Focusing and Scanning through Flexible Multimode Fibers without Access to the Distal End

TL;DR

This paper introduces a robust, endoscopic method for focusing and scanning through flexible multimode fibers without needing access to the fiber's distal end, enabling lensless two-photon micro-endoscopy.

Contribution

It presents a new in-situ technique for controlling light focusing through uncharacterized, bent multimode fibers using proximal measurements, overcoming previous sensitivity issues.

Findings

Successful controlled focusing through bent fibers

Proximal light patterns reveal distal light distribution

Potential for lensless two-photon micro-endoscopy

Abstract

Multimode fibers (MMFs) are attractive ultra-thin replacements for state-of-the-art endoscopes, but the phase randomization in propagation through MMFs poses a major hurdle for imaging and focusing of light. Recently, this challenge has been addressed by pre-measuring the compensation for the fiber's complex input-output modes relations. Unfortunately, the sensitivity of this approach to fiber bending and temperature variations renders it inappropriate for many applications. Here, we demonstrate a truly endoscopic robust method for controlled in-situ focusing and scanning through a flexible uncharacterized MMF, whereby all the instrumentation is situated at the proximal end. We show that in graded-index (GRIN) fibers, light patterns at the proximal end allow retrieving information about the distal light distribution. We utilize these properties and two-photon fluorescence for robust controlled focusing through bended GRIN fibers. Our results carry potential for lensless two-photon micro-endoscopy.