Research on the Supersensitive Detecting Technology of Backward Raman Scattering Signal in Optic Fiber
ABSTRACT Unlike the Brillouin scattering, the anti-Stokes Raman scattering i n optic fiber is unrelated with the strain, but isonly the function of the absolute temperature. The frequency shift caused by Raman scattering is about 13.95 Thz. So the Raman scattering is easier to be picked up than Brillouin scattering. It has certain advantage while being used as the signal of the distributed optic-fiber temperature sensor. But it is weaker than the Brillouin scattering, the peak photocurrent produced in APD is of the order of nA, near or even lower than the noise current of the APD. The N/S after being photo-electric transformed is usually lower than 1. The means of Fourier transform and wavelet transform is not effective in dealing with such kind of signal. Through analyzing the feature of the anti-Stokes Raman scattering signal and using the wavelet, the anti-Stokes Raman scattering signal after being cumulated & averaged was picked up. The supersensitive detecting below the "noise current" of the APD was carried out with the senstivity: 0.104 nA x K(-1), lower than the "noise current" of the APD 2 classes.
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ABSTRACT: Noise in the optical fiber sensing system, produced in laser, transmission, demodulation and environment, reduces the Signal-to-Noise Ratio (SNR) and the measuring accuracy of the whole system . In this paper, a statistical approach based on Kalman-filter is undertaken to removal noise of the measured object real time, and then to improve the accuracy of the fiber sensing system. The temperature induced by fiber sensing is modeled as a discrete-time state variable by a Gauss-Markov random process with the Gaussian white and additive noise in the linear dynamic system. Based on Bayesian MAP Inference and minimum mean-square error criterion (MMSE), gain of the kalman-filter and the state error covariance can be regulated by Measurement Update equations to correct posteriori state estimate. Such recursive algorithm can finally get the optimum estimator of the state through time. The performances of the model and the algorithm are investigated in the DOFS temperature sensing system. Variance is used to evaluate its performance in noise removal. At the same time, the experimental results of the method proposed is compared with original measurement data analysis. The algorithm performs more improvement in accuracy of the fiber sensing system, and implements the real-time measurement.Proceedings of SPIE - The International Society for Optical Engineering 10/2012; DOI:10.1117/12.977936 · 0.20 Impact Factor
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ABSTRACT: Basic principles, development trends and applications status of distributed optical fiber Raman temperature sensor (DTS) are introduced. Performance parameters of DTS system include the sensing optical fiber length, temperature measurement uncertainty, spatial resolution and measurement time. These parameters have a certain correlation and it is difficult to improve them at the same time by single technology. So a variety of key techniques such as Raman amplification, pulse coding technique, Raman related dual-wavelength self-correction technique and embedding optical switching technique are researched to improve the performance of the DTS system. A 1 467 nm continuous laser is used as pump laser and the light source of DTS system (1 550 nm pulse laser) is amplified. When the length of sensing optical fiber is 50 km the Raman gain is about 17 dB. Raman gain can partially compensate the transmission loss of optical fiber, so that the sensing length can reach 50 km. In DTS system using pulse coding technique, pulse laser is coded by 211 bits loop encoder and correlation calculation is used to demodulate temperature. The encoded laser signal is related, whereas the noise is not relevant. So that signal-to-noise ratio (SNR) of DTS system can be improved significantly. The experiments are carried out in DTS system with single mode optical fiber and multimode optical fiber respectively. Temperature measurement uncertainty can all reach 1 degrees C. In DTS system using Raman related dual-wavelength self-correction technique, the wavelength difference of the two light sources must be one Raman frequency shift in optical fiber. For example, wavelength of the main laser is 1 550 nm and wavelength of the second laser must be 1 450 nm. Spatial resolution of DTS system is improved to 2 m by using dual-wavelength self-correction technique. Optical switch is embedded in DTS system, so that the temperature measurement channel multiply extended and the total length of the sensing optical fiber effectively extended. Optical fiber sensor network is composed.Guang pu xue yu guang pu fen xi = Guang pu 04/2013; 33(4):865-71. DOI:10.3964/j.issn.1000-0593(2013)04-0865-07 · 0.27 Impact Factor