Weijia Bao

Ultrahigh Sensitive Temperature Sensor Based on Fabry–Pérot Interference Assisted by a Graphene Diaphragm

A miniature Fabry-Pérot interferometer is proposed and experimentally demonstrated for temperature measurement. The cavity of the interferometer is a small air gap between the end-face of a single-mode fiber and an ultrathin graphene. A well-defined interference spectrum is obtained, and the dip wavelengths are employed to demonstrate the temperature change. Due to the smart intrinsic characteristics of the graphene film, the proposed device can measure the temperature up to 1008°C, and shows an ultrahigh sensitivity of 1.56 and 1.87 nm/°C at the temperature range of 500°C-510°C and 1000°C-1008°C, respectively, making it a good candidate for high-temperature monitoring in the harsh environment.

Fiber Bragg Grating Sensors for the Oil Industry

With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

Sensing Characteristics for a Fiber Bragg Grating Inscribed Over a Fiber Core and Cladding

Femtosecond laser exposure is used to fabricate a fiber Bragg grating (FBG) over a fiber core cladding on a short section of photon-sensitive thin-core fiber (TCF) using a side-illumination technique. The cladding modes are excited in the fusion region due to the core mismatch between the leading-in single-mode fiber and the TCF, and one resonant cladding mode and core mode are then transmitted downstream as separate wavelengths by the FBG. Furthermore, the transmission intensities for the two resonant dips could be controlled by changing the focal-line position of laser beam. Because the cladding modes have unique field shapes, they react to perturbations both inside and outside the fiber, which makes them good candidates for measuring fiber bending and ambient refractive index variation.

Orientation-Dependent Displacement Sensor Using an Inner Cladding Fiber Bragg Grating

An orientation-dependent displacement sensor based on grating inscription over a fiber core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents fiber bending dependence, together with a strong orientation dependence because of asymmetrical distribution of the “cladding” FBG along the fiber cross-section.

Highly sensitive fiber-optic accelerometer by grating inscription in specific core dip fiber

A highly sensitive fiber-optic accelerometer based on detecting the power output of resonances from the core dip is demonstrated. The sensing probe comprises a compact structure, hereby a short section of specific core (with a significant core dip) fiber stub containing a straight fiber Bragg grating is spliced to another single-mode fiber via a core self-alignment process. The femtosecond laser side-illumination technique was utilized to ensure that the grating inscription region is precisely positioned and compact in size. Two well-defined core resonances were achieved in reflection: one originates from the core dip and the other originates from fiber core. The key point is that only one of these two reflective resonances exhibits a high sensitivity to fiber bend (and vibration), whereas the other is immune to it. For low frequency (<10 Hz) and weak vibration excitation (<0.3 m/s2) measurement, the proposed sensor shows a much higher resolution (1.7 × 10−3 m/s2) by simply monitoring the total power output of the high-order core mode reflection. Moreover, the sensor simultaneously provides an inherent power reference to eliminate unwanted power fluctuations from the light source and transmission lines, thus providing a means of evaluating weak seismic wave at low frequency.

High-Temperature Properties of a Thin-Core Fiber MZI With an Induced Refractive Index Modification

A fiber-optic Mach-Zehnder interferometer is proposed and demonstrated experimentally. The device consists of a short piece of thin-core fiber (TCF) sandwiched between two standard single mode fibers. A microscope direct-writing technique with femtosecond laser exposure is utilized to induce a refractive index change near the interface between the core and cladding of the TCF. This added refractive index profile is helpful for the coupling of the core-to-cladding modes, resulting in a well-defined interference spectrum. The ultra-high temperature property of the proposed interferometer is demonstrated up to 900 °C with a sensitivity of 94.3 pm/°C over repeated measurements. And the repeated operations of thermal annealing are employed to improve the thermal stability and measurement performance of the interferometer.