Fiber Optic Sensor Displacement Test

Fiber optic displacement sensors measure displacement by detecting changes in light intensity, phase, or wavelength caused by mechanical movement affecting the optical fiber.

Overview of Fiber Optic Displacement Sensors

Fiber optic displacement sensors (FODS) are devices that convert mechanical displacement into measurable optical signals. They are widely used in industrial, civil, and medical applications due to their high sensitivity, small size, immunity to electromagnetic interference, and non-contact measurement capability . The main types of fiber optic displacement sensors include intensity-modulated sensors, interferometric sensors, and fiber Bragg grating (FBG) sensors.

Intensity-Modulated Sensors

In intensity-based displacement sensors, the displacement of a target changes the amount of light transmitted or reflected through the optical fiber. A multimode fiber is often used as the probe because its large core radius and high numerical aperture allow it to collect significant light from the target . The transmitted or reflected light is measured by a photodetector, and the displacement is inferred from the variation in light intensity. This method is simple, cost-effective, and suitable for both lateral and axial displacement measurements .

Interferometric Sensors

Interferometric fiber optic sensors, such as Michelson, Mach–Zehnder, and Fabry–Perot interferometers, measure displacement by detecting phase changes in light caused by variations in the optical path length . In a fiber optic microprobe interferometer, a modulated laser source sends light through the fiber to a sensing probe. The light reflected from the target interferes with a reference beam, producing an interference signal. The phase difference of this signal is directly related to the displacement of the target, allowing ultra-precise measurements down to nanometer resolution .

Fiber Bragg Grating (FBG) Sensors

FBG-based displacement sensors operate on the principle that strain or displacement applied to the fiber changes the grating period, which shifts the reflected wavelength of light . By monitoring the wavelength shift, the displacement can be accurately determined. FBG sensors are particularly useful for continuous monitoring in civil structures, deep boreholes, or geotechnical applications, and multiple sensors can be multiplexed along a single fiber for distributed measurements .

Macro-Bending Loss Sensors

Another approach is the macro-bending loss sensor, where displacement is converted into a controlled bend in the fiber. The bending causes light loss proportional to the displacement, which is measured by a photodetector . This method provides a wide measurement range and high sensitivity, making it suitable for large-scale structural monitoring.

Summary

The principle of fiber optic displacement testing relies on detecting changes in light properties—intensity, phase, or wavelength—caused by mechanical displacement. The choice of sensor type depends on the required measurement range, resolution, environmental conditions, and application. Intensity-based sensors are simple and versatile, interferometric sensors offer ultra-high precision, and FBG sensors enable distributed and continuous monitoring. Macro-bending sensors provide robust, large-range displacement detection for civil and industrial structures .