Ai Zhou

Two-Axis Bending Sensor Based on Cascaded Eccentric Core Fiber Bragg Gratings

A two-axis bending sensor based on cascaded eccentric core fiber Bragg gratings (ECFBGs) is presented and demonstrated. The sensor is fabricated by independently writing two FBGs in two ECFs, which are spliced with orthogonal core-offset directions. Each ECFBG is most sensitive at the core-offset direction and insensitive at the perpendicular direction. When one ECFBG is in the state of most insensitive to bend, the other ECFBG must be in the most sensitive state. Therefore, the cascaded ECFBG structure can easily realize 2-D bending measurement. The sensors spectral responses to the curvature at different bending directions are experimentally investigated. The maximum curvature sensitivities of the two ECFBGs are 37 and 39 pm/m-1, respectively, within the curvature range from 0 to 8.18 m-1.

A phase-shifted long period fiber grating based on filament heating method for simultaneous measurement of strain and temperature

We proposed and fabricated a long period fiber grating (PS-LPG) for simultaneous measurement of strain and temperature. The is fabricated by inscribing micro-tapers on a pre-tapered fiber using the filament heating method. When the temperature or strain varies, the wavelengths of both transmission notches shift in the same direction, whereas the depths of the notches change in different directions. Therefore, the sensitivity of the PS-LPG can be improved by detecting the depth difference between the two notches, and simultaneous temperature and strain measurements can be achieved from the variation of the wavelength and the depth difference. Experimental results show that the wavelength sensitivities can reach a maximum of respectively −1.5 nm m −1 and for strain and temperature, and the depth sensitivities are as high as −4.6 dB m −1 and −0.06 dB °C−1, respectively.

Dynamic range beyond 100 dB for polarization mode coupling measurement based on white light interferometer.

This paper presents a method to improve the dynamic range of white light interferometer (WLI) based polarization mode coupling (PMC) measurement system beyond 100 dB. The limitation of interference beat noise is overcame by analyzing in detail the inherent noises that have impacts on the detection sensitivity. An improved PMC measurement system and method are proposed for testing ultra-high polarization extinction ratio (PER) of polarization-related devices. The method can improve dynamic range dramatically through eliminating interference beat noise and enhancing the tested interference intensity simultaneously, which are verified theoretically and experimentally. In addition, a Y-junction with ~80 dB PER of LiNbO3 chip corresponding to a weak signal is tested as an application example. The results demonstrate that the high PER interferogram can be identified clearly and steadily with standard deviation 0.9 dB (3σ) @ ~80 dB. This proposed method is highly beneficial in fabrication and evaluation for polarization devices with ultra-high PER.

Simultaneous measurement of temperature and bend by using an eccentric core fiber Bragg grating cascaded with a Fabry-Perot cavity

A simultaneous temperature and bend sensor based on an eccentric core fiber Bragg grating (ECFBG) cascaded with a Fabry-Perot cavity is presented and demonstrated. The hybrid structure is composed of a piece of hollow core fiber, which is sandwiched in between a piece of multimode fiber and an ECFBG. The curvature sensitivities of the ECFBG at the two opposite most sensitive directions are 52.2 pm/m⁻¹ and −51.7 pm/m⁻¹ respectively, and those of the FPI are 58.6pm/m⁻¹ and −61.4 pm/m⁻¹, respectively. The temperature sensitivities of the two parts are very different. The temperature sensitivity of the ECFBG is 10.3 pm/°c, and that of the FPI is as low as 0.7 pm/°c.

High Sensitive Directional Torsion Sensor Based on a Segmented Long-Period Fiber Grating

A segmented long period fiber grating (LPFG)-based torsion sensor with both high sensitivity and ability to discriminate torsion direction is proposed and experimentally investigated. The sensor is made up of three spatial orthogonal short LPFGs by using a CO2 laser notched method. Experimental results show that the torsion sensitivities can reach −0.54 and 0.33 dB/(rad/m) in dip depth and 0.30 and −0.14 nm/(rad/m) in resonance wavelength for clockwise and counterclockwise, respectively, in a single polarization state. Torsion rate and torsion direction can be measured simultaneously by the sensor. Moreover, the temperature sensitivity of the sensor has been determined as 0.067 nm/°C. Therefore, the proposed segmented-LPFG would have great potential applications in engineering and fiber sensors.

Ultra-highly sensitive gas pressure sensor based on dual side-hole fiber interferometers with Vernier effect

We have presented and demonstrated a fiber optic gas pressure sensor with ultra-high sensitivity based on Vernier effect. The sensor is composed of two integrated parallel Mach-Zehnder interferometers (MZIs) which are fabricated by fusion splicing a short section of dual side-hole fiber (DSHF) in between two short pieces of multimode fibers (MMFs). Femtosecond laser is applied for cutting off part of the MMF and drilling openings on one air hole of the DSHF to achieve magnified gas pressure measurement by Vernier effect. Experimental results show that the gas pressure sensitivity can be enhanced to about -60 nm/MPa in the range of 0-0.8 MPa. In addition, the structure possesses a low temperature cross-sensitivity of about 0.55 KPa/°C. This presented sensor has practically value in gas pressure detection, environmental monitoring and other industrial applications.

An in-fiber Mach-Zehnder interferometer based on dual side-hole fiber for highly sensitive measurement of curvature

An in-fiber Mach-Zehnder interferometer (MZI) based on dual side-hole fiber (DSHF) was demonstrated for highly sensitive measurement of curvature. The MZI-based bending sensor is fabricated by fusion splicing a piece of DSHF in between two standard single mode fibers (SMF) with cladding alignment. Due to the existence of the two air holes and the asymmetrical cross-section of the DSHF, the DSHF-based MZI is a core-cladding interferometer which is sensitive to directional bending. The bending characteristics are investigated experimentally within the curvature range of 0–8.172m−1. The bending sensitivities of the sensor are respectively 1.464 nm/m−1 and −1.394 nm/m−1 at their two opposite bending directions.

Simultaneous strain and directional bending sensor based on eccentric-core fiber Bragg grating

A fiber sensor based on cascaded eccentric-core fiber Bragg grating (ECFBG) and single mode fiber Bragg grating (SMFBG) is proposed and experimentally demonstrated to realize bending and axial strain measurement simultaneously. The ECFBG is sensitive to both bending and axial strain, and the SMFBG is insensitive to bend but sensitive to axial strain. The maximum bending sensitivities of the ECFBG are 36 pm/m⁻¹ and −37 pm/m⁻¹ at the bending direction of 0° and 180°, respectively. The strain sensitivities of the ECFBG and SMFBG are 0.76 pm/με and 0.72 pm/με, respectively.

High-order polarization mode crosstalk effect: a calibration scheme of white light-based optical coherence domain polarimetry

We propose a calibration scheme of the white light interferometer based optical coherence domain polarimetry (OCDP), which could be used to measure the ultra-weak polarization mode crosstalk (PMC) or the ultra-high polarization extinction ratio (PER) of different polarization optical devices. The calibration depends on the first and second order PMC effect of different polarization devices in series. The first and second PMCs between 0 and -90dB, established by five pieces of polarization maintaining fiber (PMF) and a Y-waveguide, is used to prove its feasibility.

Measurement for polarization mode dispersion of LiNbO3 integrated waveguide modulator used white light interferometry

We present a method to measure the polarization mode dispersion (PMD) of the LiNbO3 multifunctional integrated waveguide modulator (MFIWM) which is consist of a Y-waveguide, two extended polarization maintaining (PM) fibers, lead-in PM pigtail and lead-out PM pigtail. This method is based on an all-fiber time-domain scanning white light interferometer and utilizes fast Fourier transform (FFT) technology to obtain interferometric phase. The PMD of each part of MFIWM was measured and distinguished accurately. It’s demonstrated that, the PMD of Y-waveguide is 13.5 ps/nm/km@1555nm which is approximately 40~160 times of the PMD of PM fibers in the MFIWM under test.