Plastic Optical Fiber Sensor for Displacement Monitoring with Dual-Wavelength Compensation of Power Fluctuations

The paper describes the use of a dual-wavelength technique for compensating power fluctuations in intensity-based plastic optical fiber sensors for displacement measurements. The sensor consists of a fiber that collects the reflected light from a target and retrieves its distance from the signal attenuation. Besides a measurement signal at 450 nm that travels back and forth the displacement region and is attenuated according to the target position, a compensation signal at 650 nm propagates along the fiber and is back-reflected at the fiber tip by a dichroic filter. The compensation signal undergoes power fluctuations due to environmental stress of the fiber and is therefore used as a reference to correct the measurement signal. A demonstrator, which includes a sensor, the acquisition circuitry and signal processing software, was built and tested in laboratory and proved that the measurement signal is capable of monitoring displacements in the range (0 divided by 10) mm even at low power, while the reference signal is not influenced by the position of the reflecting target and can therefore be used as reliable reference.