Fiber optic sensors are primarily classified into intrinsic, extrinsic, and hybrid models, each differing in how the fiber interacts with the measured quantity.

1. Intrinsic Fiber Optic Sensors

Intrinsic sensors use the optical fiber itself as the sensing element. The measurand, such as strain, temperature, or pressure, directly affects the light traveling through the fiber. Changes in intensity, phase, polarization, or wavelength of the light are measured to determine the physical quantity. These sensors are highly sensitive and suitable for structural health monitoring, aerospace, and industrial applications where the fiber can be embedded within the material being monitored .

2. Extrinsic Fiber Optic Sensors

Extrinsic sensors employ external transducers to interact with the measurand, while the fiber primarily serves as a light transmission medium. The fiber carries light to and from the sensing element, which modulates the light outside the fiber. This model is ideal for remote sensing in harsh environments, such as measuring temperature inside jet engines or electrical transformers, where direct electrical sensors are impractical .

3. Hybrid Fiber Optic Sensors

Hybrid sensors combine intrinsic and extrinsic principles, allowing the fiber to both transmit light and participate in sensing. This approach can enhance sensitivity and flexibility, making it suitable for complex industrial and biomedical applications where multiple parameters need to be monitored simultaneously .

4. Specialized Models

• Fiber Bragg Grating (FBG) Sensors: These intrinsic sensors use periodic variations in the fiber's refractive index to reflect specific wavelengths. FBG sensors are widely used for strain, temperature, and pressure monitoring with high resolution and multiplexing capability .
• Fiber Optic Current Sensors: These sensors detect changes in light polarization caused by magnetic fields (Faraday Effect) and are used in power generation, oil and gas, and aerospace applications for precise current measurement .
• Distributed Sensors: These sensors measure parameters along the entire length of the fiber, enabling continuous monitoring of pipelines, bridges, and large structures .

Summary

The main models of fiber optic sensors—intrinsic, extrinsic, and hybrid—are distinguished by whether the fiber itself or an external transducer interacts with the measurand. Specialized sensors like FBG, current, and distributed sensors extend the capabilities of these models, offering high sensitivity, EMI immunity, and suitability for harsh or remote environments . These classifications guide the selection of sensors for applications in industrial automation, structural health monitoring, aerospace, and biomedical fields.