Fiber optic sensing technology has revolutionized the way industries monitor strain, temperature, and other critical parameters. Two primary types of distributed fiber optic sensing technologies are scattering-based sensors and fiber Bragg grating (FBG)-based sensors. Understanding their differences, advantages, and applications is essential for selecting the best solution for a given use case.
Scattering-Based Fiber Optic Sensors
Scattering-based sensors leverage naturally occurring backscattering of light within an optical fiber. These backscattering signals—Raman, Rayleigh, and Brillouin—arise due to random imperfections in the fiber’s lattice structure. Because they don’t require engineered reflective components like FBGs, scattering-based systems can use standard telecom-grade fiber with specialized terminations and coatings.
Key Characteristics
- Spatial Resolution: Typically on the order of meters.
- Refresh Rate: Generally 10–30 seconds, which decreases as sensing length increases
- Signal-to-Noise Ratio: Low compared to FBG-based sensors
- Best Use Cases: Applications requiring long sensing lengths (kilometers) and where real-time measurements and high spatial resolution are not essential.
Types of Scattering-Based Technologies
Raman Scattering
Raman-based sensors, commonly referred to as Distributed Temperature Sensing (DTS) systems, rely on thermally induced vibrations. They are exclusively sensitive to temperature changes and are widely used in:
- Oil and gas pipeline monitoring
- Security and leak detection.
- Linear heat detection systems
Rayleigh Scattering
Rayleigh-based sensors exhibit higher intensity than Raman signals and are more sensitive to strain than temperature. Their primary application is Distributed Acoustic Sensing (DAS), which is particularly valuable in:
- Downhole oil and gas monitoring
- Vibration and intrusion detection
Brillouin Scattering
Brillouin-based sensors are unique in their ability to measure both strain and temperature while covering the longest sensing distances among scattering technologies. They are most commonly used for:
- Structural health monitoring
- Oil and gas pipeline and railway monitoring
- Applications requiring long-distance, low-density sensing
Fiber Bragg Grating (FBG)-Based Sensors
FBG sensors function by reflecting specific wavelengths of light while transmitting all others. When subjected to strain or temperature changes, the reflected wavelength shifts, allowing for precise measurements.
Key Characteristics
- High Signal-to-Noise Ratio: Provides more accurate measurements, especially in applications where the sensors encounter motion or vibrations
- Modular and Flexible Installations: Suitable for dynamic environments with complex fiber routing.
- Best Use Cases: Applications requiring real-time, high-resolution, and high-speed data acquisition over sensing lengths up to 100 meters
Demodulation Techniques in FBG Sensing
Two primary methods are used to interpret optical signals in FBG-based systems: Wavelength Division Multiplexing (WDM) and Optical Frequency Domain Reflectometry (OFDR).
Wavelength Division Multiplexing (WDM)
WDM assigns each FBG sensor a unique wavelength, limiting the number of sensors per fiber channel. While WDM allows high refresh rates, it has significant drawbacks:
- Limited number of sensors per fiber due to wavelength overlap concerns.
- Customization is required for each fiber installation, increasing lead times.
- Best for applications requiring only a few high-speed sensing points.
Optical Frequency Domain Reflectometry (OFDR)
OFDR enables high-resolution, spatially continuous sensing by writing all FBGs at the same wavelength. This method provides several advantages over WDM:
- Higher spatial resolution
- More sensing locations per fiber
- Consistently high sampling rates, even with many sensors
- Easier deployment as the sensing fiber can be used without customization.
Choosing the Right Fiber Optic Sensors For Your Applications
When to Choose Scattering-Based Sensors
Scattering-based sensors are ideal for applications requiring long-range monitoring with minimal sensor density. If real-time data and high spatial resolution are not critical, scattering technologies like Raman, Rayleigh, and Brillouin are excellent choices for:
- Oil and gas pipeline monitoring.
- Railway and structural health monitoring.
- Large-scale temperature mapping.
When to Choose FBG-Based Sensors
FBG-based sensors are best suited for applications that demand high precision, fast sampling rates, and adaptability to complex installation environments. OFDR-based systems offer unparalleled spatial resolution and versatility for:
- Aerospace and structural testing
- Real-time strain and temperature monitoring
- Advanced research applications
Conclusion
Both scattering-based and FBG-based fiber optic sensors have their unique strengths and applications. For industries requiring long-range sensing with minimal resolution needs, scattering technologies provide a viable solution. However, for applications demanding high precision, high-speed data acquisition, and extensive sensor coverage, OFDR-based FBG sensing is the superior choice.
As fiber optic sensing technology continues to evolve, the versatility and performance of OFDR-based systems are setting new standards in industries ranging from aerospace to energy infrastructure. Whether monitoring structural integrity, environmental conditions, or industrial processes, using the right fiber optic sensors ensure accurate and reliable data collection for informed decision-making.
Discover how Sensuron’s advanced fiber optic strain and temperature sensing systems are transforming industries through real-world applications at: https://www.sensuron.com/case-studies-new/
