Neuroscience research related to functionality, connectivity and metabolism of neuronal circuits, individual neuronal cells and sub-cellular structures, nowadays, experiences a burgeoning need to develop techniques for the detailed investigation inside the complexity of the living matter. Particularly, high-resolution observations combined with an extended depth of penetration in tissue represents an ongoing challenge.
Holographic control of light propagation in complex media opens a promising way to overcome this technological barrier via exploiting multimode fibres as hair-thin, minimally-invasive endoscopes. This concept allows for more than one order of magnitude reduction of the instrument’s footprint and a significant enhancement of imaging resolution, compared with current minimally invasive endoscopes.
Here, we demonstrate a compact and high-speed system for fluorescent imaging at the tip of a fibre. The instrument’s performance reaches micron-scale resolution across a field of view 50 micrometres, yielding 7-kilopixel image information at a rate of 3.5 frames per second. The resolution limit is dictated only by the numerical aperture of the fibre probe, and the contrast/pureness of the focal points, utilised for raster-scanning regime, approach the theoretical limits for phase-only holographic wavefront shaping.
The achieved performance allowed for in-vivo observations of neuronal somata and processes, residing deep inside the visual cortex and hippocampus of an animal model with minimal damage to the tissue surrounding the fibre penetration area.
We believe that this demonstration represents an important step towards implementations of various advanced forms of imaging through multimode fibre based endoscopes to address numerous key challenges in neuroscience.
Digital micro-mirror devices (DMDs) have recently emerged as practical spatial light modulators (SLMs) for applications in photonics, primarily due to their modulation rates, which exceed by several orders of magnitude those of the already well-established nematic liquid crystal (LC)-based SLMs. This, however, comes at the expense of limited modulation depth and diffraction efficiency. Here we compare the beam-shaping fidelity of both technologies when applied to light control in complex environments, including an aberrated optical system, a highly scattering layer and a multimode optical fibre. We show that, despite their binary amplitudeonly modulation, DMDs are capable of higher beam-shaping fidelity compared to LC-SLMs in all considered regimes.
We have developed an all-solid, step-index multimode fibre based on compound "soft-glasses" yielding a very-high NA reaching 0.96 at 1064nm. By further extending the methods of holographic control of light propagation in multimode fibres, we were able to mitigate the adverse effect of mode-dependent loss affecting the new fibre type. This enabled harnessing the full available NA almost completely, and demonstrating high-resolution focussing with output NAs up to 0.91 through lensless fibres. Further, we show that the NA and pureness of such foci allow stable three-dimensional optical confinement of micrometre-sized dielectric objects. Being inherently holographic, this technique is capable of generating an arbitrary number of optical tweezers, as well as precisely repositioning them independently in all directions. The versatility of the new instrument is demonstrated by simultaneous and dynamic 3D manipulation of large assemblies of dielectric microparticles, as well as manipulation of micro-objects inside optically inaccessible environments such a turbid cavity through an opening as small as 0.1mm.
Moreover, the possibility of generating aberration-free foci with NA approaching 0.9 across the fibre core opens new perspectives for high-resolution holographic micro-endoscopy, paving the way for the delivery of advanced microscopy techniques through hair-thin fibre-optic probes.
Multimode fibers are a promising tool for high resolution, low-cost, minimally invasive endoscopic imaging. The fiber can be used both to illuminate the sample, which may be buried deep inside the tissue, and to collect the backreflected light. Except for the bare fiber, no other imaging optics have to be inserted, enabling a device with a very small diameter. However, light propagating through the fiber is scrambled before it hits the sample. This renders straightforward imaging impossible, but if this scrambling is known with high accuracy, for instance because the transmission matrix has been measured, the scrambling process can be compensated before the light enters the fiber. For step index multimode fibers, where the refractive index profile consists of a cylindrical core with a constant but higher refractive index than the cladding, it has been shown that the transmission matrix can be predicted for any fiber orientation. Graded index fibers (GIF), where the refractive index profile resembles a parabola, offer numerous advantages, most prominently they are much less sensitive to bending. We measured the transmission matrix of a large GIF and show that we can fully understand the transmission matrix in terms of guided fiber modes, and simultaneously acquire accurate knowledge of the refractive index profile. We also show that although the quality of a commercially available graded index fiber is not sufficient to perform the same analysis, imaging performance of a graded index fiber is much more resilient to bending than the imaging performance of a comparable step index fiber. This demonstrates the need for a graded-index fiber with a high quality refractive index profile.
Using spatial light modulators(SLM) to control light propagation through scattering media is a critical topic for various applications in biomedical imaging, optical micromanipulation, and fibre endoscopy.
Having limited switching rate, typically 10-100Hz, current liquid-crystal SLM can no longer meet the growing demands of high-speed imaging. A new way based on binary-amplitude holography implemented on digital micromirror devices(DMD) has been introduced recently, allowing to reach refreshing rates of 30kHz.
Here, we summarise the advantages and limitations in speed, efficiency, scattering noise, and pixel cross-talk for each device in ballistic and diffusive regimes, paving the way for high-speed imaging through multimode fibres.