Types of Fiber Optic Sensing Structures

Fiber optic sensors are primarily classified into intrinsic, extrinsic, and hybrid structures, based on where and how the sensing occurs within the optical system.

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 light properties—intensity, phase, polarization, wavelength, or transit time—are measured to determine the physical quantity. These sensors are capable of distributed sensing over long distances, making them ideal for structural health monitoring, pipeline surveillance, and high-voltage environments where electrical interference is a concern . Examples include Fiber Bragg Gratings (FBG) and sensors based on Rayleigh, Raman, or Brillouin scattering.

2. Extrinsic Fiber Optic Sensors

Extrinsic sensors have external transducers that interact with the measurand, while the fiber primarily serves to transmit light to and from the sensing element. The modulation occurs outside the fiber, and the fiber acts as a communication link. This structure is useful for remote or inaccessible locations, such as measuring temperature inside aircraft engines or electrical transformers, where conventional sensors cannot operate safely . Extrinsic sensors often use multimode fibers to carry light from the transducer to the detector.

3. Hybrid Fiber Optic Sensors

Hybrid sensors combine intrinsic and extrinsic principles, where the fiber both carries light into the sensing device and participates in the sensing process. This allows for enhanced sensitivity and flexibility in applications requiring both direct fiber modulation and external transduction . Hybrid structures are used in specialized industrial and aerospace applications where complex measurements, such as current, vibration, or torque, are needed.

Summary

• Intrinsic sensors: Fiber itself senses; suitable for distributed and high-precision measurements.
• Extrinsic sensors: Fiber transmits light to an external sensor; ideal for remote or hazardous locations.
• Hybrid sensors: Combine both approaches for enhanced functionality and versatility. These structural classifications help engineers and researchers select the appropriate fiber optic sensor for applications ranging from telecommunications, aerospace, and power systems to structural health monitoring .