Common Models of Fiber Optic Displacement Sensors

Fiber optic displacement sensors can be classified by operating principle into intensity-modulated, interferometric, and phase-based types, and by construction into intrinsic, extrinsic, and hybrid sensors.Classification by Operating PrincipleIntensity-Modulated Sensors These sensors measure displacement by detecting changes in the intensity of light reflected or transmitted from a target. A multimode fiber probe collects light, and a photodiode measures the intensity variation, which correlates with the displacement between the probe and the target. This method is simple, cost-effective, and widely used for short-range, high-precision measurements in industrial and medical applications .Interferometric Sensors Interferometric sensors use the interference of light waves to detect extremely small displacements. They offer very high sensitivity and resolution but are more complex and sensitive to environmental disturbances such as vibration and temperature changes .Phase-Based Sensors These sensors detect displacement by measuring the phase shift of light traveling through the fiber. Phase changes occur due to variations in the optical path length caused by displacement. They are suitable for high-precision applications where sub-micron resolution is required .Classification by ConstructionIntrinsic Sensors In intrinsic fiber optic sensors, the fiber itself acts as the sensing element. The measurand (displacement) directly affects the light traveling within the fiber, modifying parameters such as intensity, phase, polarization, or wavelength. These sensors are capable of distributed sensing over long distances and are highly resistant to electromagnetic interference .Extrinsic Sensors Extrinsic sensors use the fiber primarily to transmit light to and from an external sensing element. The displacement is measured outside the fiber, and the fiber serves as a conduit for the optical signal. This design is simpler and allows the use of specialized transducers for specific applications .Hybrid Sensors Hybrid sensors combine features of both intrinsic and extrinsic types. The fiber carries light into and out of the sensing element, allowing flexibility in design and enabling measurements in challenging environments where both direct fiber sensing and external transduction are advantageous .ApplicationsFiber optic displacement sensors are used in precision engineering, micro-assembly, structural health monitoring, and harsh environments where electromagnetic interference or high voltage makes conventional sensors unsuitable. They are valued for their non-contact measurement capability, high sensitivity, small size, and immunity to electrical noise . By selecting the appropriate type based on operating principle and construction, engineers can optimize for range, resolution, environmental robustness, and cost.