Integrated Photonics-Based Focusing through Multimode Fibers

Abstract

A breakthrough in endoscopy imaging resolution, coupled with a drastic reduction in its invasiveness, is on the horizon with emerging techniques based on multimode fibers. These imaging techniques rely on illumination wavefront control to deliver diffraction-limited images through a thin multimode fiber (diameter ∼ 100 μm). By controlling the wavefront at the fiber’s input, a grid of focused spots can be raster scanned at the fiber’s end to form an image. However, there’s room for performance improvement in spatial light modulators commonly used for focusing through multimode fibers. The ones based on liquid crystals suffer from low sub-kHz modulation speed, while the other, digital micromirror devices, require an intricate optical setup to achieve preferred phase modulation. We propose a novel approach for focusing through a fiber by employing a photonic integrated circuit to modulate the wavefront, which brings improvements in the modulation rate and the optical setup compactness. Using an optical phased array with 128 antennas, operating at a wavelength of λ = 852 nm, we demonstrate focusing through a graded-index multimode fiber. Spot size as low as 2.3 μm, approaching the diffraction limit, and an average focus power ratio equal to 93% of the theoretical maximum are achieved. Additionally, we demonstrate 2D focus steering at the fiber’s distal end for raster-scan imaging. By focusing in different planes, we show that OPAs can enable volumetric imaging through a multimode fiber. Finally, our device was fabricated on a silicon nitride platform, offering the prospect of large-scale fabrication.