Graded-index (GRIN) lenses have been widely used for developing compact imaging devices due to the small dimensions and simple optics designs. GRIN lenses, however, have intrinsic aberration which causes a distortion of the image and thus are subject to limited resolution and blurred imaging quality. Here, we employ the high-precision wavefront measurement technique for compensation of the distortion of a GRIN lens to obtain a high-resolution and high-contrast image. In doing so, we demonstrate a high-resolution and ultra-thin endo-microscope using a GRIN. A reflection-type interferometric microscope through a GRIN lens was constructed using multiple lasers (473 nm, 532 nm, and 633 nm) as light sources. The characteristics of the aberration of the GRIN lens were measured using the digital holographic method. The distortion of the GRIN lens was removed by numerical image processing with the prior information from the pre-calibration. We apply this technique to a reflection image of biological tissues acquired by our custom-built GRIN lens probe. Consequently, a diffraction limited lateral resolution as well as improved axial resolution can be achieved. Our approach will facilitate the use of GRIN lenses for compact imaging devices without compromising optical resolution and image quality.
A graded-index (GRIN) lens is suitable for developing an ultra-thin endoscope due to its small diameter and simplicity for optics design. A GRIN lens, however, generates intrinsic optical aberration causing low resolution and poor imaging quality. Recently, wavefronts of light can be measured with very high precision and the optical aberration can be corrected in numerical ways even for the case of highly scattering media. In this study, based on the high precision wavefront sensing and numerical image processing techniques, we demonstrate a high-resolution and ultra-thin endo-microscope using a GRIN rod lens as a core imaging optics. We constructed a reflection-type interferometric microscope through a GRIN rod lens using a p-polarized Nd:YAG laser (532 nm) as a light source. By recording and processing blank transmission images as a function of various illumination states, the characteristics of the aberration generated by the GRIN lens were obtained. After this pre-calibration, we could numerically compensate the aberration induced onto a reflection image of an object. Consequently, a diffraction limited lateral resolution as well as improved axial resolution could be achieved. Our approach will fascinate the use of GRIN lenses for compact and high-resolution imaging devices including ultra-thin endo-microscopes.