In recent years, the mileage of highways and urban traffic tunnels in China has increased rapidly, bringing challenges to tunnel fire safety. The hazards caused by traffic tunnel accidents at home and abroad have aroused great attention to tunnel fire safety. Most fires in tunnels are open fires with thick smoke, with a heat release rate of several megawatts, and a fire can form within minutes. According to extensive tests (including road vehicles and metro trains), a fire usually fully forms in 8-10 minutes; for large freight trucks, about 20-30 minutes; when fuel leaks and forms a pit/pool, a fire fully forms within 1-3 minutes. Tests of "tunnel fire" cases show that after about 5 minutes, the temperature of a vehicle fire rises to about 200℃. Research results indicate that when a serious fire occurs in a tunnel, in the first 4 minutes, a temperature rise of more than 50℃ per minute can be detected in the hot air layer above the fire point. In addition, tunnel ventilation will produce specific effects. Tests generally show that with a longitudinal wind greater than 2m/s, the flame of a car or train drifts upward and the temperature at the top only reaches 60 degrees Celsius; and at a location 20 meters away from the fire, the top temperature will drop below 50 degrees Celsius.

Because the tunnel is long and narrow with usually low visibility, plus the "black hole" effect when entering, tunnels are accident-prone sections where various fires occur. Due to the "tubular" spatial structure of tunnels, even a very small fire can induce a big fire. Therefore, the policy of "prevention first, combining prevention with firefighting" should be implemented in tunnels, and effective fire prevention measures should be taken to prevent and reduce fire hazards. For temperature-measuring fire detectors, differential temperature alarm based on temperature rise gradient is the preferred alarm method for tunnel fire protection, followed by fixed-temperature warning and alarm. Among the existing fire detection schemes, due to safety, long-distance transmission, reliability and other reasons, none can well solve the fire detection problem of this industry. The fiber Bragg grating fire detector is a new generation of fiber optic fire detector in the world, with intrinsic safety and explosion proofing, strong anti-electromagnetic interference, good electrical insulation, lightning protection, high accuracy, light weight and small size, and the ability to conveniently use wavelength division multiplexing to connect multiple FBGs in one fiber for distributed measurement. Therefore, the FBG fire detector has received worldwide attention and is being widely applied. At present, there is no unified industry standard for traffic tunnel fire protection, and traditional temperature measurement technologies all have certain limitations. But no matter what technology is adopted, only timely and accurate fire detection and early warning can minimize fire losses — such fiber optic measurement and sensing technology is the fire alarm technology we really pursue.