Extended focal depth Fourier domain optical coherence microscopy with a Bessel-beam - LP02 mode - from a higher order mode fiber.

Dipankar Sen,Lars Grüner-Nielsen,Aart J Verhoef,Alexei V Sokolov,Rainer A Leitgeb,Anton Classen,Alma Fernández,Shahriar Esmaeili,Holly C Gibbs,Philip Hemmer,Andrius Baltuska

doi:10.1364/boe.442081

Abstract

We present a robust fiber-based setup for Bessel-like beam extended depth-of-focus Fourier-domain optical coherence microscopy, where the Bessel-like beam is generated in a higher order mode fiber module. In this module a stable guided LP02 core mode is selectively excited by a long period grating written in the higher order mode fiber. Imaging performance of this system in terms of lateral resolution and depth of focus was analyzed using samples of suspended microbeads and compared to the case where illumination is provided by the fundamental LP01 mode of a single mode fiber. Illumination with the LP02 mode allowed for a lateral resolution down to 2.5 µm as compared to 4.5 µm achieved with the LP01 mode of the single mode fiber. A three-fold enhancement of the depth of focus compared to a Gaussian beam with equally tight focus is achieved with the LP02 mode. Analysis of the theoretical lateral point spread functions for the case of LP01 and LP02 illumination agrees well with the experimental data. As the design space of waveguides and long-period gratings allows for further optimization of the beam parameters of the generated Bessel-like beams in an all-fiber module, this approach offers a robust and yet flexible alternative to free-space optics approaches or the use of conical fiber tips.

Highlights

Optical coherence tomography (OCT) is a three-dimensional imaging modality which has proven to be a powerful tool for biological imaging and healthcare diagnostics [1,2,3]
We present a robust fiber-based setup for Bessel-like beam extended depth-of-focus Fourier-domain optical coherence microscopy, where the Bessel-like beam is generated in a higher order mode fiber module
Illumination with the LP02 mode allowed for a lateral resolution down to 2.5 μm as compared to 4.5 μm achieved with the LP01 mode of the single mode fiber

Summary

Introduction

Optical coherence tomography (OCT) is a three-dimensional imaging modality which has proven to be a powerful tool for biological imaging and healthcare diagnostics [1,2,3]. A fundamentally different, and very attractive, approach to extend the DOF beyond the limit given by Gaussian optics that was proposed for FD-OCT is based on wavefront engineering [9,27,28,29] to generate so-called diffraction-less (self-healing) beams (most prominently Bessel beams), to be used instead of the traditional Gaussian beam illumination. Because of this feature Bessel beams are considered to be advantageous for imaging within scattering media. We compare the performance of the system with the fiber-generated Bessel-like LP02 beam to the performance of the same system where the HOM fiber is replaced by a single mode fiber providing a Gaussian-like beam

Optical setup

Simulations

Results and discussion