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The working principle of distributed fiber optic temperature sensing fire detection system OTDR

With the increasing maturity of fiber optic manufacturing technology, fiber optic temperature sensing technology, which uses fiber optic as the transmission medium for signals and utilizes its advantages such as long service life, corrosion resistance, low optical signal attenuation, long-distance transmission, and no interference from surrounding magnetic fields, lightning strikes, etc., is gradually being applied to fire detection systems in subway tunnels or other areas of the subway. At present, fire detection systems based on fiber optic temperature sensing technology mainly include distributed fiber optic temperature sensing fire detection systems and fiber optic grating temperature sensing fire detection systems.

Temperature measurement principle of distributed fiber optic temperature sensing fire detection system

The working principle of the distributed fiber optic temperature sensing fire detection system is to use optical time-domain reflection technology (OTDR) to measure the backward Raman scattering light signals at various points when laser pulses are transmitted in the fiber optic; There are Stokes light and anti Stokes light in Raman scattering light signals, among which the intensity of anti Stokes light is sensitive to environmental temperature, which can be used to achieve temperature measurement and position determination of continuous spatial temperature fields, thereby obtaining the temperature field distribution along the fiber path; In order to eliminate the influence of light source fluctuations and other factors, the ratio of Stokes light intensity to anti Stokes light intensity (which is only related to temperature and not to the bending radius and incident light parameters of the optical fiber) is detected. Then, OTDR technology is used to achieve real-time monitoring of the spatial temperature field distribution, ultimately achieving distributed optical fiber temperature sensing fire monitoring and alarm.

Structure of Distributed Fiber Optic Temperature Sensing Fire Detection System

Based on OTDR technology and the principle of backward Raman scattering, a distributed fiber optic temperature sensing fire detection system has been constructed. When the laser pulse generated by the laser is transmitted in the fiber, it will generate Raman scattering signals

The anti Stokes light and Stokes light scattering signals propagate backward along the optical fiber, are divided into two paths for extraction by a wavelength division multiplexer, and then converted into electrical signals by a photoelectric converter After signal processing and demodulation, temperature information is extracted from it through computer processing. By setting a threshold for temperature alarm, the display, alarm, and fire linkage functions of the fire alarm system are achieved.

The distributed fiber optic temperature sensing fire detection system can provide alarms at any temperature point and continuous dynamic monitoring signals. The detection range of this fire detection system can reach several tens of kilometers, achieving distributed fiber optic monitoring of environmental temperature changes at various points along the road; Wide temperature measurement range, temperature resolution can reach 0.1 ℃, and temperature accuracy can reach ± 0.5 ℃; It can provide a continuous dynamic monitoring signal and can set multi-level temperature point alarms and temperature difference alarms.

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