Fiber optic sensors are classified into three main categories: intrinsic, extrinsic, and hybrid sensors.

1. Intrinsic Fiber Optic Sensors

Intrinsic sensors use the optical fiber itself as the sensing element. The physical quantity being measured—such as pressure, temperature, or strain—directly affects the light traveling within the fiber. The fiber's properties, like intensity, phase, polarization, or wavelength of light, are modulated by the measurand, and the light remains within the fiber throughout the sensing process. These sensors are ideal for distributed sensing over long distances and are widely used in structural health monitoring and industrial applications .

2. Extrinsic Fiber Optic Sensors

Extrinsic sensors rely on an external transducer or sensing element. The optical fiber serves primarily as a light conduit, transmitting light to and from the external sensor. The modulation of the light occurs outside the fiber, in a “black box” device that converts the physical quantity into an optical signal. These sensors are particularly useful for remote or hard-to-reach locations, such as measuring internal temperatures in jet engines or transformers .

3. Hybrid Fiber Optic Sensors

Hybrid sensors combine features of both intrinsic and extrinsic types. The fiber carries light into and out of the sensing device, and both the fiber and the external transducer contribute to the sensing process. This approach allows for enhanced flexibility and sensitivity, making hybrid sensors suitable for complex applications where both distributed and point measurements are required . These three categories provide a framework for understanding fiber optic sensor design and selecting the appropriate type for specific applications, ranging from industrial monitoring to aerospace and structural health systems.