[Research on the supersensitive detecting technology of backward Raman scattering signal in optic fiber].

Key Laboratory of Instrumentation Science and Dynamic Measurement, Engineering Technology Research Center of Shanxi Province for Photoelectric Information and Instrument, North University of China, Taiyuan 030051, China.
Guang pu xue yu guang pu fen xi = Guang pu (Impact Factor: 0.27). 06/2009; 29(5):1300-3. DOI: 10.3964/j.issn.1000-0593(2009)05-1300-04
Source: PubMed

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: 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.
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