Principle of Fiber Optic Temperature Measurement in Cable Channels

Fiber optic temperature measurement in cable channels relies on detecting temperature-induced changes in light properties within optical fibers, enabling precise, distributed, and EMI-immune monitoring.

Overview

Fiber optic temperature sensors use optical fibers as the sensing medium, transmitting light along the fiber to detect temperature variations without electrical signals at the sensing point, making them ideal for high-voltage or EMI-prone cable channels . These systems can operate as point sensors or distributed sensors, depending on whether temperature is measured at discrete locations or continuously along the cable length .

Point Sensing

Point-type sensors, such as fiber Bragg gratings (FBGs) or phosphor-coated probes, measure temperature at specific locations:

• FBG sensors: A Bragg grating reflects a specific wavelength of light. Temperature changes cause thermal expansion and refractive index variations, shifting the reflected wavelength. An interrogator analyzes this shift to determine the local temperature .
• Phosphor-based probes: A light pulse excites a phosphor coating at the probe tip, and the decay time of the afterglow correlates with temperature. This method provides self-referencing, drift-free measurements without electrical energy at the sensing point .

Distributed Temperature Sensing (DTS)

For monitoring along entire cable channels, DTS systems use standard optical fibers as linear sensors:

• A laser pulse is sent through the fiber, and light interacts with the glass structure, producing backscattered signals via Raman or Brillouin scattering .
• The Stokes and anti-Stokes components of Raman scattering vary with temperature, while Brillouin scattering involves a frequency shift proportional to temperature and strain.
• The position of the temperature reading is determined from the time delay of the backscattered light (Optical Time Domain Reflectometry, OTDR) or via frequency-domain analysis (Optical Frequency Domain Reflectometry, OFDR), .
• This allows continuous temperature profiling along kilometers of cable with spatial resolution down to 1 meter and high accuracy (±1 °C), .

Advantages in Cable Channels

• Electromagnetic immunity: No electrical signals are used, preventing interference from high-voltage cables .
• Safety: Fully dielectric and non-conductive, suitable for explosive or high-voltage environments.
• Distributed monitoring: Detects local overheating or hot spots along cable trays, enabling preventive maintenance and fire risk mitigation .
• Long-range capability: DTS systems can monitor tens of kilometers of cable with a single fiber.

Summary

In cable channels, fiber optic temperature measurement works by translating temperature-induced changes in light properties into measurable signals, either at discrete points (FBG or phosphor probes) or continuously along the fiber (DTS). This approach provides real-time, accurate, and safe temperature monitoring, crucial for power systems, industrial installations, and high-voltage environments .