Multi-wavelength confocal displacement sensing using a highly dispersive flat-field concave grating.

A multi-wavelength confocal displacement sensor based on a flat-field concave grating (FFCG) was proposed and designed; the large dispersion and small volume of the FFCG make it an ideal candidate for replacing the complex dispersive lens group. The designed displacement sensor was calibrated by displacement meter, and the characteristics were measured. Consequently, for the proposed displacement sensor, the displacement range of 6.8 mm was measured with the R-square linearity evaluation coefficient of 0.998, and the sensitivity preceded 17.1 nm/mm. The resolution of the displacement sensor was characterized by 70 µm, as well as a full width at half maximum (FWHM) fluctuating around 1.63 nm, indicating high precision and accuracy in displacement measurement. Moreover, the stability and reliability of the sensor were verified within 20 min, with no significant wavelength shifts, and gentle power fluctuations of 557.73 counts at 520 nm and 563.67 counts at 545.05 nm, respectively.