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 ÷ 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.