Huiyong Guo

A time- and wavelength-division multiplexing sensor network with ultra-weak fiber Bragg gratings

A time- and wavelength-division multiplexing sensor network based on ultra-weak fiber Bragg gratings (FBGs) was proposed. The low insertion loss and the high multiplexing capability of the proposed sensor network were investigated through both theoretical analysis and experimental study. The demodulation system, which consists of two semiconductor optical amplifiers and one high-speed charge-coupled device module, was constructed to interrogate 2000 serial ultra-weak FBGs with peak reflectivity ranging from -47 dB to -51 dB and a spatial resolution of 2 m along an optical fiber. The distinct advantages of the proposed sensor network make it an excellent candidate for the large-scale sensing network.

Large-capacity multiplexing of near-identical weak fiber Bragg gratings using frequency-shifted interferometry

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.

Large WDM FBG Sensor Network Based on Frequency-Shifted Interferometry

We combine wavelength-division multiplexing (WDM) and frequency shifted interferometry (FSI) to interrogate a large-scale ultra-weak fiber Bragg grating (FBG) array. Based on Sagnac interference, FSI can location resolve sensors without using pulses or fast detection, therefore considerably lowers the system cost. By combining FSI with WDM, higher spatial resolution can be achieved. We demonstrated simultaneous interrogation of 363 FBG sensors, grouped into 121 identical units of 3 FBGs of different central wavelengths and spaced two meters apart. Based on the performance of the 363 grating system, we show the potential for interrogating 3207 sensors with good signal-to-noise ratio. Stability test and temperature sensing were carried out, and the obtained temperature resolution was ± 0.4 ° C. The results indicate the proposed scheme can greatly enhance the multiplexing capacity and meet the requirements of large-scale optical fiber networks.

Ultra-weak FBG and its refractive index distribution in the drawing optical fiber

For the online writing of ultra-weak fiber Bragg gratings (FBGs) in the drawing optical fibers, the effects of the intensity profile, pulse fluctuation and pulse width of the excimer laser, as well as the transverse and longitudinal vibrations of the optical fiber have been investigated. Firstly, using Lorentz-Loren equation, Gladstone-Dale mixing rule and continuity equation, we have derived the refractive index (RI) fluctuation along the optical fiber and the RI distribution in the FBG, they are linear with the gradient of longitudinal vibration velocity. Then, we have prepared huge amounts of ultra-weak FBGs in the non-moving optical fiber and obtained their reflection spectra, the measured reflection spectra shows that the intensity profile and pulse fluctuation of the excimer laser, as well as the transverse vibration of the optical fiber are little responsible for the inconsistency of ultra-weak FBGs. Finally, the effect of the longitudinal vibration of the optical fiber on the inconsistency of ultra-weak FBGs has been discussed, and the vibration equations of the drawing optical fiber are given in the appendix.

Analysis of the Spectrum Distortions of Weak Fiber Bragg Gratings Fabricated In-Line on a Draw Tower by the Phase Mask Technique

Identical weak optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflection spectra of the in-line-fabricated weak FBGs and compared them with the simulated spectra of weak FBGs. By comparing the experimental and theoretical weak FBG spectra, it is found that the different kinds of spectrum distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp. This analysis is helpful to improve the FBG array fabricating process by phase mask in-line on a draw tower. And we got good FBG arrays which have good central wavelength consistency, good reflectivity consistency and minor spectral distortion.

Optic fiber hydrogen sensor based on high-low reflectivity Bragg gratings and WO3-Pd-Pt multilayer films

A novel optic fiber hydrogen sensor is proposed in this paper. Two Bragg gratings with different reflectivity were written in single mode fiber with phase mask method by 248 nm excimer laser. The end-face of singe mode fiber was deposited with WO3-Pd-Pt multilayer films as sensing element. The peak intensity of low reflectivity FBG is employed for hydrogen characterization, while that of high reflectivity FBG is used as reference. The experimental results show the hydrogen sensor still has good repeatability when the optic intensity in the fiber is only 1/3 of its initial value. The hydrogen sensor has great potential in measurement of hydrogen concentration.

Ultra-highly sensitive gas pressure sensor based on dual side-hole fiber interferometers with Vernier effect

We have presented and demonstrated a fiber optic gas pressure sensor with ultra-high sensitivity based on Vernier effect. The sensor is composed of two integrated parallel Mach-Zehnder interferometers (MZIs) which are fabricated by fusion splicing a short section of dual side-hole fiber (DSHF) in between two short pieces of multimode fibers (MMFs). Femtosecond laser is applied for cutting off part of the MMF and drilling openings on one air hole of the DSHF to achieve magnified gas pressure measurement by Vernier effect. Experimental results show that the gas pressure sensitivity can be enhanced to about -60 nm/MPa in the range of 0-0.8 MPa. In addition, the structure possesses a low temperature cross-sensitivity of about 0.55 KPa/°C. This presented sensor has practically value in gas pressure detection, environmental monitoring and other industrial applications.

High spatial resolution multiplexing of fiber Bragg gratings using single-arm frequency-shifted interferometry

We demonstrate a high spatial resolution multiplexing scheme for fiber Bragg grating (FBG) sensors based on single-arm frequency-shifted interferometry (SA-FSI). The SA-FSI system uses an incoherent broadband source, a slow detector, and an electro-optic modulator (EOM). By sweeping the frequency of EOM and taking the fast Fourier transform (FFT) of the interference signal, we resolved the locations of FBGs distributed both in parallel and in series along fiber links despite their reflection spectral overlap. Eighteen weak FBGs (∼5% reflectivity) separated by ∼0.1 m were clearly resolved experimentally, sweeping EOM modulation frequency in the range of 2–11 GHz.

Distributed acoustic sensing system based on continuous wide-band ultra-weak fiber Bragg grating array

A distributed acoustic sensing system (DAS) with low-coherence ASE and Michelson interferometer based on continuous width-band ultra-weak fiber Bragg grating (UW-FBG) array is proposed and experimentally demonstrated. The experimental result shows that the proposed system has better performance in detecting acoustic waves than the conventional hydrophone.

Theoretical calculations of crosstalk and time delay in identical FBG array in PM fiber

The spectral shadowing, multiple reflections and polarization crosstalk in an identical FBG array in PM fiber were theoretically studied. For a PM fiber FBG array (PMF-FBG array) with FBG peak reflectivity of -30 dB, grating spacing of 2 m and total fiber length of 2000 m, the reflection power of the PMF-FBG changed from -30.0 dB to -36.9 dB at the end of the array because of the spectral shadowing and the multiple reflections. Equations for the signal time delay of the PMF-FBG array were given. Higher birefringence in the PM fiber will result in a longer time delay.