The two main types of fiber optic sensors are intrinsic and extrinsic sensors.

Intrinsic Fiber Optic Sensors

Intrinsic sensors have the sensing element built directly into the optical fiber itself. The fiber acts as both the transmission medium and the sensor, meaning that the physical quantity being measured—such as strain, temperature, pressure, or vibration—directly affects the light traveling through the fiber. Changes in light properties like intensity, phase, polarization, or wavelength are used to detect the measurand. These sensors are particularly useful for distributed sensing over long distances, allowing continuous monitoring along the fiber's length .

Extrinsic Fiber Optic Sensors

Extrinsic sensors use the optical fiber primarily as a light guide to transmit signals to and from an external sensing element. The actual sensing occurs outside the fiber, in a transducer or “black box” that converts the physical quantity into a modulated light signal. The fiber simply carries this light to a detector for analysis. Extrinsic sensors are ideal for measuring quantities in hard-to-reach or hazardous locations, such as the internal temperature of aircraft engines or transformers, where the fiber can safely transmit the signal from the remote sensor .

Summary

• Intrinsic sensors: Fiber itself is the sensor; light modulation occurs within the fiber. Best for distributed sensing.
• Extrinsic sensors: Fiber transmits light to/from an external sensor; modulation occurs outside the fiber. Best for remote or inaccessible locations. Both types offer advantages like immunity to electromagnetic interference, high sensitivity, and the ability to operate in harsh environments, making them widely used in structural health monitoring, aerospace, power systems, and industrial applications .