Novel concept of GRIN optical systems for high resolution microendoscopy: Part 1. Physical aspects

Confocal and multi-photon imaging systems are currently miniaturized to fit them to endoscopic size requirements of probe diameters often less than 2.8 mm. High resolution in lateral dimensions of less than I micron and in axial dimensions of less than 10 microns is desired to resolve sub-cellular details of in-vivo tissue. GRIN rod lens systems have been widely used as high numerical aperture objectives in these applications because of their small size, good image quality and favourable geometry with plane optical surfaces, which allows easy assembly. Image generation has been enabled by combination with coherent imaging fiber bundles, GRIN relay lenses and scanning single fibers or photonic band gap fibers. With previous GRIN systems of maximum NA of 0.50, a resolution limit laterally of approx. 1.0 micron and axially of 5.5 to 10 microns was obtained in the case of two-photon excitation. Here, we introduce a novel concept of a GRIN lens objective system with significantly higher NA yielding resolution improved by a factor of two (lateral) and four (axial). The image quality of initial 1.0 mm GRIN components will be characterized by the signal analysis of 0.2 micron fluorescent beads in the multi-photon scheme and by conventional image tests using test grids with transmitting illumination. Potentials for further miniaturization and for changing the direction of view will be discussed.