Tunnel fires are not regarded as usual, however, they also can perform great damage to lives and properties if they take place. The process of fire detection inside the lengthy and curved tunnel is challenging. Nevertheless, fiber optic sensors based on fiber Bragg grating or FBG technology allows detecting tunnel fires and FBG sensors are considered to be a novel methodology that determines not only the presence but also locates the fire inside the tunnel.
To be more precise, FBG sensors are installed across the inner sides of the tunnel where they control it by the optical spectrum analyzer or wavelength division multiplexing sensor interrogator at the exit. “The change in the center wavelength from the original spectrum at the output denotes the temperature change (fire occurrence) inside the tunnel.” FBG fiber optic sensors offer output with more precision, herewith, fiber Bragg gratings can hold very high-temperature values.
It should be noted that it is possible to detect fires applying various ways, for instance, installing temperature sensors inside the tunnel or controlling the tunnel through the camera. Temperature sensors enable to sense of tunnel fires but their location presents difficulties. Moreover, such a temperature sensor as a thermistor can not stand very high temperatures (1000˚C) as well as there is a problem of self-healing.
FBG sensors, in their turn, provide a highly efficient process of sensing and locating. The thing is that fiber Bragg gratings in fiber optic sensors lead to a narrow range of wavelengths to shift and the rest of it to transmit through it. The center of the reflected wavelength is Bragg’s Wavelength. The features of FBG sensors allow measuring temperature or strain changes in the structures. Additionally, FBG fiber optic sensors have a greater temperature steadiness ability, more immune to electromagnetic interferences, longer lifetime, explosion safe and it is possible to be multiplexed.
Fiber Bragg grating is a short part of optical fibers that reflects a specific light wavelength and transmits all other wavelengths. FBGs operate as an optical notch filter. The operating principle is based on the Bragg grating patterns inside the fiber that perform the reflection. The design of fiber Bragg gratings includes holographic interference or a phase mask to undergo a short length of optical fiber to a periodic distribution of light intensity.
The developed FBG sensors have been already tested and demonstrated the following results: the sensitivity value of the fiber optic sensor achieves 20 pm/˚C. Also, it is possible to increase the accuracy of fire location inside the tunnel by increasing the number of FBG fiber optic sensors for the considered tunnel length of 4 km. Thus, FBG technology can help to perform operations very fast and can save many lives and properties.
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