Improved chromatic confocal displacement-sensor based on a spatial-bandpass-filter and an X-shaped fiber-coupler

To improve the positioning capabilities of a chromatic confocal displacement measurement system, we introduce a super-resolving pupil filtering element (referred to as spatial-bandpass-filter), an X-shaped fiber-coupler, and a modified peak-extraction algorithm. The spatial-bandpass-filer decreases the spherical aberration of conventional refractive lenses and transmission loss, without changing the dispersion range. The new optical design minimizes the effect of light-source-variation as well as reflectance-variation of the sample surface against the wavelength by adding a reference arm with an X-shaped fiber-coupler and normalization algorithm. This enables rapid re-purposing to obtain data for crosstalk, reference and measurement signals. A prototype setup was built and the experimental results show that the new chromatic dispersion system can reach a position stability of ± 0.36 μm using a commercial cost-effective white-light source. It reaches an axial displacement resolution of up to 1 μm and a peak-fit error below ± 0.2% for a range of 1.05 mm.