An aspherical microlens assembly for deep brain fluorescence microendoscopy

Fluorescence microendoscopy is becoming a standard technique in neuroscience for visualizing neuronal activity in the deep brain. Gradient refractive index (GRIN) lenses are increasingly used for fluorescence microendoscopy; however, they inherently suffer from strong aberrations and distortion. Aspherical lenses change their radius of curvature with distance from the optical axis and can effectively eliminate spherical aberrations. The use of these lenses has not been fully explored in deep brain fluorescence microendoscopy due to technical difficulties in manufacturing miniature aspherical lenses. In this study, we fabricated a novel microendoscope lens assembly comprised two nested pairs of aspherical microlenses made by precision glass molding. This assembly, which was 0.6 mm in diameter and 7.06 mm in length, was assembled in a stainless steel tube of 0.7 mm outer diameter. This assembly exhibited marked improvements in monochromatic and chromatic aberrations compared with a conventional GRIN lens, and is useful for deep brain fluorescence microendoscopy, as demonstrated by two-photon microendoscopic calcium imaging of R-CaMP1.07-labeled mouse hippocampal CA1 neurons. Our aspherical-lens-based approach offers a non-GRIN-lens alternative for fabrication of microendoscopic lenses.