Dynamic bending compensation while focusing through a multimode fiber

Salma Farahi,Demetri Psaltis,David Ziegler,Christophe Moser,Ioannis N Papadopoulos

doi:10.1364/oe.21.022504

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.

Highlights

Imaging inaccessible structures has become possible by modern endoscopy with applications in medical diagnosis and treatment as well as in manufacturing where the visualization of hidden structures is important to increase the quality of manufactured goods
The calculated phase is displayed on a Spatial Light Modulator (SLM), which assigns the information to a plane wave, that in turn forms the phase conjugate optical field
We suggest the following process for imaging: first, the fiber conformation is determined by correlation with the intensity speckle pattern stored in a database, second the ensemble of phase patterns used for raster scanning corresponding the fiber conformation are recalled form the database and displayed sequentially on the SLM

Summary

Introduction

Imaging inaccessible structures has become possible by modern endoscopy with applications in medical diagnosis and treatment as well as in manufacturing where the visualization of hidden structures is important to increase the quality of manufactured goods. The first is the miniaturization of optical elements (micro GRIN lenses) [1,2,3] and the second is a fiber-based approach [1,4,5,6] for flexible endoscopes. A conventional endoscope is a fiber bundle where each fiber corresponds to a single pixel of the captured image. The image resolution is limited by the fibers core-to-core separation. There are approaches using a single mode fiber for excitation and a multimode fiber for light collection of reflected or fluorescent signal whereby miniaturized actuators scan a focused light spot at the distal tip [1]

Results

Discussion

Conclusion