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Tunnel Fiber Optic Grating Temperature Sensor

Fiber and communications infrastructure knowledge brief Published 12 Sep 2021

Prepared for preliminary research by the ENSAA knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

Fiber optic grating sensors, particularly Fiber Bragg Gratings (FBG), enable accurate, long-distance, and real-time temperature monitoring in tunnels, supporting fire detection and structural health monitoring.

Overview of Fiber Optic Grating Sensors

Fiber optic grating sensors, such as FBG temperature sensors, operate by reflecting specific wavelengths of light that shift in response to temperature changes. This allows precise measurement of temperature along the fiber without electrical interference, making them ideal for harsh tunnel environments with high voltage, humidity, or electromagnetic noise. These sensors are intrinsically safe, lightweight, and flexible, allowing installation in confined or complex tunnel geometries.

System Architecture and Signal Transmission

A typical tunnel temperature monitoring system uses a fiber optic grating sensor array connected to a central acquisition unit. For example, systems like the SuperHawk1002T fiber grating sensor interface with FPGA-based controllers (e.g., FACE-TD200) to acquire and process temperature data. The sensors can be multiplexed along a single optical fiber, reducing cabling complexity and enabling long-distance monitoring over several kilometers with minimal signal loss.

The optical signals are digitized, and temperature data is calculated based on the Bragg wavelength shift, allowing real-time monitoring and automated alarms for abnormal temperature changes.

Installation in Tunnels

Fiber optic grating sensors are typically installed along the tunnel ceiling or walls, depending on the fire risk and structural monitoring requirements. Fixing methods include cable ties or suspension from the ceiling, with recommended spacing (e.g., 1. 5 meters apart) to ensure accurate readings.

For long tunnels, multiple fibers may be used to cover different sections or sides of the tunnel cross-section, ensuring comprehensive coverage. The system can provide distributed temperature profiles with measurement points every 0. 5 meters, enabling early detection of fires or overheating in power cables.

Applications

Fire Detection: Fiber optic grating sensors detect rapid temperature rises from vehicle fires or electrical faults, triggering alarms before the fire spreads. Power Cable Monitoring: Online monitoring of tunnel power cables ensures safe operation under high current and voltage conditions. Structural Health Monitoring: FBG sensors measure strain and temperature in tunnel linings, helping detect potential structural issues and ensuring long-term safety.

Advantages

High accuracy and repeatability in temperature measurement. Electromagnetic interference resistance and intrinsic safety in explosive or high-voltage environments. Long-distance signal transmission with low attenuation, suitable for tunnels several kilometers long.

Real-time monitoring and automated alarms, enabling proactive safety management. Multiplexing capability, allowing multiple sensors on a single fiber for cost-effective installation. Fiber optic grating temperature sensing systems provide a robust, reliable, and scalable solution for tunnel safety, fire detection, and structural monitoring, combining high precision with operational flexibility.

Tunnel Fiber Optic Grating Temperature Sensor - ENSAA OPTICAL NETWORKS [PDF]

Tunnel Fiber Optic Grating Temperature Sensor

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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