Dynamic bending compensation while focusing through a multimode fiber

Abstract

Multimode fiber endoscopes have recently been shown to provide sub-micrometer resolution, however, imaging through a multimode fiber is highly sensitive to bending. Here we describe the implementation of a coherent beacon source placed at the distal tip of the multimode fiber, which can be used to compensate for the effects of bending. In the first part of this paper, we show that a diffraction limited focused spot can be generated at the distal tip of the multimode fiber using the beacon. In the second part, we demonstrate focusing even when the fiber is bent by dynamically compensating for it. The speckle pattern at the proximal fiber end, generated by the beacon source placed at its distal end, is highly dependent on the fiber conformation. We experimentally show that by intensity correlation, it is possible to identify the fiber conformation and maintain a focus spot while the fiber is bent over a certain range. Once the fiber configuration is determined, previously calibrated phase patterns could be stored for each fiber conformation and used to scan the distal spot and perform imaging.

Single multimode fiber endoscope

“…On the other hand, the excitation of multiple transverse modes with different propagation velocities produces a speckle field, product of the interference among all of the propagating modes, dismissing the MMFs as a standalone imaging device. Lately, a variety of new ideas using digital phase conjugation [3,4] and wavefront shaping techniques [5][6][7][8][9] have demonstrated the possibility of controlling the propagation of these modes showing their imaging potential. Papadopoulos et al [3] use digital optical phase conjugation to calibrate a MMF and create a focus in every point at the tip of the fiber, converting the MMF into a scanning fluorescence microscope.…”

“…Still, the requirement of accessing the distal tip, makes it difficult to implement in vivo. A second creative solution uses a single mode fiber and a holographic film in conjunction with the MMF to create a virtual beacon source at the distal tip of the fiber [4] enabling digital phase conjugation compensation of fiber bending. Furthermore, recent advances in theoretical models [10] have shown the possibility of predicting the bending angle based on the analysis of the transmission matrix of the fiber, and its potential for image correction.…”

Ultra-thin rigid endoscope: two-photon imaging through a graded-index multi-mode fiber

“…As has already been noted in several articles on lensless endoscopes based on MMF, the transmission matrix is extremely sensitive to twists and bends of the MMF [19,20]. Hence, the MMF has to remain static-this also applies for our ultra-thin rigid endoscope.…”

Towards new applications using capillary waveguides

“…Our CWG, in fact, would fit in a 23 gauge needle. Another possibility is to run calibrations for different bending states in order to create a discrete number of phase lookup tables, as proposed in [26]. Recently it was also demonstrated that is possible to calculate the right phase pattern in order to project and scan a focused spot through a MMF without any calibration step [27].…”