Dual-range axial chromatic focal splitting in confocal metrology enabled by a hybrid diffractive–refractive bifocal lens

Spectral confocal metrology exploits the axial chromatic dispersion of optical systems for measuring surface profiles and thicknesses of partially reflective and transparent samples. Previous approaches are limited to a single axial range of chromatic focal shift, spanning from the shortest to the longest wavelength, as the depth-sensing range. Here, we present for the first time, to our knowledge, an approach in which a bifocal hybrid lens enables two independently adjustable, distinct axial regions of chromatic focal shift to serve as depth-measurement ranges. By exploiting two independently addressable axial spectral regions, the approach enables an extended overall depth-measurement range, provides control over the axial positions of the spectral foci, and permits tailoring of their relative intensities via the spectral diffraction efficiency. Consequently, the design flexibility is significantly increased, allowing adaptation to a wide range of applications. These concepts are demonstrated using simple bifocal hybrid optical designs optimized through ray-tracing simulations.