Yangzi Zheng

Magnetic Field Sensing With Reflectivity Ratio Measurement of Fiber Bragg Grating

A simple magnetic field sensor by using an optical fiber Bragg grating (FBG) cascaded by a cleaved optical fiber end, which is surrounded with magnetic fluid (MF), is experimentally demonstrated. Magnetic field changes Fresnel reflectivity of the fiber end face through refractive index change of the MF, leading to a modulation to the reflection background in the reflection spectrum of the FBG. By comparing intensity difference between the FBG reflection peak, which is not sensitive to the external magnetic field, and the reflection background, magnetic field measurement is achieved successfully. High accuracy and stability can be realized because influence of power level fluctuation of the optical source on measurement results can be eliminated. Furthermore, temperature can be measured simultaneously from reflection wavelength shift of the FBG that is helpful to further compensate the effect of temperature on magnetic field measurement.

Magnetic field sensor with optical fiber bitaper-based interferometer coated by magnetic fluid

An optical fiber magnetic field sensor is demonstrated using a waist-enlarged fusion bitaper-based optical fiber Mach-Zehnder interferometer (MZI) coated by magnetic fluid (MF). Since refractive index (RI) of the MF is sensitive to magnetic field, differential effective RI of the sensing fiber and hence transmission spectrum of the MZI are modulated. In addition, structural pattern state of the nanoscaled magnetic particles in the MF changes with magnetic field strength, leading to variation in contrast of the interference pattern through scattering-induced loss of the cladding modes, which interfere with the core mode. By monitoring wavelength shift or intensity change of a transmission dip of the MF-coated MZI, magnetic field strength measurement is realized with sensitivity up to -24 pm/Oe or 0.085 dB/Oe for wavelength and intensity measurements, respectively.

Miniature temperature sensor with germania-core optical fiber

A miniature all-fiber temperature sensor is demonstrated by using a Michelson interferometer formed with a short length of Germania-core, silica-cladding optical fiber (Ge-fiber) fusion-spliced to a conventional single-mode fiber (SMF). Thanks to the large differential refractive index of the Ge-fiber sensing element, a reasonably small free spectral range (FSR) of 18.6 nm is achieved even with an as short as 0.9 mm Ge-fiber that may help us increase the measurement accuracy especially in point sensing applications and, at the same time, keep large measurement temperature range without overlapping reading problem. Experimental results show that high sensitivity of 89.0 pm/°C is achieved and the highest measurement temperature is up to 500°C.

Simultaneous Measurement of Tilt Angle and Temperature With Pendulum-Based Fiber Bragg Grating Sensor

A fiber optic tilt senor for simultaneous measurement of 2-D tilt angle and temperature is proposed and demonstrated experimentally. The sensing head consists of two fiber Bragg gratings (FBGs) attached on a specially designed columnar pendulum. Reflection peaks of the FBGs split and resonant wavelengths shift with tilt and temperature variations. A maximum tilt angle sensitivity of 0.074 nm/°, accuracy of 0.18°, and temperature sensitivity of 0.021 nm/°C have been achieved within the tilt angle range of 0°-40°.

Miniature pH Optical Fiber Sensor Based on Fabry–Perot Interferometer

A polyvinyl alcohol (PVA)/poly-acrylic acid (PAA) hydrogel coated fiber-optic intrinsic Fabry-Perot interferometer (IFPI) has been demonstrated as a pH sensor. The proposed Fabry-Perot pH interferometric sensor consists of a section of hollow-core photonic crystal fiber (HCPCF) sandwiched between the lead-in single mode fiber (SMF) and sensing SMF. The principle of the proposed sensor is based on the swelling effect of PVA/PAA hydrogel sensing film at the tip of sensing SMF, which induces optical path modulation when exposed to varying pH solutions. The pH sensor exhibits a relatively high sensitivity of 11 nm/pH ranging from the value of 4.1 to 6.9 with good repeatability and stability.

Fiber Optic Fabry–Perot Optofluidic Sensor With a Focused Ion Beam Ablated Microslot For Fast Refractive Index and Magnetic Field Measurement

A compact fiber-optic Fabry–Perot interferometric sensor with an embedded microfluidic channel for static magnetic field measurement is proposed and implemented. Fabrication of the Fabry–Perot structure involved cascading two silica capillary tubes with different inner diameters, followed by milling of a microslot directly on the sidewall of the large inner diameter silica capillary tube, by focused ion beam micromachining. The microslot, together with the hollow structure of the small inner diameter silica capillary tubes forms an open Fabry–Perot cavity to function as a microfluidic channel. The microslot was found to improve the response time of the Fabry–Perot sensor to changes in refractive index (RI) of the cavity. Magnetic fluid was introduced in the Fabry–Perot cavity, thereby, enabling implementation of an optofluidics system for magnetic field measurement. The sensing scheme is based on the magnetic field strength induced RI change of the magnetic fluid which in turn modulates the interference spectrum of the sensor. Wavelength shifts were monitored real time for the continuous measurement of magnetic field strength. The sensor calibration curves with respect to magnetic field were plotted and the sensor exhibits a relative high magnetic field sensitivity of ∼418.7 pm/Oe relative to the dip wavelength around 1590 nm.

Optical fiber Fabry-Perot interferometer with pH sensitive hydrogel film for hazardous gases sensing

An optical fiber Fabry-Perot interferometer (FPI) coated with polyvinyl alcohol/poly-acrylic acid (PVA/PAA) hydrogel film for toxic gases measurement has been developed. Splicing a short section of hollow core fiber between two single mode fibers forms the FPI. Dip-coated pH-sensitive PVA/PAA hydrogel film on the fiber end performs as a receptor for binding of volatile acids or ammonia, which makes the sensing film swelling or shrinking and results in the dip wavelength shift of the FPI. By demodulating the evolution of reflection spectrum for various concentrations of volatile acids, a sensitivity of 20.8 nm/ppm is achieved with uniform linearity.

Miniature pH sensor based on optical fiber Fabry-Perot interferometer

An optical fiber Fabry-Perot interferometer (FPI) coated with polyvinyl alcohol/poly-acrylic acid (PVA/PAA) hydrogel is proposed for pH value measurement. The FPI is formed by splicing a section of hollow-core photonic crystal fiber (HCPCF) between two standard single mode fibers. Its optical path different is modulated by the coated PVA/PAA hydrogel sensing film through swelling effect which depends on pH value of the tested solution. High sensitivity up to 11nm/pH is achieved within the pH value range between pH 4.1 and pH 6.9.

Compact temperature sensor by using a specialty fiber of Germania core and silica cladding

A miniature temperature sensor with sensitivity of 87.8 pm/0C is demonstrated by using a Michelson interferometer formed with a 0.7-mm specialty optical fiber with Germania core and silica cladding spliced to a single mode fiber.

Intensity-modulated refractometer with long period fiber grating cascaded by chirped fiber grating

An intensity-modulated refractometer is proposed and experimentally demonstrated by using a long period fiber grating (LPG) cascaded with a chirped-fiber Bragg grating (CFBG). The reflection wavelength band of the CFBG was properly selected to contain the most sensitive spectral part of the LPG. As a result, intensity of the reflected signal was modulated linearly by refractive index (RI) of surrounding liquid outside the LPG. RI measurement in a range from 1.33 to ~1.45 was realized with enhanced sensitivity up to 48.93 μW/R.I.U.