Jiejun Zhang

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.

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.

A compact all fiber refractive index sensor based on modal interference

In this paper, we propose and experimentally demonstrate a compact all fiber refractometer based on fiber modal interference. Different from other published program mostly based on the interference between high order modes in the Multi-mode fiber (MMF), we use the interference between core and cladding modes in the Single-mode fiber (SMF). The sensing head can be simply fabricated by splicing a section of SMF between two MMFs. Due to the different cladding modes contribution into the interference, the transmission spectrum dips with specific interference order shifts differently with the changes of the environmental parameters. The sensor shows good response to the refractive index (RI) variation with the highest sensitivity of -96.85nm/RIU (refractive index unit).

Tunable dual-wavelength double-ring fiber laser and its application in highly sensitive temperature sensing

A novel tunable dual-wavelength double-ring fiber laser with single longitudinal mode operation is proposed and demonstrated. By beating the dual-wavelength lasing output at different temperature, a high sensitivity of 1.4750 GHz/K is achieved.

FBG-microfiber-FBG cascade Fabry-Perot interferometer for simultaneous measurement of temperature and refractive index

A microfiber Fabry-Perot interferometer (FPI) which employs two fiber Bragg Gratings (FBGs) as reflection mirrors and a short length microfiber as its cavity is proposed and fabricated. Theoretic study shows that the reflection spectrum of such microfiber FPI is consisted of two parts - interference fringes induced by FPI and reflection band induced by FBGs. Temperature affects both parts while ambient refractive index only influences the first part, i.e. microfiber FPI has different response to temperature and RI. Therefore, Dual-parameter measurement is experimentally demonstrated by tracking the FSR variation and the central wavelength shift of the reflection spectrum of microfiber FPI.

High sensitivity temperature sensor using microfiber based Mach-Zehnder interferometer

A high sensitive temperature sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. Temperature measurement is achieved by immerging a section of microfiber into the refractive index (RI) liquid with a high thermo-optic coefficient. A slight change of ambient temperature will lead to the enhanced variation of the liquid index. Due to the evanescent field of microfiber, microfiber effective refractive index will be changed, and subsequently the optical length. Thus, by measuring the free spectral range (FSR) of the MZI, the temperature sensor can achieve a high sensitivity of 6.44nm/°C at the temperature of 20.6°C.

Dual-polarization DBR fiber laser sensor for liquid-level measurement

A dual-polarization distributed Bragg reflector (DBR) fiber grating laser for liquid level measurement is proposed and experimentally demonstrated. The laser cavity is used as the sensing device. The liquid level is obtained by measuring the beat frequency generated between the two lasing polarizaion modes. The result shows the beat frequency shifts linearly with the water level and a sensitivity of 241 kHz/mm has been obtained. The sensor is simple, easy to fabricate and with high sensitivity to liquid level variation, which could be very helpful in practical applications.