Mathematical modeling and experimental validation of the impact of the incident angle in optical precision metrology

This work presents a simple and non-contact method to determine the angular displacement of a target measured with a bifurcated Optical Fiber Displacement Sensor (OFDS). Target angular displacements are estimated from the intensity-modulation variations of the reflected light. The OFDS consists of a central single-mode optical fiber (OF) and two concentric multimode OF rings. The method has been validated to analyze the alignment stability of a mechanical structure, such as the blades of an aircraft. Results reveal that theoretical model and experimental data are in good agreement, and that the rotation angle of the target is a trade-off value since increasing it enhances the sensitivity of the sensor, at the cost of decreasing its linear working area. In fact, compared to the results when the target and the OFDS are at 90º, results indicate that the sensitivity of the device can be enhanced 4 times by positioning the target at an incident angle of 10º, although in return, its linear working region decreases 60 %. Moreover, the blind region decreases to 50 %.