Three-core fiber optic sensors use multiple cores within a single fiber to enable high-sensitivity measurements of physical parameters like bending, refractive index, temperature, and shape.

Design and Structure

Three-core fiber optic sensors typically consist of three distinct cores embedded within a single optical fiber, which can be arranged in microstructured or hole-assisted configurations. In microstructured fibers, one core may be exposed in a hollow channel, another enclosed in an inner tube, and the third covered by cladding, forming a Mach-Zehnder interferometer when spliced with multi-mode fibers at both ends. Light propagating through the three cores interferes, producing a spectrum that can be analyzed using Fast Fourier Transform to extract sensing information such as refractive index and temperature with high sensitivity (-1319.56 nm/RIU for refractive index and 0.32 nm/°C for temperature) . Hole-assisted three-core fibers enhance sensitivity to vector bending, achieving maximum bending sensitivities of -27.282 nm/m in specific directions, making them suitable for precise structural monitoring .

Working Principle

The interference of light in the three cores forms the basis of sensing. Variations in external parameters like bending, temperature, or refractive index alter the optical path lengths in each core, shifting the interference pattern. By analyzing these shifts, the sensor can detect minute changes in the measured quantity. Multi-core fibers can also incorporate Fiber Bragg Gratings (FBGs) for shape sensing, enabling real-time 3D reconstruction of fiber deformation with sub-millimeter accuracy (<0.1 mm) .

Applications

Three-core fiber optic sensors are used in a variety of fields:

• Structural health monitoring: Detecting strain, bending, and deformation in bridges, aircraft, and pipelines .
• Medical robotics and minimally invasive surgery: Real-time shape sensing of flexible instruments in confined spaces .
• Environmental sensing: Measuring temperature, pressure, and refractive index in liquids or gases .
• Robotics and navigation: Providing spatial awareness and feedback for autonomous systems .

Advantages

• High sensitivity due to multi-core interference.
• Compact and robust design suitable for embedded applications.
• Simultaneous multi-parameter sensing (e.g., temperature and refractive index).
• Reduced post-processing requirements compared to conventional fiber sensors, as the sensing structure is integrated into the fiber . Three-core fiber optic sensors represent a versatile and precise platform for modern sensing applications, combining interferometric sensitivity with multi-core fiber flexibility to address challenges in engineering, medical, and environmental monitoring.