Xinpu Zhang

Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer

An optical fiber temperature and torsion sensor has been proposed by employing the Lyot-Sagnac interferometer, which was composed by inserting two sections of high-birefringence (HiBi) fiber into the Sagnac loop. The two inserted sections of HiBi fiber have different functions; while one section acts as the temperature sensitive region, the other can be used as reference fiber. The temperature and twist sensor based on the proposed interferometer structure have been experimentally demonstrated. The experimental results show that the envelope of the output spectrum will shift with the temperature evolution. The temperature sensitivity is calculated to be −17.99 nm/°C, which is enlarged over 12 times compared to that of the single Sagnac interferometer. Additionally, the fringe visibility of the spectrum will change due to the fiber twist, and the test results reveal that the fringe visibility and twist angle perfectly conform to a Sine relationship over a 360° twist angle. Consequently, simultaneous torsion and temperature measurement could be realized by detecting the envelope shift and fringe visibility of the spectrum.

Novel birefringence interrogation for Sagnac loop interferometer sensor with unlimited linear measurement range

A novel demodulation method for Sagnac loop interferometer based sensor has been proposed and demonstrated, by unwrapping the phase changes with birefringence interrogation. A temperature sensor based on Sagnac loop interferometer has been used to verify the feasibility of the proposed method. Several tests with 40 °C temperature range have been accomplished with a great linearity of 0.9996 in full range. The proposed scheme is universal for all Sagnac loop interferometer based sensors and it has unlimited linear measurable range which overwhelming the conventional demodulation method with peak/dip tracing. Furthermore, the influence of the wavelength sampling interval and wavelength span on the demodulation error has been discussed in this work. The proposed interrogation method has a great significance for Sagnac loop interferometer sensor and it might greatly enhance the availability of this type of sensors in practical application.

High-Resolution Refractive Index Sensing With Dual-Wavelength Fiber Laser

A novel refractive index (RI) sensor has been demonstrated based on a simple dual-wavelength fiber ring laser by incorporating two fiber Bragg grating (FBG) filters. The grating filters are made of two FBGs with different cladding thicknesses. By inserting the filters into the fiber ring cavity, a stable dual-wavelength fiber laser can realize at room temperature. The cladding-etched FBG (thinned) acted as the RI sensing elements, while the normal one as temperature reference. As the RI increases, the laser wavelength corresponding to the thinned FBG moves toward longer wavelength, while that corresponding to the normal FBG is unchanged. Then, RI changes can be determined by measuring the separation of the two lasing wavelengths. Moreover, compared with the conventional thinned FBG-based sensors, the fiber laser sensor has a narrow bandwidth, which is beneficial to high precision sensing. The RI measurement resolution of proposed sensor is about 1.44 × 10-5 RIU.

Fiber Tip Thermometer

We present a novel fiber tip thermometer based on a compact fiber interferometer structure. The sensor tip is formed by splicing the end of two abreast conventional single-mode fibers (SMFs) together, one SMF as the lead-in fiber and the other as the lead-out fiber. Through controlling the splicing parameters, a series of sensor tips with different shapes were made. Due to fiber end enlargement induced by discharge, the coupling occurs between two parallel fiber cores. In addition, the light reflected by the fused end face can be coupled back the fiber core, where the core–core coupling light interfere with the reflected light. Therefore, the different optical paths of the lights form a modal interferometer. A sensor prototype is fabricated and temperature sensing is realized by measuring the wavelength shift of the resonance dips in the reflection spectrum. In the temperature range of 60 °C–300 °C, the corresponding temperature sensitivities are 11 and 10.93 pm/°C for the heating and cooling processes, respectively. Considering that the wavelength resolution of the optical spectrum analyzer is 20 pm, the resolution of the temperature measurement is estimated to be 1.8 °C. Because of its compactness, ease of fabrication, good sensitivity, easy connectivity to other in-fiber optical components, and low cost, this thermometer could find various applications in temperature sensing.

Annealing properties of fiber Bragg grating UV-inscribed in boron-germanium codoped fiber

In this work, we mainly focus on the investigation of the feasibility of production of high-temperature stable fiber Bragg grating (FBG) based on reduplicative alternate annealing and hydrogen loading. The experimental results also can demonstrate the significance of the presence of hydrogen to the thermal regeneration of FBGs. The gratings are characterized and variations are compared after each stage, including UV fabrication, annealing, and reduplicative hydrogen-preloaded annealing. In different stages, the spectral and annealing responses of FBG are, respectively, investigated, as temperature increases, the Bragg wavelength consistently shifts to longer wavelengths; nevertheless, the reflection variations are distinctly discrepant. After reduplicative alternate annealing and hydrogen loading, the thermal stability is tremendously improved, and a reborn, stable grating is formed.

The Longitudinal Force Measurement of CWR Tracks with Hetero-Cladding FBG Sensors: A Proof of Concept

A new method has been proposed to accurately determine longitudinal additional force in continuous welded rail (CWR) on bridges via hetero-cladding fiber Bragg grating (HC-FBG) sensors. The HC-FBG sensor consists of two FBGs written in the same type of fiber but with different cladding diameters. The HC-FBGs have the same temperature sensitivity but different strain sensitivity because of the different areas of the cross section. The differential strain coefficient is defined as the relative wavelength differences of two FBGs with the change of applied longitudinal force. In the verification experiment in the lab, the HC-FBGs were attached on a section of rail model of which the material property is the same as that of rail on line. The temperature and differential strain sensitivity were calibrated using a universal testing machine. As shown by the test results, the linearity between the relative wavelength difference and the longitudinal additional force is greater than 0.9999. The differential strain sensitivity is 4.85 × 10−6/N. Moreover, the relative wavelength difference is not affected by the temperature change. Compared to the theoretical results, the accumulated error is controlled within 5.0%.