Jianghai Wo

Refractive index sensor using microfiber-based Mach–Zehnder interferometer

A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159  μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.

Twist sensor based on axial strain insensitive distributed Bragg reflector fiber laser

A novel fiber-optic twist sensor based on a dual-polarization distributed Bragg reflector (DBR) fiber grating laser is proposed and experimentally demonstrated. By beating the signal between the two polarizations of the laser which operates at 1543.154 nm, a signal of 30.78 MHz in frequency domain is observed. The twist will change the fiber birefringence, and resulting in the beat frequency variation between the two polarization modes from the fiber laser. The result shows the beat frequency shifts as a Sinc function curve with the twist angle and both the measuring curve period and twist sensitivity depend on the twist length of the laser cavity. A high twist sensitivity of 6.68 MHz/rad has been obtained at the twist length of 17.5 cm. Moreover, the sensor is insensitive to the environmental temperature, as well as strain along the fiber axis with ultralow beat frequency coefficients, making temperature and axial strain compensation unnecessary.

Microfiber Fabry–Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor

We propose a novel fiber Fabry-Perot interferometer (FPI) that incorporates a length of microfiber as its cavity and two fiber Bragg gratings (FBGs) as reflectors. The microfiber FPI is simply fabricated by flame-heated taper-drawing the central spot of an FBG into a section of microfiber. Ambient refractive index (RI) influences the effective index of microfiber, and thus the free spectrum range of the microfiber FPI, resulting in RI sensing. A dual-wavelength fiber laser based on the microfiber FPI is constructed, enabling radio frequency interrogation with high resolution. RI sensitivity of 911 MHz/RIU is experimentally demonstrated for microfiber FPI with equivalent diameter of 1.455 μm. Simulation results indicate that the sensitivity can be further enhanced by reducing the diameter of the microfiber.

Hybrid TDM/WDM-Based Fiber-Optic Sensor Network for Perimeter Intrusion Detection

A distributed fiber-optic sensor system is proposed and demonstrated for long-distance intrusion-detection, which employs the hybrid time/wavelength division multiplexing architecture. The sensing elements are single-mode telecommunication fiber which can be hanged on the fence and hedge or buried along the monitored perimeter. The pulsed light generated by the superluminescent diode was filtered by the filter which has multichannel (m=6), and then amplified by erbium-doped fiber amplifiers. A 1×(n+1) (n=20) splitter of which every port has a fiber delay coil except the first port splits the amplified light. The fiber delay coils have different lengths, which generate different delay time and produce n time zones. By utilizing the m channel dense wave-length division multiplexing modules, every fiber sensing unit (OSU)-based unbalance Mach-Zehnder Sagnac interferometer technology occupy a time zone and a wavelength. By utilizing 20 time zones and 6 wavelengths, the system contains up to 120 OSUs, of which the distributed sensing distance is from 0 to 500 m. So, the whole sensing distance of this system could reach 60 km. The system has been demonstrated to stably run over six months with the false alarm rate of less than 4%.

Simultaneous wavelength and frequency encoded microstructure based quasi-distributed temperature sensor

A novel microstructure based temperature sensor system using hybrid wavelength-division-multiplexing /frequency-division-multiplexing (WDM/FDM) is proposed. The sensing unit is a specially designed microstructure sensor both frequency and wavelength encoded, as well as low insertion loss which makes it have the potential to be densely multiplexed along one fiber. Moreover, the microstructure can be simply fabricated by UV light irradiation on commercial single-mode fiber. Assisted with appropriate demodulation algorithm, the temperature distribution along the fiber can be calculated accurately. In theory, more than 1000 sensors can be multiplexed on one fiber. We experimentally demonstrated the feasibility of the scheme through building a sensor system with 9 microstructures multiplexing and with temperature resolution of 0.4°C.

Microfiber Fabry–Perot Interferometer for Dual-Parameter Sensing

We propose and demonstrate a microfiber Fabry–Perot interferometer (MFPI) fabricated by taper-drawing microfiber at the center of a uniform fiber Bragg grating (FBG). The MFPI employing the two separated sections of FBG as reflectors and a length of microfiber as its cavity is derived. Theoretic study shows that the reflection spectrum of such MFPI is consisted of two parts – interference fringes induced by multi-beam interference and reflection spectrum envelope induced by FBGs. Temperature affects both interference fringes and reflection wavelength of FBGs while ambient refractive index (RI) only influences the interference fringes, i.e., MFPI has different response to temperature and RI. Therefore, MFPI for simultaneous sensing of RI and temperature is experimentally demonstrated by tracking a reflection peak of interference fringes and the Bragg wavelength of the FBGs, which are respectively assisted by frequency domain processing and Gaussian fitting of the optical spectrum. Consequently, wavelength measurement resolution of 0.5 pm is realized.

Highly sensitive liquid-level sensor based on dual-wavelength double-ring fiber laser assisted by beat frequency interrogation

A highly sensitive liquid-level sensor based on dual-wavelength single-longitudinal-mode fiber laser is proposed and demonstrated. The laser is formed by exploiting two parallel arranged phase-shift fiber Bragg gratings (ps-FBGs), acting as ultra-narrow bandwidth filters, into a double-ring resonators. By beating the dual-wavelength lasing output, a stable microwave signal with frequency stability better than 5 MHz is obtained. The generated beat frequency varies with the change of dual-wavelength spacing. Based on this characteristic, with one ps-FBG serving as the sensing element and the other one acting as the reference element, a highly sensitive liquid level sensor is realized by monitoring the beat frequency shift of the laser. The sensor head is directly bonded to a float which can transfer buoyancy into axial strain on the fiber without introducing other elastic elements. The experimental results show that an ultra-high liquid-level sensitivity of 2.12 × 10(7) MHz/m within the measurement range of 1.5 mm is achieved. The sensor presents multiple merits including ultra-high sensitivity, thermal insensitive, good reliability and stability.

Fiber Sensor Based on Interferometer and Bragg Grating for Multiparameter Detection

An integrated in-fiber Mach-Zehnder interferometer (MZI) and fiber Bragg grating (FBG) sensor that can simultaneously detect localized temperature, strain, and ultrasonics is proposed and demonstrated. Using a 60-mm single-mode fiber with a 12-mm FBG spliced to two 3-mm multimode fibers and lead-in and lead-out single-mode fibers, the sensor can discriminate temperature and strain from the responses of the MZI and the FBG to the two static parameters. Ultrasonic signals are directly recorded as high-frequency voltage vibration by demodulation of Bragg wavelength shift using a corresponding sensing system. The overall system shows high practical potential for use in nondestructive testing.

Wideband Microfiber Fabry–Pérot Filter and Its Application to Multiwavelength Fiber Ring Laser

A microfiber Fabry-Pérot (MFP) filter consisting of two microfiber Sagnac loop mirrors as the reflectors and a section of microfiber as the cavity is proposed and fabricated. Owing to the high coupling efficiency induced by the large evanescent field of the microfiber, a broadband comb spectrum with high extinction ratio and flat amplitude can be obtained. The MFP with the extinction ratio of 15 dB as well as the free spectrum range of 0.18 nm is fabricated by bending and twisting a microfiber tapered from a single mode fiber. Consequently, the MFP is applied to an erbium-doped fiber ring laser as the wavelength filter. Assisted by a section of highly nonlinear fiber to suppress the mode competition, 42-wavelength lasing oscillations are achieved at room temperature.

Noninvasive respiration movement sensor based on distributed Bragg reflector fiber laser with beat frequency interrogation

Abstract. A distributed Bragg reflector fiber laser-based respiration movement monitoring system has been proposed and experimentally demonstrated. To fabricate the sensing element for respiration monitoring, a fixture that consists of a plastic plate, a section of elastic textile is employed to experience and transfer the belly expansion induced pressure onto the cross-section of the laser cavity. By tracing the change of the beat signal that generates between two polarization lasing modes, the information of the respiration movement can be extracted in real time. Experimental studies have demonstrated that the system is able to detect both respiration waveform and rate simultaneously. Moreover, the recorded results show that the different gestures as well as the physiology conditions can be distinguished by monitoring the amplitude and period change of the waveform. It is anticipated that the proposed fiber laser-based sensor would be highly suitable for respiratory monitoring.