Delivery of focused short pulses through a multimode fiber

TL;DR

This paper introduces a method to deliver focused ultrafast pulses through multimode fibers by compensating for spatial distortions and modal dispersion, enabling high-quality multiphoton imaging.

Contribution

It presents a selective phase conjugation technique that excites modes with similar group velocities to minimize temporal broadening of ultrafast pulses in multimode fibers.

Findings

Delivered 500 fs pulses with minimal broadening

Achieved pulse duration 1/30th of uncorrected pulses

Confirmed two-photon absorption at the output

Abstract

Light propagation through multimode fibers suffers from spatial distortions that lead to a scrambled intensity profile. In previous work, the correction of such distortions using various wavefront control methods has been demonstrated in the continuous wave case. However, in the ultra-fast pulse regime, modal dispersion temporally broadens a pulse after propagation. Here, we present a method that compensates for spatial distortions and mitigates temporal broadening due to modal dispersion by a selective phase conjugation process in which only modes of similar group velocities are excited. The selectively excited modes are forced to follow certain paths through the multimode fiber and interfere constructively at the distal tip to form a focused spot with minimal temporal broadening. We demonstrate the delivery of focused 500 fs pulses through a 30 cm long step-index multimode fiber. The achieved pulse duration corresponds to approximately 1/30th of the duration obtained if modal dispersion was not controlled. Moreover, we measured a detailed two-dimensional map of the pulse duration at the output of the fiber and confirmed that the focused spot produces a two-photon absorption effect. This work opens new possibilities for ultra-thin multiphoton imaging through multimode fibers.