Inside Fiber Optic Sensors: Categories, Materials, and Core Functional Traits

Fiber optic sensors are broadly classified into several categories based on sensing principle, sensor location, and structure:

Intrinsic vs. Extrinsic Sensors:
- Intrinsic sensors use the optical fiber itself as the sensing element, where physical changes affect the light guided within the fiber.
- Extrinsic sensors use the fiber to transmit light to and from an external sensing element, with the fiber itself not affected by the measurand.

Point, Integral, and Distributed Sensors:
- Point sensors measure parameters at discrete points.
- Integral sensors gather information over a fiber segment.
- Distributed sensors can measure along the entire length of the fiber, enabling detection over large distances.

By Light Interaction Type:
- Intensity-based: Sensors detect changes in light intensity due to external factors.
- Phase-based: Sensors measure phase shifts in light signals caused by environmental changes.
- Wavelength-based: Fiber Bragg Gratings (FBGs) reflect specific wavelengths that shift according to strain or temperature.

By Sensor Configuration:
- Thru-beam type: Separate transmitter and receiver fibers.
- Reflective type: Single unit with fibers arranged parallel, coaxial, or separate for transmitter and receiver.

The optical fiber core and cladding materials largely determine sensor performance and application suitability. Common materials include:

• Plastic Optical Fibers (POF): Made of acrylic resin cores within protective sheaths. Advantages include lightweight, flexibility, cost-effectiveness, suitable for short-range and low-cost sensing.
• Glass Optical Fibers: Made of high-purity silica glass with core diameters typically 10 to 100 microns. Advantages include higher temperature tolerance (up to 350°C), superior signal transmission, suitable for harsh environments.
• Flexible Polymer Materials: Thermoset or thermoplastic elastomers (e.g., PDMS - polydimethylsiloxane), biocompatible hydrogels, natural polymers such as spider silk and silk fibroin. These provide mechanical flexibility, stretchability, and can be processed via advanced fabrication methods like 3D printing, with critical optical properties for performance.

Key functional traits that define fiber optic sensors include:

• Light Transmission and Guiding: Optical fibers guide light via total internal reflection between core and cladding with different refractive indexes. External changes affect light transmission and are detected.
• Sensitivity to Physical Parameters: Sensors detect strain, temperature, pressure, vibrations, chemical concentrations, displacement by monitoring changes in light intensity, phase, polarization, or wavelength reflected within the fiber.
• Immunity to Electromagnetic Interference: Operation using light signals ensures sensors are unaffected by electromagnetic noise, ideal for sensitive environments.
• Small Size and Flexibility: Thin, flexible fibers allow embedding in tight or harsh spaces, including biomedical uses.
• Durability and High Reliability: Glass fibers offer durability for extreme conditions; plastic fibers give flexibility.
• Types of Detection Configurations: Parallel, coaxial, and separate transmitter/receiver fiber arrangements suit various sensing needs and optical fiber types.