow-core photonic bandgap fiber (HB-PCF). The HB-PCF plays the role of birefringent cavity in the reflective fiber Lyot filter and works as the sensor head in the gas pressure sensor. Experiment results show that the responses of the sensor to gas pressure and temperature are 3.94 nm/MPa and −0.009 nm/°C, indicating that the proposed gas pressure is sensitive to gas pressure rather than temperature. Coupled with the advantages of simple structure, easy manufacture, high sensitivity and temperature independent, the proposed reflective fiber Lyot filter-based gas pressure sensor holds great potential application in the field of gas pressure monitoring.

Two-mode fiber based directional torsion sensor with intensity modulation and 0° turning point

In this paper, we report a novel in-fiber Mach-Zehnder interferometer based directional torsion sensor, in which a section of two-mode fiber is sandwiched between two single mode fibers by over core-offset splicing technique. The variety of fringe visibility demonstrates the strong dependence on offset and fiber length. For the first time the near zero visibility at 0° rotating state is obtained by fine offset-modulation. The experimental results show that, with 0° turning point, the counter-clockwise to the clockwise direction can be recognized by the reversal from peak to dip of fringes. Moreover, a competitive sensitivity of 21.485 dB/(rad/cm) is gained with high linearity and low-temperature crosstalk in the range from -40 rad/m to 40 rad/m. Without any pre-twisting, our fiber torsion sensor is small size, ease of fabrication, cost efficiency and very potential in the applications of industrial and artificial intelligence.

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.

High-speed and high-precision torsion sensor based on polarization-induced microwave photonic phase shift measurement

We propose and experimentally demonstrate a technique to achieve high-speed and high-precision torsion sensing based on polarization-induced microwave photonic phase shift measurement. In the proposed system, a section of single-mode fiber (SMF) is used as a sensor that is incorporated into a microwave photonic phase shifter (MPPS). The MPPS consists of a laser source, a polarization modulator (PolM), an optical bandpass filter (BPF), a SMF, a polarizer, and a photodetector (PD). The phase shift of the microwave signal is a linear function of the twist angle of the SMF. An IQ detection module is utilized to measure the microwave phase shift. The performance of the proposed torsion sensor is experimentally evaluated. A measurement range of over 180° and a sensing accuracy of 0.03° are realized at an ultrahigh speed of 1 MHz, with low strain and temperature cross-sensitivity.

Strain independent twist sensor based on uneven platinum coated hollow core fiber structure

Optical fiber based twist sensors usually suffer from high cross sensitivity to strain. Here we report a strain independent twist sensor based on an uneven platinum coated hollow core fiber (HCF) structure. The sensor is fabricated by splicing a section of ~4.5-mm long HCF between two standard single mode fibers, followed by a sputter-coating of a very thin layer of platinum on both sides of the HCF surface. Experimental results demonstrate that twist angles can be measured by monitoring the strength change of transmission spectral dip. The sensor's cross sensitivity to strain is investigated before and after coating with platinum. It is found that by coating a platinum layer of ~9 nm on the HCF surface, the sensor's cross sensitivity to strain is significantly decreased with over two orders of magnitude less than that of the uncoated sensor sample. The lowest strain sensitivity of ~2.32×10^-5 dB/𝜇𝛆 has been experimentally achieved, which is to the best of our knowledge, the lowest cross sensitivity to strain reported to date for optical fiber sensors based on intensity modulation. In addition, the proposed sensor is capable of simultaneous measurement of strain and twist angle by monitoring the wavelength shift and dip strength variation of a single spectral dip. In the experiment, strain and twist angle sensitivities of 0.61 pm/𝜇𝛆 and 0.10 dB/° have been achieved. Moreover, the proposed sensor offers advantages of ease of fabrication, miniature size, and a good repeatability of measurement.

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).

Intensity-demodulated torsion sensor based on thin-core polarization-maintaining fiber

An intensity-demodulated torsion sensor is designed and realized, which consists of a polarization ring as the sensing part and a section of thin-core polarization-maintaining fiber as the demodulation part. An intensity map of a sinusoidal change can be obtained at some specific wavelengths, and the experimental results correspond to the theoretical analysis well. The maximum sensitivity is about 0.29 dB/deg at the wavelength of 1584.6 nm, and the minimum sensitivity is about 0.10 dB/deg at the wavelength of 1510.2 nm. Meanwhile, the temperature characteristic is measured in the experiment. More broadly, the proposed structure can be used in an integrated smart device for loose-screw detection in devices in aeronautics and astronautics.

Highly sensitive torsion sensor with femtosecond laser-induced low birefringence single-mode fiber based Sagnac interferometer

A highly sensitive optical fiber torsion sensor with femtosecond laser-induced low birefringence SMF-based Sagnac interferometer (SI) is proposed and experimentally demonstrated in this paper. A straight-line waveguide positioned horizontally with respect to the fiber core is inscribed by the femtosecond laser in the cladding of the SMF, which leads to the asymmetry stress distribution in the SMF, and then gives rise to the low birefringence in the SMF. Compared with most of the previous reported SI based torsion sensors, there is no splicing joint in the femtosecond laser-induced low birefringence SMF-based SI, which lowers the transmission loss and makes the SI based torsion sensor more robust simultaneously. The experiment result shows that the proposed torsion sensor exhibits a torsion sensitivity of up to 3.2562 nm/degree, with the high torsion resolution of 0.003 degree. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low, to -0.000055 degree/°C and 0.000013 degree/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, theoretical analysis are carried out to compare with the experimental results to demonstrate the feasibility and good consistency.