Fiber optic sensing systems can be classified based on sensor location, sensing principle, and configuration, including intrinsic, extrinsic, hybrid, point, integral, and distributed types.

Classification by Sensor Location

• Intrinsic Sensors: The optical fiber itself acts as the sensing element. Physical changes such as strain, temperature, or pressure directly modulate the light within the fiber, and the light remains inside the fiber throughout the measurement process .
• Extrinsic Sensors: The fiber transmits light to and from an external sensing element. The fiber is not affected by the measurand; it primarily serves as a light conduit .
• Hybrid Sensors: Combine features of intrinsic and extrinsic sensors, where the fiber both carries light and interacts with the sensing element .

Classification by Measurement Scope

• Point Sensors: Measure parameters at discrete locations along the fiber .
• Integral Sensors: Gather information over a segment of the fiber, providing an average measurement over that length .
• Distributed Sensors: Measure continuously along the entire length of the fiber, enabling detection over large distances and providing spatially resolved data .

Classification by Sensing Principle

• Intensity-Based Sensors: Detect changes in light intensity caused by external factors such as bending, strain, or temperature .
• Phase-Based Sensors: Measure phase shifts in the light signal induced by environmental changes .
• Wavelength-Based Sensors: Use devices like Fiber Bragg Gratings (FBGs) to reflect specific wavelengths that shift according to strain or temperature .

Classification by Sensor Configuration

• Thru-Beam Type: Separate transmitter and receiver fibers, where light passes through the sensing region .
• Reflective Type: Single unit with fibers arranged parallel, coaxial, or separate for transmitter and receiver, reflecting light back to the detector .

Applications

Fiber optic sensors are widely used in structural health monitoring, temperature and pressure measurement, chemical detection, and security applications. Distributed sensing systems are particularly valuable for monitoring pipelines, power cables, bridges, and other critical infrastructure over long distances . This classification framework helps in selecting the appropriate fiber optic sensing system based on the measurement requirements, environmental conditions, and desired spatial resolution.