Xue-gang Li

High Sensitive Modal Interferometer for Temperature and Refractive Index Measurement

A novel sensitivity-enhanced optical fiber sensor has been presented and demonstrated to measure liquid temperature and refractive index (RI) based on the reflective single-mode-multimode-single-mode fiber structure. The sensing part is packaged in a segment of capillary tube by hot glue. Due to the high coefficient of thermal expansion of hot glue, the increasing temperature bends the fiber in the tube. The measurement is achieved via monitoring the spectral shift of the modal interference between the core and the cladding modes in the ended single-mode fiber, which is well carved to provide a mirror interface. The Fresnel reflection occurred in the interface is utilized to measure liquid RI and temperature. The highest sensitivity of -92.6 pm/°C and -67.9 dB/RIU are achieved, respectively. This structure has many advantages, such as high sensitivity, expectation for simultaneous measurement of liquid temperature and RI, convenient operation, and low cost of fabrication.

Temperature-Insensitive Optical Fiber Curvature Sensor Based on SMF-MMF-TCSMF-MMF-SMF Structure

A new optical fiber curvature sensor dominated by three-core single mode fiber (TCSMF) is proposed and demonstrated. The sensing structure is formed by cascading multimode fiber (MMF), TCSMF and MMF together, namely, the SMF-MMF-TCSMF-MMF-SMF structure. Three core modes in the TCSMF interfere with each other and then form the interference spectrum. The interference phase difference is changed due to the elastic-optic effect as bending deformation inflicted on the fiber. Thus the spectrum can be shifted accordingly. The two short segments of MMF can significantly improve the coupling efficiency between the fundamental mode and three core modes, and get high fringe visibility. Experiments show that the highest curvature sensitivity of -28.29 nm/m-1 is achieved in the range of 2.79-3.24 m-1. Moreover, this sensor possesses the temperature-insensitive characteristic on account of the similar thermo-optic coefficient among three cores.

Simultaneous Measurement of RI and Temperature Based on a Composite Interferometer

A novel double-parameter sensor based on a composite interference structure has been designed and implemented in this letter. It can be regarded as a cascade of the conventional single-mode fiber (SMF)-multimode fiber-photonic crystal fiber-SMF structure, which forms a reflected Fabry-Perot interferometer and a transmitted Mach-Zehnder interferometer at the same time. In addition, a simultaneous measurement of temperature and refractive index has been achieved by monitoring the correlative moves of reflected spectrum and transmitted spectrum. The fact that these two spectra do not interfere with each other makes it easier to obtain a demodulation method that does not involve complex fast Fourier transform and matrix computation. The sensitivities of temperature and refractive index can reach up to 27.5 pm/°C and 108 nm/RIU, respectively.

Small Curvature Sensor Based on Butterfly-Shaped Mach–Zehnder Interferometer

A novel and small curvature sensor based on the butterfly-shaped Mach–Zehnder interferometer is proposed and experimentally demonstrated. The sensing element is a tapered hollow-core fiber sandwiched between two single-mode fibers. The fusion-collapsed region around the first fusion interface excites the high-order modes, and the butterfly-shaped structure couples the high-order modes back into the core and interferes with the fundamental mode in the second fusion-collapsed interface. Simulation of the butterfly-shaped structure is carried out by the beam propagation method to determine an optimized size of sensing element. The experimental results show that the light intensity variation of the interference spectrum is almost linearly proportional to the change of curvature, and the curvature sensitivity and resolution of the proposed sensor can be up to −10.9041 dB/m−1 and 0.000917 m−1, respectively, in the range from 0.387 to 1.285 m−1. The proposed curvature sensor is compact in size, high sensitive, and inexpensive. The excellent electromagnetic interference resistance and fabrication simplicity make the device an attractive candidate for curvature measurement in harsh environments.

Measurement of RI and Temperature Using Composite Interferometer With Hollow-Core Fiber and Photonic Crystal Fiber

A novel, highly sensitive double-parameter sensor based on the composite interference structure has been designed and implemented in this paper. It can be regarded as a cascade of the traditional single-mode fiber (SMF)-hollow-core fiber (HCF)-photonic crystal fiber (PCF)-SMF structure, which forms a reflected Fabry-Perot interferometer and a transmitted Mach-Zehnder interferometer at the same time. The HCF is filled with ethanol, which is sensitive to temperature. Then the simultaneous measurement of the temperature and the refractive index (RI) can be achieved by monitoring the reflected spectrum and transmitted spectrum. The fact that these two spectrums do not interfere with each other makes it easier to obtain a demodulation method for simultaneous temperature and RI measurement that does not involve complex fast Fourier transform and matrix computation. The sensitivities of the temperature and the RI can reach up to 0.454 nm/°C and 178.7 nm/RIU, respectively.

High sensitivity all-fiber Sagnac interferometer temperature sensor using a selective ethanol-filled photonic crystal fiber

ABSTRACT An all-fiber Sagnac temperature sensor based on 3 dB tapered coupling and ethanol selective-filled photonic crystal fiber was demonstrated theoretically and experimentally. The ethanol selectivity-filled photonic crystal fiber has noncircular symmetry and thus exhibits birefringence. The ethanol selective-filled photonic crystal fiber, acting as the sensing element, was inserted in a Sagnac loop interferometer to measure temperature. The output spectra of Sagnac interferometer applied different temperatures were measured and analyzed. Experimental results have shown the temperature sensitivity of the Sagnac interferometer is 1.65 nm/°C in the range of 25–33°C.

Measurement of Magnetic Field and Temperature Based on Fiber-Optic Composite Interferometer

A novel, highly sensitive dual-parameter sensor based on a composite interference structure has been designed and implemented in this paper. The sensor was composed of a Fabry–Pérot interferometer, where the microcavity was filled by magnetic fluid (MF) and a Mach–Zehnder interferometer, where the photonic crystal fiber was filled with ethanol. The refractive index of the MF and ethanol would be changed with the increase of magnetic field and temperature. Hence, simultaneous measurement of temperature and refractive index can be achieved by monitoring the reflected spectrum and transmitted spectrum. The sensitivities of MF and ethanol can reach up to 0.033 nm/Gs and 0.236 nm/°C, respectively.

Applications of Modal Interferences in Optical Fiber Sensors Based on Mismatch Methods

Modal interference is a major topic in optical fiber sensing technology that has been applied to measure temperature, refractive index, curvature, displacement, strain, humidity, pH, and other parameters. In this article, basic sensing principles for the development of modal interferometers are introduced based on core-cladding and core modes. The structures, principles, and applications of the sensors are comprehensively reviewed and potential and future prospects are discussed.

In-Fiber Mach–Zehnder Interferometer Based on Up-Taper Fiber Structure With Er3+ Doped Fiber Ring Laser

A robust optical fiber modal Mach-Zehnder (MZ) interferometer is proposed, which is embedded in a fiber ring laser system as a tunable filter to improve the detection limit (DL) due to its narrow full width half maximum (FWHM) and high signal-to-noise radio (SNR). The sensing element is composed of two cascaded up-taper joints fabricated by shoving fusion. After filtering, the laser spectrum possesses the sensing characteristics that the interferometer possessed. Temperature and refractive index (RI) are investigated as measurands in experiments and the response sensitivities are 92.1 pm/°C and 44.8 nm/RIU, respectively. Accordingly, the DLs for these two measurands are calculated as 0.19 °C and 3.86 × 10-4 (RIU). Moreover, the axis strain response is demonstrated to be much lower than many modal interferometers even under the help of fiber ring laser. Thus, the property contributes to the resistance of the outer force and disturbance. Compared to the simple sensing device, the proposed fiber ring laser sensor significantly improves the DL, which is one of the most crucial parameters. It is the first time to combine the fiber ring laser device with the up-taper MZ interferometer as far as we known. It is a feasible balanced way to improve the sensing capability without the expense of mechanical strength of sensing element.