Improved chromatical and field correction of high-NA GRIN-based endomicroscopic imaging systems for new biophotonics applications

Endomicroscopic objectives have been used for linear confocal as well as nonlinear quasi-confocal imaging processes for many years, especially in medical and neuroscience applications. State-of-the-art devices achieve sub-cellular resolutions by combining plano-convex lenses with special high-NA silver-doped GRIN lenses and diffractive optical elements for their chromatic correction. NAs of 0.8 are achieved while keeping the outer diameter of the mounted objective as thin as 1.4 mm. Recently developed designs correct two major drawbacks of the state-of-the-art devices and are presented in comparison with their precursors. With these developments, the diffraction-limited field of view is increased by 350% in diameter for monochromatic corrected devices and even more for the polychromatic corrected ones. Besides, solutions for chromatic corrected objectives are presented which avoid diffractive optical elements and rather make use of achromatic lenses. The design concepts of those recently developed objectives are presented here and corresponding prototypes are evaluated by confocal and quasi-confocal experiments as well as by wavefront measurements.