Rayleigh Scattering in Optical Fiber

International Zone (English) OFSCN Knowledge Base (Public)
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How does it serve as the signal source for distributed fiber sensing (OTDR/OFDR)?

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Rayleigh scattering serves as a fundamental signal source for distributed fiber sensing techniques like Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR) due to its inherent nature within optical fibers.

Here’s how it works:

  1. Microscopic Inhomogeneities: Optical fibers, despite their high purity, have microscopic density fluctuations and compositional variations frozen into their glass structure during manufacturing. These inhomogeneities are much smaller than the wavelength of light.
  2. Scattering Mechanism: When light propagates through the fiber, it encounters these tiny inhomogeneities. This causes a portion of the light to be scattered in all directions, a phenomenon known as Rayleigh scattering.
  3. Backscattered Signal: A small fraction of this scattered light is directed back towards the source, known as Rayleigh backscatter. This backscattered light acts as a continuous “signature” from every point along the fiber.
  4. Distributed Sensing:
    • OTDR: In OTDR, a short optical pulse is launched into the fiber. As the pulse travels, it generates Rayleigh backscatter. By measuring the arrival time and intensity of the backscattered light, and knowing the speed of light in the fiber, the OTDR instrument can determine the location and magnitude of events (like splices, connectors, or breaks) or changes in loss along the entire length of the fiber.
    • OFDR: OFDR uses a continuously swept frequency light source. The backscattered light from different points along the fiber arrives back at the detector with a phase shift proportional to the distance traveled and the frequency sweep. By performing a Fourier Transform on the detected signal, the system can precisely map the scattering profile and detect minute changes in strain or temperature with very high spatial resolution along the fiber.

The continuous generation of Rayleigh backscatter along the fiber’s entire length allows for truly distributed measurements, making the fiber itself the sensor.

OFSCN offers specialized optical fibers, such as the OFSCN® 85°C Seamless Steel Tube Fiber Cable and OFSCN® 200°C Seamless Steel Tube Fiber Cable, which can be used as distributed fiber optic sensing cables or sensors, compatible with sensing equipment based on Rayleigh, Raman, or Brillouin scattering.

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