Perry Ping Shum

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 7159u2009u2009μm/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.

Highly sensitive refractive index sensor based on cascaded microfiber knots with Vernier effect

We propose and experimentally demonstrate a refractive index (RI) sensor based on cascaded microfiber knot resonators (CMKRs) with Vernier effect. Deriving from high proportional evanescent field of microfiber and spectrum magnification function of Vernier effect, the RI sensor shows high sensitivity as well as high detection resolution. By using the method named Drawing-Knotting-Assembling (DKA), a compact CMKRs is fabricated for experimental demonstration. With the assistance of Lorentz fitting algorithm on the transmission spectrum, sensitivity of 6523nm/RIU and detection resolution up to 1.533 × 10(-7)RIU are obtained in the experiment which show good agreement with the numerical simulation. The proposed all-fiber RI sensor with high sensitivity, compact size and low cost can be widely used for chemical and biological detection, as well as the electronic/magnetic field measurement.

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.

Experimental Demonstration of Multipoint Temperature Warning Sensor Using a Multichannel Matched Fiber Bragg Grating

A novel time-division-multiplexing fiber Bragg grating (FBG) sensor for multipoint temperature warning sensor is proposed and demonstrated. A multiwavelength pulsed laser based on a multichannel matched FBG is employed. The sensor array consists of multiple uniform FBGs at different positions and with different nominal wavelengths. When the temperature exceeds the threshold at a certain position, the light at the corresponding time slot and wavelength can be detected. The sensor provides a simple and flexible solution to locate the abnormal temperature increase with different tolerable thresholds at different locations.

Relative phase noise induced impairment in M-ary phase- shift-keying coherent optical communication system using distributed fiber Raman amplifier

We show for the first time, to the best of our knowledge, that, in a coherent communication system that employs a phase-shift-keying signal and Raman amplification, besides the pump relative intensity noise (RIN) transfer to the amplitude, the signals phase will also be affected by pump RIN through the pump-signal cross-phase modulation. Although the average pump power induced linear phase change can be compensated for by the phase-correction algorithm, a relative phase noise (RPN) parameter has been found to characterize pump RIN induced stochastic phase noise. This extra phase noise brings non-negligible system impairments in terms of the Q-factor penalty. The calculation shows that copumping leads to much more stringent requirements to pump RIN, and relatively larger fiber dispersion helps to suppress the RPN induced impairment. A higher-order phase-shift keying (PSK) signal is less tolerant to noise than a lower-order PSK.

Improved location algorithm for multiple intrusions in distributed Sagnac fiber sensing system

An improved algorithm named twice-FFT for multi-point intrusion location in distributed Sagnac sensing system is proposed and demonstrated. To find the null-frequencies more accurately and efficiently, a second FFT is applied to the frequency spectrum of the phase signal caused by intrusion. After Gaussian fitting and searching the peak response frequency in the twice-FFT curve, the intrusion position could be calculated out stably. Meanwhile, the twice-FFT algorithm could solve the problem of multi-point intrusion location. Based on the experiment with twice-FFT algorithm, the location error less than 100m for single intrusion is achieved at any position along the total length of 41km, and the locating ability for two or three intrusions occurring simultaneously is also demonstrated.

Comb Filter-Based Fiber-Optic Methane Sensor System With Mitigation of Cross Gas Sensitivity

A remote fiber-optic methane gas sensor system is proposed and demonstrated with accurate gas concentration measurement and good mitigation of cross gas sensitivity. We use a polarization-maintaining photonic crystal fiber (PM-PCF)-based Sagnac loop filter to slice the spectrum of a broadband light source so as to precisely match several absorption lines of the methane gas within the near-infrared band. Meanwhile, a compact and cost-effective gas cell with multiple reflections is designed to enhance the interaction between the light beam and the methane gas to be detected, which also subsequently increase the system sensitivity. Due to the insensitive temperature dependence of the PM-PCF-based comb filter, we can obtain gas concentration measurement with a sensitivity of ~410 ppm. Moreover, by intentionally pumping the acetylene gas into the gas cell during the methane gas concentration measurement, the power variation caused by the interfering gas with 100% concentration is only equals to 0.7% of the power variation induced by the 100% concentration methane gas. Thus, effective mitigation of cross gas sensitivity is experimentally verified. The proposed fiber-optic methane gas sensor system is verified with low cost, compact size, potential capability of multipoint detection, and high sensitivity.

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.