Fiber Optic Sensing Technologies

These systems are utilized for monitoring various physical parameters like temperature, strain, and pressure with high precision and reliability. Within the FOS market, several different technologies are available, each offering unique advantages and addressing specific application needs.

Types of Fiber Optic Sensing Technologies

Both technologies differ in their approaches to data acquisition and are chosen based on specific performance requirements.

Scattering-Based Sensing

Scattering-based techniques leverage naturally occurring imperfections in the optical fiber to measure temperature and strain. They offer fully distributed sensing along the fiber and are capable of long-range monitoring.

Key Characteristics

  • Spatial Resolution: Generally on the order of meters. Some Rayleigh backscatter systems are capable of millimeter-level resolution.
  • Sampling Rate: Lower compared to FBG sensors, typically ranging from 10 to 30 seconds, which decreases with increasing sensing length.
  • Applications: Best suited for scenarios where real-time, high-resolution measurements are not critical but longer sensing lengths and distributed measurements are required.

Fiber Bragg Grating (FBG) Sensing

As opposed to inherent scattering, FBG sensors are inscribed into the optical fiber, acting as wavelength-selective mirrors that reflect specific wavelengths of light. When subjected to strain or temperature changes, the reflected wavelength shifts, enabling precise measurements.

  • Higher Signal-to-Noise Ratio: Provides reliable data, even in dynamic and harsh environments.
  • Versatility:Can be used in either point-based or fully distributed systems depending on the application’s requirements.
Fiber Bragg Grating (FBG) Sensing

Applications

FBG systems are widely used for structural health monitoring, aerospace structural testing, and temperature sensing in environments where traditional electronic sensors may fail.

Comparing Scattering-Based and FBG Technologies

Scattering-Based Sensors FBG Sensors
Raman Rayleigh Brillouin WDM OFDR
Sensing Type Fully Distributed Fully Distributed Fully Distributed Single Point or Quasi-distributed Fully Distributed
Typical Spatial Resolution 1 meter 5 to 10 meters 1 meter Centimeters Millimeters to Centimeters
Typical Max Number of Sensors Varies, dependent on sensing length Varies, dependent on sensing length Varies, dependent on sensing length 8 to 20 per fiber Hundreds to thousands, depending on sensing length
Typical Max Sensing Length Tens to hundreds of kilometers 40 to 50 kilometers Tens of kilometers Meters to kilometers 13 meters per fiber
Typical Sampling Rate 100 to 250 MHz 1 kHz to tens of kHz 1 Hz to kHz, dependent on sensing length 1 kHz to Tens of kHz 10 to 200 Hz
Signal to Noise Ratio Low Low Low High High
Typical Applications Distributed temperature sensing for pipelines Relatively static strain or temperature sensing (pipelines, structural monitoring) Very long distance strain or temperature sensing (pipelines, railways) Quasi-distributed strain and temperature sensing for applications requiring fewer sensors Fully distributed strain and temperature sensing for applications requiring high resolution (structural monitoring, aerospace testing)

Scattering-Based Sensing

Primary types of Scattering-Based Sensing

Raman Scattering (DTS)

Primarily used for temperature measurements. It relies on thermally induced changes in the Raman backscatter signature. Common applications include pipeline monitoring for leak detection and linear heat detection in fire safety systems.

Rayleigh Scattering

This technique is used to sense both strain and temperature. Inherent variations in the fiber's refractive index cause backscattering which varies with changes in strain or temperature. It is most often used in Distributed Acoustic Sensing (DAS) for applications like seismic monitoring and downhole oil and gas exploration.

Brillouin Scattering

Capable of measuring both strain and temperature, Brillouin systems offer the longest sensing range among scattering techniques. These are ideal for applications like railway track monitoring, perimeter security, and pipeline integrity assessments, where long-distance coverage is required.

Choosing the Right Technology

Selecting the most suitable fiber optic sensing technology depends on application requirements, including the spatial resolution, acquisition rate, sensing length, and environmental conditions.

Scattering-Based Systems

Ideal for applications requiring long-range monitoring with low to moderate spatial resolution and low acquisition rates, such as pipeline integrity checks or large-scale structural monitoring.

FBG-Based Systems

Ideal for applications needing high numbers of sensors over relatively shorter distances, such as aerospace testing and structural health monitoring.

Interrogation Techniques

Fiber Bragg Grating (FBG) Sensing

Wavelength Division Multiplexing (WDM)

This is the most common type of FBG interrogation. This technique assigns different Bragg wavelengths to each FBG sensor along the fiber to discern which sensor is which. It is commonly used when relatively few discrete sensing points are needed at high sampling rates. However, it is significantly limited by the number of sensors per fiber due to wavelength overlap risks.

Optical Frequency Domain Reflectometry (OFDR)

Unlike WDM, OFDR uses the same wavelength for all sensors, enabling spatially continuous measurements. OFDR is capable of much higher spatial resolution when compared with WDM and is not limited on the maximum number of sensors per fiber. This technique is ideal for applications requiring real-time measurements and high spatial resolution.

Final Thoughts

As industries continue to push the boundaries of innovation, traditional sensors like strain gauges and thermocouples are increasingly being replaced by more advanced, optical fiber-based solutions. These technologies not only enhance measurement capabilities but also offer additional robustness and detailed distributed measurements that are often needed for more challenging applications.

With continuous advancements, fiber optic sensing technologies are gradually becoming an integral part of instrumentation systems, providing the precision and reliability needed to meet modern engineering challenges. Whether it's monitoring the structural health of a bridge, assessing stress profiles along an aircraft wing in real-time, or detecting leaks in oil pipelines, fiber optic sensors are paving the way for safer, more efficient operations across various industries.