Quasi-distributed fiber gratings use arrays of fiber Bragg gratings along a single optical fiber to enable multi-point sensing with high precision and fast response.

Overview

Quasi-distributed fiber gratings are arrays of fiber Bragg gratings (FBGs) integrated along a single optical fiber, allowing simultaneous measurement at multiple points. Unlike fully distributed sensing, which monitors the entire fiber continuously, quasi-distributed systems provide discrete sensing locations while maintaining the advantages of multiplexing and compact deployment . These systems are widely used for temperature, strain, vibration, and humidity monitoring in applications ranging from structural health monitoring to environmental sensing .

Sensing Mechanism

Each FBG reflects a specific wavelength of light that shifts in response to strain or temperature changes. By monitoring the wavelength shifts of multiple gratings along the fiber, quasi-distributed systems can map spatial variations of the measured parameter. Advanced coatings, such as MXene/PI composites, can enhance sensitivity for specific applications like humidity sensing, while dedicated temperature-compensation gratings help decouple temperature effects from other measurements .

Fabrication and Interrogation

FBG arrays can be fabricated using femtosecond laser writing or online writing techniques, allowing precise control over grating spacing and reflectivity . Interrogation methods include stepped incoherent optical frequency domain reflectometry (IOFDR) and wavelength scanning of tunable lasers, which provide high spatial resolution (down to centimeters) and low-noise detection . These techniques enable the interrogation of tens of gratings with minimal optical power, supporting high-density quasi-distributed sensing.

Applications

• Structural Health Monitoring: Detecting strain, vibration, or icing loads on composite insulators and transmission lines .
• Environmental Sensing: Measuring humidity, temperature, or other parameters in real-time with high accuracy .
• Industrial Systems: Monitoring fuel cells, pipelines, or mechanical components where multi-point sensing is critical .
• Dynamic and Static Sensing: Simultaneous monitoring of vibrations and static parameters using phase demodulation techniques .

Advantages

• Multiplexing Capability: Multiple FBGs can be interrogated along a single fiber, reducing cabling complexity.
• High Sensitivity and Accuracy: Coatings and compensation gratings improve measurement precision.
• Fast Response: Suitable for dynamic monitoring applications.
• Compact and Flexible Deployment: Compatible with arrays and multi-point installations in confined spaces . Quasi-distributed fiber gratings represent a versatile and scalable sensing technology, bridging the gap between point sensors and fully distributed fiber optic systems, with applications in civil, industrial, and environmental monitoring.