Liyu Zhang

Bending Vector Sensor Based on a Sector-Shaped Long-Period Grating

A novel optical fiber grating vector bending sensor head is proposed. The asymmetric characteristics are embedded in the sensor head by tilting the grating planes at increasing angles, as moving away from the center grid of the structure. A high-frequency-CO2-laser is used to fabricate the sensor head. Due to the characteristics of the asymmetric structure, the bending sensitivity for the curvature range from -2 to 0 m-1 reaches 8.53 nm/ m-1, and for the curvature range from 0 to 2 m-1 reaches 2.82 nm/ m-1, respectively.

Design for a Single-Polarization Photonic Crystal Fiber Wavelength Splitter Based on Hybrid-Surface Plasmon Resonance

A novel single-polarization photonic crystal fiber wavelength splitter based on hybrid-surface plasmon resonance is proposed. A full-vector finite-element method is applied to analyze the guiding properties. Numerical simulations show that the proposed splitter, which is only several hundred microns in length, gives single polarization in the 1.31-μm and 1.55-μm bands. The loss of the unwanted polarized mode is 102.6 and 245.0 dB/cm in the two aforementioned communication windows, respectively, and the corresponding insertion loss is as low as 3.5 and 1.7 dB/cm, respectively. Moreover, the dependence of the bandwidth on the fiber length is given, and according to that function, the bandwidth can reach 40 nm (1.31-μm band) and 100 nm (1.55-μm band) when the fiber length is up to 1 mm. Additionally, the tolerances for a realistic fabrication are analyzed. In the last part, we discuss other methods to deal with an anticrossing phenomenon in detail.

Simultaneous strain and temperature measurement by cascading few-mode fiber and single-mode fiber long-period fiber gratings

A hybrid optical fiber structure of two cascading long-period fiber gratings (LPFGs), respectively, inscribed on a segment of few-mode fiber (FMF) and single-mode fiber (SMF), is proposed and experimentally demonstrated. This structure could be used for simultaneous measurement of strain and temperature. The FMF-LPFG exhibits an opposite temperature response and higher strain sensitivity compared to that of the SMF-LPFG, which can improve the measurement resolutions. Experimentally, the strain and temperature sensitivities of the proposed sensor are -2.9  pm/με and -17.6  pm/°C, respectively, for the FMF-LPFG; for the SMF-LPFG, these are -1.47  pm/με and 46.4  pm/°C, respectively. The maximum errors are ±7.98  με and ±0.54°C for strain and temperature, respectively.

CO 2 -Laser-Induced Long Period Fiber Gratings in Few Mode Fibers

A novel long period fiber grating (LPFG) is inscribed in few mode fibers by CO2-laser-irradiation method. The experiment results and theoretical calculation demonstrate that the LPFGs are originated from the strong coupling between the fundamental core mode LP01 and the high-order core mode LP11. The resonant wavelength is found to shift toward the lower wavelength with temperature or force increasing, which is quite different from LPFGs fabricated in common single mode fibers. The gratings are quite sensitive to strain and slightly insensitive to temperature. Such structures are easy to be fabricate and can be promising candidates as strain sensors.

A Fiber Bending Vector Sensor Based on M–Z Interferometer Exploiting Two Hump-Shaped Tapers

A novel, compact, and simple in-line Mach-Zehnder interferometer (MZI) formed by cascading two hump-shaped tapers in a segment of single-mode fiber is proposed and fabricated. This optical fiber device has recognizable features for different bend directions while high sensitivities for ambient temperature and axial force. In the range of 0-1.2 m-1, the sensitivities of the sensor with the length of 13.4 mm are 10.224 and -4.973 nm/m-1 for opposite bending directions 0° and 180°, respectively. The responses for temperature and force of the MZI are also investigated in experiment. The results show that the shift of the dip wavelength is linearly proportional to the change of temperature or force. The sensitivities of temperature and force are 0.049 nm/°C in the range of 20°C-80°C and -3.643 nm/N in the range of 0-0.98 N.

In-fiber torsion sensor based on dual polarized Mach-Zehnder interference

This paper presents a novel optical fiber torsion sensor based on dual polarized Mach-Zehnder interference (DPMZI). Unlike the conventional fiber sensor, the proposed sensor is composed of a sensor part and a demodulator. The demodulator is made by a bared single mode fiber (SMF) loop, and the sensor part is a segment of a coated SMF placed before the loop. A mathematical model is proposed based on DPMZI mechanism and from the model when the sensor part is twisted, the E-field rotational angle will bring a quasi-linear impact on the resonance dip wavelength in their matched detecting range. A proof-of-concept experiment was performed to verify the theoretical prediction. From the experimental data, a sensitivity of -0.3703, -1.00962, and -0.59881 nm•m/rad is achieved with the determining range of 12.0936, 7.6959, and 10.4444 rad/m respectively. The sensor which is composed only of the SMF has the advantages of low insertion loss (~-2dB), healthy structure, low manufacture cost, and easy assembly and application.

Fiber torsion sensor based on a twist taper in polarization-maintaining fiber.

A novel optical fiber torsion sensor head is proposed. A section of polarization-maintaining fiber (PMF) is spliced between single mode fiber (SMF), and a twist taper is fabricated by a commercial electric-arc fusion splicer in the middle of the PMF. The asymmetric characteristics are obtained by the twist taper so that a fiber torsion sensor with directional discrimination is fabricated. Due to the characteristics of the asymmetric structure, the torsion sensitivity for the twist rate from 0 rad/m to -8 rad/m reaches 2.392 nm/rad·m-1, and for the twist rate from 0 rad/m to 8 rad/m reaches 1.071 nm/rad·m-1 respectively.

Mach–Zehnder Interferometer Based on Interference of Selective High-Order Core Modes

We report a novel Mach-Zehnder interferometer based on interference of selective high-order core modes in few mode fibers (FMFs). The proposed interferometer is composed of a section of FMF sandwiched between single-mode fibers with small lateral core-offsets. The theoretical and experimental results indicate that the interference patterns are coupled between core modes LP21 and LP02. Because LP21 and LP02 both reside in the fiber core, few of the guided light can propagate outside the fiber through the evanescent wave, which makes the sensor insensitive to surrounding refractive index (RI). The experimental results show that the proposed structure has a high strain sensing sensitivity of 3.35 pm/μu within the range of 0-1000 μu at the wavelength of 1513 nm. RI insensitivity makes the device convenient for strain measurement in humid environment.

Real time and simultaneous measurement of displacement and temperature using fiber loop with polymer coating and fiber Bragg grating

An all-fiber sensor scheme for real time and simultaneous displacement and temperature measurement is presented and demonstrated. The sensor head is formed by cascading a fiber loop with polymer coating with a fiber Bragg grating. The compatibility of the two components is fully utilized. A sensor resolution of 0.14314 V/μm in displacement and 0.00795 nm/°C in temperature are experimentally achieved within a displacement range of 0-50 μm and a temperature range of 20 °C-75 °C, respectively. The fiber loop with the protection of polymer coating is mechanically reliable, which means the sensor head also suits measuring dynamic displacement. A 500 Hz mechanical micro-vibration is successfully measured by the proposed sensor experimentally. In the last part, we perform a test making the sensor reach its maximum deformation and find the surviving sensor still possesses the same responsiveness as before.

Polarization Rotator Based on Hybrid Plasmonic Photonic Crystal Fiber

A photonic crystal fiber polarization rotator (PR) is proposed based on a novel mechanism-mode interference by two hybrid waveguide modes, rather than adjusting optical property in the light-guided part. The proposed PR is achieved by introducing two defects and an ancillary Au nanowire into the background lattice, and under the optimized parameters, the PR can offer almost a 100% polarization conversion ratio in the wavelength of 1.55-μm band (~100-nm bandwidth), compact length ~162.7 μm, low modal insertion loss of ~1.2 dB, and high coupling efficiency over 0.92 for both TE and TM mode based on the numerical simulation result of the full-vector finite-element method.