[Show abstract][Hide abstract] ABSTRACT: In deep tissue photoacoustic imaging, the spatial resolution is inherently
limited by acoustic diffraction. Moreover, as the ultrasound attenuation
increases with frequency, resolution is often traded-off for penetration depth.
Here we report on super-resolution photoacoustic imaging by use of multiple
speckle illumination. Specifically, we show that the analysis of second-order
fluctuations of the photoacoustic images combined with image deconvolution
enables resolving optically absorbing structures beyond the acoustic
diffraction limit. A resolution increase of almost a factor 2 is demonstrated
experimentally. Our method introduces a new framework that could potentially
lead to deep tissue photoacoustic imaging with sub-acoustic resolution.
[Show abstract][Hide abstract] ABSTRACT: Multimode fibers (MMFs) are attractive ultra-thin replacements for
state-of-the-art endoscopes, but the phase randomization in propagation through
MMFs poses a major hurdle for imaging and focusing of light. Recently, this
challenge has been addressed by pre-measuring the compensation for the fiber's
complex input-output modes relations. Unfortunately, the sensitivity of this
approach to fiber bending and temperature variations renders it inappropriate
for many applications. Here, we demonstrate a truly endoscopic robust method
for controlled in-situ focusing and scanning through a flexible uncharacterized
MMF, whereby all the instrumentation is situated at the proximal end. We show
that in graded-index (GRIN) fibers, light patterns at the proximal end allow
retrieving information about the distal light distribution. We utilize these
properties and two-photon fluorescence for robust controlled focusing through
bended GRIN fibers. Our results carry potential for lensless two-photon
[Show abstract][Hide abstract] ABSTRACT: We present a method to measure the spectrally-resolved transmission matrix of
a multiply scattering medium, thus allowing for the deterministic
spatiospectral control of a broadband light source by means of wavefront
shaping. As a demonstration, we show how the medium can be used to selectively
focus one or many spectral components of a femtosecond pulse, and how it can be
turned into a controllable dispersive optical element to spatially separate
different spectral components to arbitrary positions.
[Show abstract][Hide abstract] ABSTRACT: Noninvasive label-free imaging of biological systems raises demand not only for high-speed three-dimensional prescreening of morphology over a wide-field of view but also it seeks to extract the microscopic functional and molecular details within. Capitalizing on the unique advantages brought out by different nonlinear optical effects, a multimodal nonlinear optical microscope can be a powerful tool for bioimaging. Bringing together the intensity-dependent contrast mechanisms via second harmonic generation, third harmonic generation and four-wave mixing for structural-sensitive imaging, and single-beam/single-pulse coherent anti-Stokes Raman scattering technique for chemical sensitive imaging in the fingerprint region, we have developed a simple and nearly alignment-free multimodal nonlinear optical microscope that is based on a single wide-band Ti:Sapphire femtosecond pulse laser source. Successful imaging tests have been realized on two exemplary biological samples, a canine femur bone and collagen fibrils harvested from a rat tail. Since the ultra-broad band-width femtosecond laser is a suitable source for performing high-resolution optical coherence tomography, a wide-field optical coherence tomography arm can be easily incorporated into the presented multimodal microscope making it a versatile optical imaging tool for noninvasive label-free bioimaging.
[Show abstract][Hide abstract] ABSTRACT: This paper investigates experimental means of measuring the transmission
matrix (TM) of a highly scattering medium, with the simplest optical setup.
Spatial light modulation is performed by a digital micromirror device (DMD),
allowing high rates and high pixel counts but only binary amplitude modulation.
We used intensity measurement only, thus avoiding the need for a reference
beam. Therefore, the phase of the TM has to be estimated through signal
processing techniques of phase retrieval. Here, we compare four different phase
retrieval principles on noisy experimental data. We validate our estimations of
the TM on three criteria : quality of prediction, distribution of singular
values, and quality of focusing. Results indicate that Bayesian phase retrieval
algorithms with variational approaches provide a good tradeoff between the
computational complexity and the precision of the estimates.
[Show abstract][Hide abstract] ABSTRACT: We present an approach for two-dimensional (2D) imaging through a single
single-mode or multimode fiber without the need for scanners. A random
scattering medium placed next to the distal end of the fiber is used to encode
the collected light from every input spatial position with a different random
spectral signature. We demonstrate 2D imaging of objects illuminated by a
white-light fiber-coupled LED from a single measured spectrum. The technique is
insensitive to fiber bending, an advantage for endoscopic applications.
[Show abstract][Hide abstract] ABSTRACT: The recent theory of compressive sensing leverages upon the structure of signals to acquire them with much fewer measurements than was previously thought necessary, and certainly well below the traditional Nyquist-Shannon sampling rate. However, most implementations developed to take advantage of this framework revolve around controlling the measurements with carefully engineered material or acquisition sequences. Instead, we use the natural randomness of wave propagation through multiply scattering media as an optimal and instantaneous compressive imaging mechanism. Waves reflected from an object are detected after propagation through a well-characterized complex medium. Each local measurement thus contains global information about the object, yielding a purely analog compressive sensing method. We experimentally demonstrate the effectiveness of the proposed approach for optical imaging by using a 300-micrometer thick layer of white paint as the compressive imaging device. Scattering media are thus promising candidates for designing efficient and compact compressive imagers.
[Show abstract][Hide abstract] ABSTRACT: A new coherent anti-Stokes Raman spectroscopy (CARS) technique is reported for real-time detection and classification of several chemical constituents, utilizing a single detector and a single beam of shaped femtosecond pulses. The technique is based on rapidly switching between differently shaped pulses that either maximize or minimize the targeted vibrational lines excitation, thus creating temporally modulated 'bright' and 'dark' profiles in the total CARS signal that are measured by a single photomultiplier tube and demodulated by a multi-channel lock-in amplifier. Using a two-dimensional spatial light modulator displaying 24 different pulse shapes, we demonstrate pulse shaping at 80 kHz and chemically specific microscopy with pixel dwell times of less than 0.5 ms.
Journal of Modern Optics 06/2014; 61(10):872-876. DOI:10.1080/09500340.2013.867080 · 1.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Diffraction-limited imaging through complex scattering media is a long sought
after goal with important applications in biomedical research. In recent years,
high resolution wavefront-shaping has emerged as a powerful approach to
generate a sharp focus through highly scattering, visually opaque samples.
However, it requires a localized feedback signal from the target point of
interest, which necessitates an invasive procedure in all-optical techniques.
Here, we show that by exploiting optical nonlinearities, a diffraction-limited
focus can be formed inside or through a complex sample, even when the feedback
signal is not localized. We prove our approach theoretically and numerically,
and experimentally demonstrate it with a two-photon fluorescence signal through
highly scattering biological samples. We use the formed focus to perform
two-photon microscopy through highly scattering, visually opaque layers.
[Show abstract][Hide abstract] ABSTRACT: We implement the photoacoustic transmission-matrix approach on a
two-dimensional photoacoustic imaging system, using a 15 MHz linear ultrasound
array. Using a black leaf skeleton as a complex absorbing structure, we
demonstrate that the photoacoustic transmission-matrix approach allows to
reveal structural features that are invisible in conventional photoacoustic
images, as well as to selectively control light focusing on absorbing targets,
leading to a local enhancement of the photoacoustic signal.
[Show abstract][Hide abstract] ABSTRACT: We investigate experimentally the use of speckle illumination for photoacoustic imaging. In particular, we demonstrate that otherwise invisible features are revealed through high-frequency signals fluctuations from different speckle realizations.
[Show abstract][Hide abstract] ABSTRACT: Imaging with optical resolution through and inside complex samples is a
difficult challenge with important applications in many fields. The fundamental
problem is that inhomogeneous samples, such as biological tissues, randomly
scatter and diffuse light, impeding conventional image formation. Despite many
advancements, no current method enables to noninvasively image in real-time
using diffused light. Here, we show that owing to the memory-effect for speckle
correlations, a single image of the scattered light, captured with a standard
high-resolution camera, encodes all the information that is required to image
through the medium or around a corner. We experimentally demonstrate
single-shot imaging through scattering media and around corners using
incoherent light and various samples, from white paint to dynamic biological
samples. Our lensless technique is simple, does not require laser sources,
wavefront-shaping, nor time-gated detection, and is realized here using a
camera-phone. It has the potential to enable imaging in currently inaccessible
[Show abstract][Hide abstract] ABSTRACT: We report the implementation of the photo-acoustic transmission-matrix (PATM) method with a linear ultrasound array. The PATM allows optimizing light delivery to specified regions of the image and revealing features otherwise hidden by limited view.
[Show abstract][Hide abstract] ABSTRACT: This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/OL.38.005188 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.
In high-frequency photoacoustic imaging with uniform illumination, homogeneous photoabsorbing structures may be invisible because of their large size or limited-view issues. Here we show that, by exploiting dynamic speckle illumination, it is possible to reveal features that are normally invisible with a photoacoustic system comprised of a 20 MHz linear ultrasound array. We demonstrate imaging of a ∅5 mm absorbing cylinder and a 30 μm black thread arranged in a complex shape. The hidden structures are directly retrieved from photoacoustic images recorded for different random speckle illuminations of the phantoms by assessing the variation in the value of each pixel over the illumination patterns.
[Show abstract][Hide abstract] ABSTRACT: We experimentally and numerically study the potential of
photoacoustic-guiding for light focusing through scattering samples via
wavefront-shaping and iterative optimization. We experimentally demonstrate
that the focusing efficiency on an extended absorber can be improved by
iterative optimization of the high frequency components of the broadband
photoacoustic signal detected with a spherically focused transducer. We
demonstrate more than 8-fold increase in the photoacoustic signal generated by
a 30 microns wire using a narrow frequency band around 60MHz. We numerically
confirm that such optimization leads to a smaller optical focus than using the
low frequency content of the photoacoustic feedback.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate how to measure accurately the transmission matrix of a complex medium. With this information, we show how to focus light, recover an image, and even perform efficient reconstruction of a sparse object.
Computational Optical Sensing and Imaging; 06/2013