What are the types of fiber optic cables used in sensors

Fiber optic sensor cables are typically classified as single-mode or multimode fibers, with specialized coatings and constructions tailored for sensing applications in various environments.

Core Types

Single-mode (SM) fibers have a small core diameter, usually around 8–10 µm, allowing light to propagate along a single path. This supports long-distance, high-precision sensing with minimal signal distortion, making them ideal for applications requiring high accuracy and low attenuation . Multimode (MM) fibers have larger core diameters, typically 50 µm or 62.5 µm, allowing multiple light paths. MM fibers are suitable for shorter-range sensing and applications where higher light intensity is needed, such as distributed temperature or strain monitoring . Graded-index MM fibers are often preferred to reduce mode dispersion and improve signal quality .

Sensor Cable Constructions

Fiber optic sensor cables are designed to withstand environmental and mechanical stresses. Common constructions include:

• Metal-free cables: Flexible and lightweight, reducing the risk of induced voltages and suitable for sensitive electronic environments .
• Metal-armored cables: Provide robust protection against harsh conditions, rodent damage, and mechanical stress .
• Tube-in-tube and metal-tubing designs: Offer additional protection for high-temperature or cryogenic applications .

Coatings and Sheathing

The choice of coating depends on the operating environment:

• Polyacrylate coatings: Standard temperatures and general-purpose sensing .
• Polyimide or metal coatings: High-temperature or cryogenic environments .
• Sheathings: Flame-retardant non-corrosive (FRNC) or water-tight high-density polyethylene (HDPE) for environmental protection .

Sensor Types and Fiber Role

Fiber optic sensors can be intrinsic, extrinsic, or hybrid:

• Intrinsic sensors: The fiber itself acts as the sensing element, with light modulation occurring within the fiber .
• Extrinsic sensors: The fiber transmits light to and from an external transducer, with modulation occurring outside the fiber .
• Hybrid sensors: Combine intrinsic and extrinsic features, with light traveling into and out of the sensing device .

Summary

In sensor applications, the choice of fiber optic cable depends on distance, environmental conditions, and measurement precision. Single-mode fibers are preferred for high-accuracy, long-distance sensing, while multimode fibers are used for shorter-range, high-intensity applications. Specialized coatings, armored constructions, and sheathings ensure durability and reliable performance in diverse conditions .