One limitation of microendoscopy is the device footprint that should be minimal for many applications. Here we present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. With the use of a fast spatial-light modulator, it is possible to rapidly learn the transmission matrix during a prior calibration step. A focused spot can then be produced to raster-scan a sample at the distal tip of the fiber. Our setup provides both photoacoustic and fluorescence microscopic images of test samples in vitro (fluorescent beads and red blood cells) through a single multimode fiber.
Many applications of microendoscopy, including brain imaging, requires minimally invasive devices to minimize damage during insertion in the tissue. Here we present a minimally-invasive endoscope based on a multimode fiber that combines photoacoustic and fluorescence sensing. By learning the transmission matrix during a prior calibration step, a focused spot can be produced and raster-scanned over a sample at the distal tip of the fiber by use of a spatial light modulator. We demonstrate that our setup provides both photoacoustic and fluorescence microscopic images of test samples in vitro (fluorescent beads and red blood cells) through the same fiber.
We present a ultra-thin system that combines optical resolution photoacoustic microscopy and fluorescence imaging based on a multimode fiber and a fiber optical hydrophone with only 250μm cross section.
We present an ultra-thin endoscope that combines a multimode optical fiber (MMF) attached to
an optical hydrophone for simultaneous optical-resolution photoacoustic microscopy and fluorescence
imaging. The MMF is used for light delivery and fluorescence collection and the hydrophone
for acoustic detection; a digital micro-mirror device (DMD) modulates the amplitude of the optical
wavefront of a pulsed laser coupled into the MMF, controlling the illumination at the distal tip.
The DMD allows for fast calibration approaches to reach calibration and measurement times of a
few seconds.
We obtain optical-diffraction-limited images with full field illumination recording the intensity
of a series of various calibrated speckle patterns produced by different configurations of the DMD
at the input, with no wavefront shaping. The intensity fluctuations from speckle pattern to speckle
pattern encodes for the position at which the signal is emitted. The fluorescence signal from the sample is
collected with the MMF and detected with a PMT at the proximal side. For the acoustic detection,
embedding the ultrasound detection within the device avoids the absorption of high-frequency ultrasound
by the tissue and therefore removes any limitation on the insertion depth. The footprint of
the probe is 250 um x 125 um making it thinner than common GRIN lenses used for endoscopy.
To best of our knowledge, our approach provides the thinnest endoscope head capable
of obtaining optical-resolution photoacoustic and fluorescence images simultaneously.
Recent progress in controlling light propagation in multimode fibers in the linear regime, opened new opportunities for multimode fiber endoscopy. However, nonlinear light propagation in multimode fibers comprises complex intermodal interactions and rich spatiotemporal dynamics. In this work, we demonstrate a wave-front shaping approach for controlling nonlinear phenomena in multimode fibers. Using a spatial light modulator at the fiber’s input and a genetic algorithm optimization, we control a highly nonlinear stimulated Raman scattering cascade and its interplay with four wave mixing via a flexible implicit control on the superposition of modes that are coupled into the fiber. We demonstrate versatile spectrum manipulations that could be used to generate a multi-wavelength, tunable source. The wavefront shaping control allows spectral shifting and modal tuning. A theoretical analysis of modal phase matching in graded index multi-mode fibers is presented and we suggest potential bio-imaging applications.
Optical-resolution photoacoustic microscopy offers a specific contrast to optical absorption. The limiting penetration depth of current techniques due to scattering produced by tissues makes endoscopic approaches attractive for photoacoustic imaging deep inside biological structures. Conventional approaches generally involves mechanically raster scanning a focused spot over the sample and acquiring an acoustic signal for each spot. Here, we demonstrate that a full-field illumination approach with multiple known speckle patterns generated by a multimode fiber can also provide diffraction-limited optical-resolution photoacoustic images. As a proof of principle we experimentally image micro-structured test samples illuminated with reference speckle patterns measured during a calibration step. A digital micromirror device modulating the incident light coupled into a multimode fiber provides the different speckle patterns at the distal tip of the fiber where the sample is placed.
We study and compare the performance in simulations and experiments of three different approaches; the first method is based on cross-correlation between the photoacoustic signal under multiple speckle illumination with the calibrated known speckle patterns, following approaches from ghost imaging. The second method is based on computing the pseudo-inverse of the reference matrix obtained from the calibration step. A third method based on compressed sensing exploits the sparsity of the sample achieving reconstructed images with a number of speckle realizations smaller than the number of speckle grains. Additionally, speckle-illumination-based photoacoustic microscopy provides a powerful framework for the development of novel reconstruction approaches, that can demand less computation time in case of compressed sensing approaches.
We present a ultra-thin endoscopy system for optical resolution photoacoustic microscopy. The system is based on a silica capillary waveguide of two hundred microns of diameter. The silica tube acts as a multi-mode optical waveguide for the illumination, while the hollow core of the capillary carries a fiber-based optical hydrophone to detect the photoacoustic waves. Embedding the ultrasound detection within the device avoids the absorption of high-frequency ultrasound by the tissue and therefore removes any limitation on the insertion depth.
To control the illumination at the distal tip of the capillary, a digital micromirror device modulates the amplitude of the optical wavefront which is coupled into the capillary. The DMD allows for fast calibration approaches to reach calibration and measurement times of a few seconds, as compared with current approaches limited to hours. We obtain optical-diffraction-limited images with full field illumination recording the intensity of a series of various speckle patterns produced by different configurations of the DMD at the input, with no wavefront shaping. The intensity fluctuations from shot to shot codes for the position at which it is measured. Computational methods based on correlation, pseudo-inverse and compressed sensing approaches are investigated and compared with raster-scanning an optimized focus for image reconstruction. To best of our knowledge, our approach provides the thinest endoscope head capable of obtaining optical resolution photoacoustic images.
We demonstrate a single multi-mode fiber-based micro-endoscope for measuring blood flow speeds. We use the transmission-matrix wavefront shaping approach to calibrate the multi-mode fiber and raster-scan a focal spot across the distal fiber facet, imaging the cross-polarized back-reflected light at the proximal facet using a camera. This setup allows assessment of the backscattered photon statistics: by computing the mean speckle contrast values across the proximal fiber facet we show that spatially-resolved flow speed maps can be inferred by selecting an appropriate camera integration time. The proposed system is promising for minimally-invasive studies of neurovascular coupling in deep brain structures.
Recent advances in wavefront control, spatial light modulators, and computational power enable the use of a single multimode fiber as a fluorescence scanning microscope. We explore multimode fibers with different characteristics (diameter, index profile, etc.) and compare their performance regarding robustness against external perturbations and quality of the scanning focus.
We demonstrate the capabilities of a high-speed phase modulation system based on a digital micromirror device. We use the system to focus light through dynamic scattering materials. We demonstrate up to three orders of magnitude speed improvement respect to previous systems based on liquid-crystal spatial light modulators. Furthermore, the system can be adapted to maintain a focus through a perturbed multimode fiber and help convert it into a micro-endoscope.
The optical imaging depth in biological materials is limited by the scattering of light in tissue. New methods which control light propagation through scattering media have been introduced with the potential to overcome the scattering of light in biological materials. These techniques shape the incident wavefront to pre-compensate for the scattering effects of light propagation in the material and beyond. However, living biological materials have speckle decorrelation times on the millisecond timescale. This fast rate of change makes liquid crystal spatial light modulation (LC-SLM) devices too slow for this task. To achieve the required wavefront control with high modulation speeds we present binary-amplitude off-axis computer-generated holography implemented on a digital micro-mirror device (DMD). Binary amplitude off-axis holography is a method for the generation of arbitrary wavefronts, and in particular uniform-amplitude phase-modulated images. As a result, we are able to simultaneously encode phase modulated wavefronts at the high frame rate of binary amplitude DMDs. This wavefront encoding technique allows for focusing through temporally dynamic turbid materials at a rate which approaches the decorrelation time of living biological tissue. We demonstrate this technique by high speed wavefront optimization for focusing through turbid media as well as through a dynamic, strongly scattering sample with short speckle decorrelation times. With this approach we attain an order of magnitude improvement in measurement speed over the previous fastest wavefront determination method and three orders of magnitude improvement over LC-SLM methods.
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