Research And Implement Of Method And Technology Of Distributed Optical Fiber Fire Warning Based On Raman Scatter

The conventional fire detectors such as smoke detector, dual wavelength fire detector and linear temperature sensitive cable etc. can only find the fire after it happened, and haven’t the ability to measure temperature before a fire happened. Because they are“points”, a fire detecting system is made of such“points”by netting them for a huge area fire detecting. Such fire detecting systems have many electric lines, so they are complex and have less ability to contest the electromagnetic interference because of the detector has electro circuit. Mean time between failures (MTBF) of the whole fire fighting system is inverse ratio to the number of the temperature sensors. As the range detected increasing, the MTBF will be decreased further and the reliability of the system will become worse.With a single sensing temperature optic-fiber to detect fires distributively in stead of many“point”fire detectors will significantly decrease the number of elements of a fire detecting system. The reliability of it will become more reliable and the MTBF of it become more longer.Implementing the distributed optic-fiber temperature sensing (DOFTS) is the base of realization of the distributed optic-fiber fire detecting (DOFFD). The physical parameters can be used in DOFTS are Raman scattering and Brillouin scattering. Because of the Brillouin scattering is related with the temperature & stress at the same time, it is difficult to distinguish which one causing the Brillouin scattering varied while the Raman scattering is only related with the temperature. So the Raman scattering is very suitable to be used as the parameter of the DOFTS.The Raman scattering of the quartz optic-fiber is very weak, less than one thousandth of Rayleigh scattering, and is about one milliomth of the input light power. Increasing the input light power can boost the Raman scattering, but the Brillouin scattering mirror effect and phenomenon of the fuse set the upper limit of the power. In addition, a fire detecting system should have fine space resolution, thus a narrow laser pulse probe should be used in a distributed optic-fiber fire detecting system. A narrow laser pulse has a little power, thus the level of the signal transformed from the Raman scattering light is almost at the same with the level of the optic-electric transforming devices’noise, and the S/N is less than zero dB. Effectively detecting and extracting Raman scattering signal is the main goal of a DOFTS.Through analyzing, we found the backward Raman scattering can be expressed by an attenuating e exponential signal multiplied by a pulse whose position and width are unknown. This signal has wider bandwidth, and the actually Raman signal mixed with great deal shot noise. The shot noise has the feature of random, and its spectrum has the feature yet, so the conventional method of de-noising based on the spectrum decomposing isn’t suitable for our research.For a signal with a great deal random noise, de-noising by multiple times averaging (MPA) is effectively measure. But for a DOFTS, ten thousands times average would be need if better de-noising effect need be gotten. This will spend long time to de-noise when the sensing range is very huge, it isn’t acceptable for a real-time fire detecting system which must has higher demand. After mathematically analyzing the backward Raman scattering, combining the MPA and wavelet together to de-noise we achieved super-sensitive detecting with two class less than the dark current of the avalanche photodiode (APD) at a little times average. The system precision is 0.06℃(1σ) in 4 s measurement time.Spatial resolution decides the shortest distance which can be distinguished by the DOFTS between two heat sources, is a key parameter decided by the width of the laser pulse injected into the optic-fiber. The large current, narrow pulse of Diode Laser driving technology is a difficult problem for implementing DOFTS. Using the reverse blocking current feature of the LD, adopting avalanche driving technique, ampere class current, nanosecond and single sine pulse LD driving has been implemented. The spatial resolution of 2.8 meter has been achieved with a 10 ns width laser pulse which has 1 W peak power.The temperature sensitivity of stokes Raman scattering and anti-stokes Raman scattering has also be researched. We got an optimized temperature demodulating expression. The temperature demodulated with the expression isn’t related with the power of the input laser pulse probe and the optic-fiber’attenuation. Thus makes the system immunizing from the fluctuation of light source and the micro-bend loss which are difficulty to overcome.The distributed optical fiber fire warning researched can be used widely in the temperature sensing and fire detecting fields such as mines, huge building, tunnel, electric power lines, electric cable bridge etc., and also can be used where the traditional temperature sensing systems are difficulty to be implemented such as the huge concreting process temperature monitor. The research can also be used to measure the cubic temperature field such as chemic retort or high temperature puddling with special optic-fiber lying. The de-noise method of MPA combined with wavelet and the ampere class current, nanosecond single sine pulse LD driving can also be used in laser rader systems.