Search

In this study, a fine gauge length distributed temperature sensor based on adaptive morphological processing (AMP) is proposed and experimentally demonstrated. During the data processing in optical frequency-domain reflectometry (OFDR), a reduction in gauge length leads to a decrease in the number of data points participating in the cross correlation calculation. This reduction in sample size negatively impacts the correlation quality, subsequently giving rise to increased demodulation errors, which are detrimental to the accuracy of the measurements. In the data processing flow of the proposed method, the correlation results of the reference and measured data within each gauge length are arranged as a results array along the sensing distance. Based on the spatial continuity and local similarity of cross correlation results for temperature sensing, cruciform and linear structural elements are chosen to perform AMP of the array, suppressing bad points and false peaks under fine gauge length. In experiments, the temperature gradient information from 43 °C to 55 °C is reconstructed free of outliers under the gauge length of 2 mm located at the end of 40-m-long sensing fiber. The mean absolute error (MAE) and standard deviation (STD) of the temperature measurement system using AMP with a gauge length of 2 mm has an approximate error level to conventional method with 6-mm gauge length, proving that the gauge length of the system is improved by a factor of three. The experimental results demonstrate that the proposed method provides a new solution idea for realizing distributed temperature measurement with fine gauge length based on OFDR.

In this study, a distributed multi-channel fiber-optic sensor for simultaneous measurement of relative humidity (RH) and temperature with finer gauge length based on spatial-domain time-delayed multiplexing is proposed and experimentally demonstrated. Simultaneous acquisition of multi-channel signals can be realized by reconfiguring the system layout in a spatial-domain time-delayed multiplexing manner by re-routing the Rayleigh backscattered signals for split-channel processing using only extra delay fibers and couplers instead of any optical switches extending the acquisition time or acquisition unit increasing the system cost. Sensing units comprised of copper (Cu)-coated fiber and polyimide (PI)-coated fiber are used to extract the changes of RH and temperature in the environment simultaneously thanks to the distinct sensitivities on RH and temperature of the two fibers. Experimental results show that the sensing system is able to realize multi-channel simultaneous measurement of RH and temperature with gauge length up to 10 mm. Besides, the system is proved to have excellent stability and reliability in high humidity environments. This novel and simple sensing technique for multi-channel dual-parameter simultaneous measurement will provide new ideas for development of economical, reliable, and practical RH and temperature sensing network.

In this study, a high spatial resolution optical frequency domain reflectometry (OFDR) based on independent component analysis (ICA) algorithm is proposed and experimentally demonstrated. In the proposed sensing system, two-dimensional (2D) images with wavelength shift information induced by applied strains as a function of fiber position are constructed by utilizing the data arrays obtained after cross-correlation processing of the reference signal and measurement signals of each sensing fiber segment. The ICA algorithm, as an effective 2D image denoising technique, is applied to the constructed 2D images to remove random noise and improve the sensing accuracy of the system so as to realize long-range distributed strain measurement with high spatial resolution. Compared with traditional 2D image denoising methods including the Gaussian filtering (GF) method and the wavelet denoising (WD) method, ICA method makes full use of the independence of image source information and noise information, which is able to effectively suppress the intensification of noise without compromising the source information. With no modification on the OFDR hardware system, strain gradient information is successfully extracted over an effective sensing distance of 75 m with a spatial resolution up to 2 mm by using the ICA method. The calculated mean strain measurement error for the ICA method is 5.45 μϵ, which is significantly improved compared to the error of 72.73 μϵ when no image denoising method is applied and reduced by approximately half compared to the errors when traditional GF method and WD method are used. The mean standard deviations of the measured strain gradient along the sensing fiber length for the proposed method, the GF method and the WD method are reduced by 93.42%, 76.99%, and 84.56%, respectively, compared to the raw data without any denoising processing, showing excellent smoothness of the recovered strain profiles. The signal-to-noise ratio (SNR) for the proposed method is improved by 23.17 dB, which is better than 14.8 dB of GF and 19.1 dB of WD. The experimental results show that the proposed method provides a new solution for OFDR system on achieving long-distance distributed strain sensing with high spatial resolution.

In this study, a wide strain measurement range optical frequency-domain reflectometry (OFDR) system based on self-correcting 2-D cross correlation is proposed and experimentally demonstrated. Large strain events can lead to spatial mismatch caused by overstretching of the fiber, resulting in inaccurate demodulation of the cross correlation spectrum during conventional OFDR data processing. The degradation of signal similarity due to spatial mismatch can be mitigated by creating a 2-D array of reference signals (RSs) in the frequency domain with different positional deviations and performing a 2-D cross correlation operation with the measurement signal (MS) in the frequency domain. In addition, based on the obvious trend difference between the main peak and the rest of the 2-D cross correlation result with positional deviation points, a self-correcting dictionary is constructed based on the standard deviation, which further improves the demodulation accuracy of the spectrum shift. In experiments, the strain gradient information from 1000 to $8000 \, \mu \varepsilon $ is reconstructed free of outliers with a spatial resolution of 3 mm located at the end of 50-m-long sensing fiber. The mean absolute error (MAE) is reduced from 14.12 to 6.12 pm, and the standard deviation is reduced from 13.09 to 4.56 pm compared to the conventional method. This novel and simple strain measurement range extension technique will provide new ideas for the development of economical, reliable, and practical large strain measurement systems.