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Focus quality in raster-scan imaging via a multimode fiber

Zhouping Lyu, Gerwin Osnabrugge, Pepijn W. H. Pinkse, and Lyubov V. Amitonova

Open Access Open Access

Abstract

A multimode fiber (MMF) is a minimally invasive imaging probe. The most popular approach of MMF-based microscopy is raster-scan imaging, where the sample is illuminated by foci optimized on the fiber output facet by wavefront shaping (WFS). Imaging quality can be quantified by characteristic parameters of the optimized spots. We investigate the influence of the input light position on WFS through a round-core MMF with partial mode control, a situation often encountered in real life. We further demonstrate a trade-off between the shape and contrast of the foci generated on the output facet: the center input position is beneficial for high-contrast imaging, while the edge input position helps to reduce focus aberrations. These results are important for high field-of-view raster-scan imaging via an MMF.

© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

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Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (7)
Fig. 1.
Fig. 1. (a) Experimental setup (HWP, half-wave plate; PBS, polarizing beam splitter; M, mirror; L, lens; P, pinhole; DMD, digital micromirror device; BS, beam splitter; Obj, objective; MMF, multimode fiber. (b) Examples of the input fields before WFS at different input positions with zero phase on the DMD. (c), (d) Output speckle patterns measured without WFS for the input beams at (c) $d_{\text{in}} = {{0.1r}}$ and (d) $d_{\text{in}} = {{0.9r}}$.

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Fig. 2.
Fig. 2. (a) Example of the optimized phase pattern created by using 254 DMD segments for the input beams at $d_{\text{in}} = {{0.1r}}$, ${\rm{r}} = {{25}}\;\unicode{x00B5}{\rm m}$. (b) Sum of 10 foci optimized with the same parameter. Black circle shows the fiber core. (c) Zoomed-in foci at a distance of 0 µm (left), 15 µm (center), and 22.5 µm (right) from the fiber center. The corresponding ellipticities are 0.11 (left), 0.29 (middle), and 0.59 (right). The scale bar is 2 µm.

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Fig. 3.
Fig. 3. (a) Experimentally measured ellipticity $\varepsilon$, (b) power ratio PR, (c) beam waists $2w_{\min}$, and (d) $2w_{\max}$ averaged over 50 foci distributed over output facet as a function of the number of segments ($N_{\rm seg}$) on the DMD. Results of semianalytical simulations: (e) $\varepsilon$, (f) PR, (g) $2w_{\min}$, and (h) $2w_{\max}$ averaged over 50 foci distributed over output facet as a function of $N_{\rm seg}$. Different colors correspond to different distances between the input beam and the fiber center: $d_{\text{in}} = {{0.1r}}$ (black line), ${{0.3r}}$ (blue line), ${{0.5r}}$ (green line), ${{0.7r}}$ (red line), and ${{0.9r}}$ (magenta line). The black lines show the diffraction limit of 1.21 µm.

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Fig. 4.
Fig. 4. (a) Experimental and (b) simulated data for roundness, $1- \varepsilon$ (blue), and power ratio, PR (orange), averaged over 50 foci uniformly distributed on the fiber output facet and optimized by 314 DMD segments as a function of the input beam position (${d_{\text{in}}}$). The solid lines show results for the fiber core of 50 µm, fiber length of 30 cm (or 1 m) and plane wave on the DMD. Dashed lines show the results of simulation for a Gaussian beam on the DMD.

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Fig. 5.
Fig. 5. Ellipticity $\varepsilon$ (central column) and the power ratio PR (right column) of a generated focus on the fiber output facet as a function of the number of segments on the DMD ($N_{\rm seg}$) and the distance between the fiber center and the position of the optimized spot ($d_f$) for light coupled to the MMF at a distance about (a) ${{0.1r}}$, (b) ${{0.5r}},$ and (c) ${{0.9r}}$ as illustrated in the left column. White and red circles in the first column show the minimum and maximum sizes of the input beam after WFS, respectively. Yellow indicates good performance.

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Fig. 6.
Fig. 6. Semianalytical simulation of the ellipticity $\varepsilon$ and the PR averaged over 50 foci as a function of the number of segments for different parameters: Gaussian input beam, fiber with 1 m length, fiber with 105 µm diameter. Different colors correspond to different coupling distances between the input beam and the fiber center: $d_{\text{in}} = {{0.1r}}$ (black line), ${{0.3r}}$ (blue line), ${{0.5r}}$ (green line), ${{0.7r}}$ (red line), and ${{0.9r}}$ (magenta line).

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Fig. 7.
Fig. 7. Semianalytical simulation of the power ratio PR and $2w_{\min}$ averaged over 50 foci at a distance of 100 and 200 µm to the fiber facet as a function of $N_{\rm seg}$ for a fiber with 50 µm diameter and 30 cm length with a plane-wave input. Different colors correspond to different coupling distances between the input beam and the fiber center: $d_{\text{in}} = {{0.1r}}$ (black line), ${{0.3r}}$ (blue line), ${{0.5r}}$ (green line), ${{0.7r}}$ (red line), and $0.9r$ (magenta line). The horizontal black line at (b), (d) is the diffraction limit at the fiber facet.

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E out ( x , y ) = ∑ k N ⁡ α k E k ( x , y ) e − i β k L ,
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