Helical Fiber Optic Sensor

Helical fiber optic sensors are advanced optical devices that use twisted or helical fiber structures to measure torsion, strain, and bending with high sensitivity and directional resolution.

Overview

Helical fiber optic sensors exploit the helical geometry of optical fibers to detect mechanical deformations such as torsion, strain, and bending. By twisting a fiber into a helical shape, the symmetry of the fiber is broken, making it sensitive to rotational and mechanical changes that standard fibers cannot detect. These sensors are compact, immune to electromagnetic interference, and compatible with fiber-optic systems, making them suitable for aerospace, structural health monitoring, and robotics applications .

Working Principle

Most helical fiber optic sensors are based on interferometric techniques, such as the Mach–Zehnder interferometer (MZI). A typical configuration involves a segment of helical fiber (single-core, two-core, or multicore) spliced between standard single-mode fibers. Mechanical deformation alters the optical path difference between modes, producing measurable changes in the transmission spectrum. The helical structure allows the sensor to distinguish both the magnitude and direction of torsion .

Types and Designs

1. Two-Core Helical Fiber Sensors: Pre-twisted two-core fibers form a compact MZI capable of measuring torsion angle and direction. Sensitivity is maximized at specific pre-twist angles, e.g., 180°, with torsion wavelength sensitivity around 0.242 nm/(rad/m) and temperature sensitivity of 32 pm/°C .
2. Helical Waveguide Sensors: Femtosecond laser-inscribed helical waveguides in single-mode fibers create strain-independent torsion sensors. These sensors achieve high twist sensitivity (e.g., 261 pm/(rad/m)) and can unambiguously determine rotational direction without splicing multiple fiber segments .
3. Helical Seven-Core Fiber Sensors: Multicore fibers with helical tapers or seven-core structures enhance sensitivity to torsion, strain, and bending. Bidirectional torsion sensitivities can reach 2.253 nm/(rad/m) clockwise and −1.123 nm/(rad/m) counterclockwise. These designs also maintain stability against temperature variations .

Advantages

• High sensitivity to torsion, strain, and bending
• Directional resolution for clockwise and counterclockwise rotations
• Compact and robust structure suitable for harsh environments
• Electromagnetic immunity and compatibility with fiber-optic networks
• Flexible fabrication using pre-twisting or femtosecond laser inscription

Applications

• Aerospace engineering: Monitoring torsion in aircraft components
• Structural health monitoring: Detecting strain and bending in bridges or pipelines
• Robotics: Measuring joint rotation and torque in anthropomorphic robots
• Industrial sensing: Compact sensors for machinery and precision instruments Helical fiber optic sensors represent a versatile and high-performance solution for precise mechanical measurements, combining the advantages of optical fiber technology with innovative helical designs.