Ciming Zhou

Different from usually-used bulk magnetostrictive materials, magnetostrictive TbDyFe thin films were firstly proposed as sensing materials for fiber-optic magnetic field sensing characterization. By magnetron sputtering process, TbDyFe thin films were deposited on etched side circle of a fiber Bragg Grating (FBG) as sensing element. There exists more than 45pm change of FBG wavelength when magnet field increase up to 50 mT. The response to magnetic field is reversible, and could be applicable for magnetic and current sensing.

An integrated fiber Michelson interferometer (MI) based on an asymmetrical twin-core fiber (TCF) cascaded with a side-hole fiber (SHF) for multiparameter sensing is proposed and demonstrated. The asymmetrical TCF consists of a centric core and an eccentric core with same refractive index. The centric core of the TCF is aligned with the core of the SHF and the eccentric core is aligned with one air hole of the SHF. Because the eccentric core of the TCF is off the axis of the fiber, a bend or twist will cause a variation of the optical path difference of theMI, which makes theMI structure be suitable for twist and vector curvature measurements. Experimental results show that the bending sensitivities of the sensor are -6.968 and 6.978 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the bending directions of 0° and 180° in the curvature ranges of 0-10.708 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively, and the twist sensitivity of the sensor is 0.639 nm/(rad/m) in the twist ranges of 0-7.44 rad/m. The structure is also sensitive to strain and temperature, and the strain and temperature sensitivities of the interferometer are 1.36 pm/με and 10.37 pm/°C, respectively.

We report on current theoretical and experimental results of hydroacoustic sensing array based on ultra-weak fiber Bragg gratings, using a modified phase generated carrier (PGC) demodulation method with a self-referencing signal. The self-referencing signal is obtained by a sensor isolated from acoustic signals and other environmental disturbances. We report improvements over the conventional PGC methods. Using our demodulation method and with nonsensitized bare fiber (reference sensor <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) keeping separate from water, experiment testing demonstrates a minimum detectable hydroacoustic pressure of 2239 μPa/√Hz. The properties of our demodulation method are also compared with those of the conventional PGC algorithms. Both simulation and experiments indicate that our demodulation method is immune to the drifts of modulation depth <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> and of carrier frequency, and the detectable frequency range can be increased by five times compared to using conventional PGC methods with the same carrier frequency. The sensor array exhibits a particularly large response in the very low frequency region, which is of great importance for underwater seismic detection and submarine applications.

We demonstrate interrogation of a large-capacity sensor array with nearly identical weak fiber Bragg gratings (FBGs) based on frequency-shifted interferometry (FSI). In contrast to time-division multiplexing, FSI uses continuous-wave light and therefore requires no pulse modulation or high-speed detection/acquisition. FSI utilizes a frequency shifter in the Sagnac interferometer to encode sensor location information into the relative phase between the clock-wise and counter-clockwise propagating lightwaves. Sixty-five weak FBGs with reflectivities in the range of -31 ~-34 dB and with near identical peak reflection wavelengths around 1555 nm at room temperature were interrogated simultaneously. Temperature sensing was conducted and the average measurement accuracy of the peak wavelengths was ± 3.9 pm, corresponding to a temperature resolution of ± 0.4 °C. Our theoretical analysis taking into account of detector noise, fiber loss, and sensor cross-talk noise shows that there exists an optimal reflectivity that maximizes multiplexing capacity. The multiplexing capacity can reach 3000 with the corresponding sensing range of 30 km, when the peak reflectivity of each grating is -40 dB, the sensor separation 10 m and the source power 14 mW. Experimental results and theoretical analysis reveal that FSI has distinct cost and speed advantages in multiplexing large-scale FBG networks.