Endoscopy is a key technology in biomedical engineering. It enables minimal invasive optical access deep into tissue. State of the art is to use coherent fiber bundles (CFB) in conjunction with rigid lens systems. Thus, structures can be detected in a fixed distance to the probe tip. However, the lens system limits the minimum diameter of the endoscope to several millimeters. Through imperfections, core-to-core crosstalk and bending sensitivity of the fiber only the intensity can be evaluated, and the phase information of the light gets lost. By a compensation of the phase distortion a remote phased array is enabled. Therefore, it is possible to eliminate any lens at the probe tip. Phase compensation and beam steering can be assumed by an external spatial light modulator (SLM). Thus, in principle thinner, lensless, holographic endoscopes with a three-dimensional adjustable focus for imaging and illumination can be realized. Several techniques on single mode CFB and multi-mode fibers have been presented. As a drawback they require double sided access to the fiber for the calibration, while single sided, in-vivo calibrations is essential due to the bending sensitivity of the CFB.
We present the method of virtual guide star calibration by using a reflective plane with the same diameter as the CFB, only 500 μm. The design enables the generation of diffraction limited foci in a range of 150 μm x 150 μm x 1000 μm. By using a galvanometer mirror in addition to the SLM, a video rate capability of 10 Hz can be achieved for a lateral scan. The technique enables a paradigm shift for micro endoscopy and laser-assisted surgery. For the future we expect this approach will lead us to create a tool for deeper tissue imaging.