Fabin Shen

Simultaneous measurement of refractive index and temperature based on a reflection-mode long-period grating and an intrinsic Fabry-Perot interferometer sensor.

We have demonstrated a single fiber probe for simultaneous measurement of external refractive index and temperature based on two interferometers: a reflection-mode long-period grating (LPG) for refractometry and an intrinsic Fabry-Perot interferometer (IFPI) for temperature measurement. Since the output signal of the combined structure is a superposition of signals produced by both sensors, which have different spatial frequencies, the original signals needed for the index and temperature measurements can be separated and recovered through digital filters.

Sapphire-fiber-based white-light interferometric sensor for high-temperature measurements.

We present a fiber-optic temperature sensor with a single-crystal sapphire fiber as the light guide and a sapphire wafer as the sensing element. Temperature is determined by measurement of the thermal dependence of the wafers optical thickness by use of white-light interferometry. We applied digital signal processing techniques to analyze the sensors spectrum. A prototype sensor was tested to 1600 degrees C and demonstrated excellent reproducibility. An accuracy of +/- 0.2% full scale was obtained. The sensor is simple, small, and flexible, with potential advantages of batch fabrication and easy calibration.

Signal-processing algorithm for white-light optical fiber extrinsic Fabry–Perot interferometric sensors

We present a novel signal-processing algorithm for single-mode optical fiber extrinsic Fabry-Perot interferometric sensors that can achieve both high-resolution, absolute measurement of the cavity length and a large dynamic measurement range simultaneously. The algorithm is based on an accurate model of the characteristics of a fiber-optic sensor that takes into account the phase shift that is due to the coupling of light reflected at the second surface to the lead-in fiber end.

Multiplexed Fiber Fabry–Pérot Interferometer Sensors Based on Ultrashort Bragg Gratings

This letter reports a fiber Fabry-Peacuterot interferometer sensor formed by two closely spaced ultrashort uniform Bragg gratings within fiber core. The sensor has insertion loss as low as 0.002 dB which enables a considerable increase in the sensors multiplexing capability. Fifty sensors are experimentally multiplexed along a single fiber using a frequency-division-multiplexing method. The potential of multiplexing more than 500 sensors is theoretically expected. The sensors application in temperature and strain measurement is also investigated

UV-induced intrinsic Fabry-Perot interferometric fiber sensors

In this paper, we present a novel point-wise laser writing method that utilizes a focused ultraviolet (UV) laser beam and metallic masks to write local Fresnel reflectors and intrinsic Fabry-Perot interferometric (IFPI) sensors in photosensitive fibers. These UV-induced IFPI sensors have features of low reflectance and low power loss and have the potential to be densely multiplexed. We also present a sweeping laser based measurement system that measures the interference spectra and estimates the optical path distances (OPD) of IFPI sensors. We also demonstrated IFPI sensors for temperature, strain and pressure measurement. Laboratory test results show that these UV-induced IFPI sensors can have a resolution of 0.1°C for temperature measurement and 0.5 micro-strain for strain measurement, and can be used in a temperature environment as high as 600°C.

Experimental investigation of optical waveguide-based multigas sensing

In this letter, the results of an experimental investigation for simultaneous multiple gas detection utilizing an optical fiber sensor are described. The optical fiber system used for these experiments contained a section of hollow tubing and this allowed a combination of gas molecular absorption with optical waveguide technology. A fiber-optic sensing structure was designed and demonstrated which can monitor multiple gases simultaneously. The experimental results demonstrate the characteristic absorptions in the optical spectra corresponding to the narrow molecular absorption lines of the gases tested, which included acetylene and carbon monoxide.

Novel Fabry-Perot fiber optic sensor with multiple applications

A novel Intrinsic Fabry-Perot fiber-optic sensor is presented in this paper. The sensors were made through two simple steps: wet chemical etch and fusion splice. Micro air-gaps were generated inside the fibers and functioned as reflective mirrors. This procedure not only provides a simple and cost effective technology for fabricating intrinsic Fabry-Perot Interferometric (IFPI) fiber sensors, but also provides two possible IFPI structures. Both of the fiber cavity between the air-gaps or the air-gap and cleaved fiber end can be used as sensing elements. With these two structures, this sensor can be used to measure the temperature, strain, pressure, refractive index of chemicals and the thin film thickness by itself. Multi-point measurements can also be achieved by multiplexing. Furthermore, it also can be multiplexed with other sensors such as Long Period Gratings (LPG) to provide compensations for other perturbation sensing. Theoretical and experimental studies of two sensor structures are described. Experimental results show that high resolution and high sensitivity can be obtained with appropriate signal processing.

Measurement of the frequency response of a diaphragm-based pressure sensor by use of a pulsed excimer laser

We present a novel method for measuring the frequency response of a diaphragm-based optical fiber Fabry-Perot interferometric pressure sensor. The impulse response of the sensor to the radiation pressure generated by an excimer laser pulse is measured. The Fourier transform of the impulse response yields the frequency response of the pressure sensor. Experimental results show that it is a convenient and efficient method for measurement of the frequency response of diaphragm-based pressure sensors.

Frequency-Division-Multiplexed Fabry-Perot Interferometric Fiber Sensors for Temperature Monitoring in a Selective Catalytic Reduction Unit

We present the quasi-distributed temperature measurement results in a selective catalytic reduction unit of a power plant by using a frequency-division-multiplexing optical fiber measurement system with eight intrinsic Fabry-Perot interferometric fiber sensors along a single fiber. The sensor was constructed by splicing a section of multimode fiber between single mode fibers. A high resolution swept laser interrogator was used to measure the spectrogram of the reflected light from the sensors, which contains multiple frequency components in wave number domain corresponding to sensors with different cavity lengths. The temperatures were measured by estimating the optical path length of each Fabry-Perot interferometer. Field test results show that the proposed technology can potentially be used in applications of multi-point high temperature sensing.

UV-induced intrinsic Fabry-Perot interferometric sensors and their multiplexing for temperature and strain sensing

We present UV-induced intrinsic Fabry-Perot interferometric (IFPI) fiber sensors and a frequency-division-multiplexing (FDM) scheme for quasi-distributed temperature and strain sensing. We present a spectrum-based measurement system with a swept laser source to measure the fringe patterns of IFPI sensors serially arranged along a single fiber. The FDM scheme is based on the multiplexing of sub-carrier frequencies generated by the frequency-modulation of a continuouswave light source. IFPI sensors with different optical path differences (OPD) will have different sub-carrier frequencies. We use band pass filters to select individual frequency component and use frequency-estimation based signal processing algorithms to determine the OPD of each sensor. Experimental results for multiplexed temperature and strain sensing are demonstrated. The performance of the multiplexing system is discussed.