Imaging through optical multimode fibers (MMFs) has the potential to enable hair-thin endoscopes that reduce the invasiveness of imaging deep inside tissues and organs. Current approaches predominantly require active wavefront shaping and fluorescent labeling, which limits their use to preclinical applications and frustrates imaging speed. Here we present a computational approach to reconstruct depth-gated confocal images from reflectance measurements in response to a proximally raster-scanned illumination. We evidence the potential of this approach by demonstrating quantitative phase, dark-field, and polarimetric imaging. Computational imaging through MMF opens a new pathway for minimally invasive imaging in medical diagnosis and biological investigations.
Open-loop spatio-spectral control of broadband light transmission through complex media such as optical multimode fiber (MMFs) requires a priori knowledge of the multispectral transmission matrix (msTM). However, acquisition of msTMs generally requires dense sampling at multiple wavelengths over the operating spectrum. Here we report on a computational spectral memory effect in a 1m long MMF. We demonstrate that the spectral correlation length among the spectral channels of a msTM can be extended 50-fold using a constant correction matrix between adjacent channels. This insight may stimulate efficient multispectral calibration methods that mitigate physical measurement limitations.
Multimode fiber (MMF) endoscopy offers high spatial resolution in an ultra-small form factor, yet several technical challenges remain to be addressed to realize practical MMF imaging. Here, we demonstrate a strategy for confocal reflectance imaging through MMF to improve contrast owing to optical sectioning and enable volumetric imaging. Instead of physically focusing the light into the sample, it uses a series of distinct illumination patterns obtained by varying the proximal coupling of the illumination. This alleviates the need for active wave-control and translates into a speed advantage critical for practical MMF imaging.
In contrast to conventional imaging systems that map an object point by point, measurements with random sensing functions in combination with computational reconstruction may afford novel imaging architectures. Here we demonstrate imaging of axial reflectivity profiles using random temporal-spatial encoding created by modal interference in a multimode fiber (MMF). Light from a broadband source (∆λ = 60nm) centered at 1310nm is split into a sample and a reference arm. In the sample arm, light in a single spatial mode is reflected by the axial reflectivity profile of the sample and coupled back into the same spatial mode. The reference light propagates through a MMF and interferes with the sample light in an off-axis geometry on a camera for holographic recording. Since the MMF supports various guided modes with distinct propagation constants, the short-coherence sample light only interferes with the spatial modes of the reference light that have matching path length. During an initial calibration procedure, interference patterns of a mirror reflection in the sample arm are recorded for varying axial mirror positions. Once this random sensing matrix (RSM) is established, the axial reflectivity profile of an object in the sample arm can be reconstructed from a single interference pattern by the multiplication with the inverse of RSM. By using a 2m long 0.22 NA MMF and tailoring the coupling regime within the MMF, we achieved axial ranging more than a centimeter. Flexible integration of polarization sensing or multi-focus imaging in a single snapshot could be envisioned in this random imaging architecture.
The recent progress of controlling light propagation in complex media has enabled the use of plain multimode fiber (MMF) as compact optical endoscope with sub-micron spatial resolution and sub-millimeter footprint. With the knowledge of the MMF’s transmission matrix (TM), the scrambling of the light propagating through the fiber can be compensated, physically or computationally. Current MMF endoscopes require distal access to calibrate the TM, which furthermore is vulnerable to perturbations and bending of the MMF. This necessitates repeated TM calibration or a rigid geometry that limits the intrinsic advantages of MMF. For practical applications of MMF endoscopy, calibration of the TM should be conducted without direct access to the distal fiber end. Here, we experimentally demonstrate that the forward and backward transmission through the MMF are the transpose of each other, imposed by the laws of optical reciprocity. This results in a transpose-symmetric double-pass TM (TM2x). Although it can be readily measured from the proximal side, the symmetry prevents unambiguous deduction of the single-pass TM from the measurement of TM2x. We then propose a strategy to obtain the single-pass TM in arbitrary fiber geometry by measuring TM2x with distinct pre-calibrated distal fiber elements and discuss the necessary conditions for the pre-calibrated elements to allow recovery of the single-pass TM. This proximal calibration technique may offer a pathway to flexible MMF endoscopy and find use in related applications involving measurement of TMs.
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