Yanbiao Liao

Long-Period Grating Relative Humidity Sensor With Hydrogel Coating

The long-period grating (LPG) coated with the hydrogel was investigated for monitoring the relative humidity (RH) levels-the ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature. The experimental results showed that the resonant wavelength of the hydrogel-coated LPG was shifted when the RH varied from 38.9% to 100%, with a linearity of 99.5% and an accuracy of plusmn2.3% RH

Simultaneous measurement of down-hole high pressure and temperature with a bulk-modulus and FBG sensor

A free active element bulk-modulus-based fiber Bragg grating (FBG) sensor for down-hole pressure and temperature measurement is presented. With a differential measurement method and an isosceles triangle cantilever structure, problems of cross-sensitivity and chirped-signal in FBG sensors are solved. Simulation results indicate that the measurement sensitivity in measuring pressure and temperature is estimated to be /spl sim/8.5 pm/MPa in a range from standard atmosphere pressure to 100 MPa and /spl sim/27.5 pm//spl deg/C from 20 to 200/spl deg/C, respectively.

A novel wavelength detection technique for fiber Bragg grating sensors

This letter reports a minimum variance shift technique for wavelength detection in fiber Bragg grating (FBG) sensors. The technique has been demonstrated to offer high detection accuracy even when the spectrum of the FBG is in partial overlap with a neighboring FBG within a wavelength division multiplexed sensor array.

Differential FBG sensor for temperature-compensated high-pressure (or displacement) measurement

Abstract A differential fiber Bragg grating (FBG) sensor with a free active element bulk-modulus for high-pressure (or displacement) measurement is presented. Based on the differential measurement method and an isosceles triangle cantilever structure, problems of cross-sensitivity and chirped signal in FBG sensor are improved both theoretically and experimentally. Preliminary experiments indicate that temperature-compensated measurement results agree well with the theoretical analysis. Displacement measurement sensitivity is ∼1.75 pm / μm , and the estimated pressure measurement resolution can reach 0.27 MPa in case the wavelength shift measurement resolution is 0.01 nm .

Analysis and Optimization of the PGC Method in All Digital Demodulation Systems

In this paper, we analyze and optimize the PGC demodulation algorithm for optical interferometers in order to extend its usage in low-cost all digital schemes. In our low-cost system, we choose heterodyne interferometer and direct modulation on Distributed FeedBack (DFB) laser and optimize the algorithm, as well as some of the system parameters in order to have the best linearity and noise performance. We first analyze the influence of low-pass filters (LPF) in the PGC-DCM algorithm. A detailed theoretical model of LPF is deduced and reveals that a nonideal LPF is the major limit of linearity in digital systems. Experimental results show that the PGC-atan algorithm will have a 40-dB higher linearity than PGC-DCM one with a 60-order digital FIR filter. Then we analyze the influence of the intensity modulation in PGC-atan algorithm, the main source of the nonlinearity in PGC-atan algorithm, and propose a simple but effective modification. Experiment results verified that it can reduce the influence of intensity modulation by about 20 dB. Further, the impact of Light Intensity Noise (LIN) and circuit noise on the output noise level is analyzed. Equations are derived to calculate the Power Spectrum (PS) of output noise caused by LIN or circuit noise. Simulations show that the theoretical analysis is of high accuracy-less than 0.5 dB. With these equations, a new parameter-Noise Transfer Factor-is defined for better discription of the noise performance of PGC. The results show that the modulation depth C and the DC work point Phi = mean(phi(t)) have great impact on the output noise level. By changing C and Phi , the output noise may decrease as much as about 20 dB. Then three examples show how to choose the system parameter according to the characteristics of LIN or circuit noise. As well, the noise performance of PGC-DCM is analyzed. The result reveals a turning point at the output noise base, which worsens its noise performance at lower frequency. PGC-DCM has the same NTF at frequency higher than the turning frequency. A parameter Frequency Conversion Coefficient is introduce to calculate the turning point.

Cantilever optical vibrometer using fiber Bragg grating

A cantilever optical vibrometer using a fiber Bragg grating (FBG) is developed. The sensor head is made by attaching an FBG to a triangular beam to provide position-independent strain for the purpose of acceleration measurement. The vibrometer demonstrates a noise-limited acceleration resolution of 2.8×10 –4 √Hz with repeatability error of less than 2.4%.

Wavelength modulation technique for intra-cavity absorption gas sensor

Wavelength modulation spectroscopy by the use of a tunable optical filter was applied to an intra-cavity fiber laser gas sensor. The optimum pump power and the modulation amplitude that maximized the signal-to-noise ratio were found. A minimum detectable gas (C/sub 2/H/sub 2/) concentration of 1000 ppm was experimentally achieved.

In-fiber polarimeters based on hollow-core photonic bandgap fibers

In-fiber polarimeters or polarization mode interferometers (PMIs) are fabricated by cascading two CO2-laser-induced in-fiber polarizers along a piece of hollow-core photonic bandgap fiber. Since the two interfering beams are the orthogonal polarizations of the fundamental mode, which are tightly confined to the core and have much lower loss than higher order modes, the PMIs can have either short (e.g., a few millimeters) or long (tens of meters or longer) device length without significantly changing the fringe contrast and hence provide design flexibility for applications required different device lengths. As examples of potential applications, the PMIs have been experimentally demonstrated for wavelength-dependent group birefringence measurement; and for strain, temperature and torsion sensors. The PMI sensors are quite sensitive to strain but relatively insensitive to temperature as compared with fiber Bragg grating sensors. The PMIs function as good directional torsion sensors that can determine the rate and direction of twist at the same time.

Positioning Error Prediction Theory for Dual Mach–Zehnder Interferometric Vibration Sensor

The positioning mean square error (MSE) prediction theory for dual Mach-Zehnder interferometric vibration sensor is proposed. The cross-correlation algorithm is considered. By taking into account the general noise time delay model, the impact of each noise term in output interference signal to ultimate positioning MSE is to our knowledge the first time be analyzed. An equivalent signal to noise ratio term for general noise time delay model is derived. It shows that incoherent phase noise (coming from polarization fading effect) makes an equivalent contribution to positioning MSE as additive noise under assumptions. In practical prediction, cross-correlation coefficient between two channel signals can be used to estimate positioning MSE. Simulation as well as experimental results are presented to corroborate the theory.

Monitoring technology of salinity in water with optical fiber sensor

A novel optical fiber sensor used for remote monitoring of salinity in water is proposed. Based on the detection of beam deviation due to the refractive index changes of the salt water, salinity can be measured by a position-sensitive detector. The measurement principle is theoretically described. Preliminary prototype is set up, and experimental results indicate the feasibility of the method. Salinity measurement resolution can reach 0.012%o within the measurement range from distilled water to salt water with salinity of 50%o.