41 research outputs found
Digital optical phase conjugation of fluorescence in turbid tissue
We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue
A convergent Born series for solving the inhomogeneous Helmholtz equation in arbitrarily large media
We present a fast method for numerically solving the inhomogeneous Helmholtz
equation. Our iterative method is based on the Born series, which we modified
to achieve convergence for scattering media of arbitrary size and scattering
strength. Compared to pseudospectral time-domain simulations, our modified Born
approach is two orders of magnitude faster and nine orders of magnitude more
accurate in benchmark tests in 1-dimensional and 2-dimensional systems
A universal matrix-free split preconditioner for the fixed-point iterative solution of non-symmetric linear systems
We present an efficient preconditioner for linear problems . It
guarantees monotonic convergence of the memory-efficient fixed-point iteration
for all accretive systems of the form , where is an
approximation of , and the system is scaled so that the discrepancy is
bounded with . In contrast to common splitting
preconditioners, our approach is not restricted to any particular splitting.
Therefore, the approximate problem can be chosen so that an analytic solution
is available to efficiently evaluate the preconditioner. We prove that the only
preconditioner with this property has the form . This unique
form moreover permits the elimination of the forward problem from the
preconditioned system, often halving the time required per iteration. We
demonstrate and evaluate our approach for wave problems, diffusion problems,
and pantograph delay differential equations. With the latter we show how the
method extends to general, not necessarily accretive, linear systems.Comment: Rewritten version, includes efficiency comparison with shift
preconditioner by Bai et al, which is shown to be a special cas
Blind focusing through strongly scattering media using wavefront shaping with nonlinear feedback
Scattering prevents light from being focused in turbid media. The effect of
scattering can be negated through wavefront shaping techniques when a localized
form of feedback is available. Even in the absence of such a localized
reporter, wavefront shaping can blindly form a single diffraction-limited focus
when the feedback response is nonlinear. We developed and experimentally
validated a model that accurately describes the statistics of this blind
focusing process. We show that maximizing the nonlinear feedback signal does
not always result in the formation of a focus. Using our model, we can
calculate the minimal requirements to blindly focus light through strongly
scattering media.Comment: 13 pages, 4 figure
Scanning a focus through scattering media without using the optical memory effect
Wavefront shaping makes it possible to form a focus through opaque scattering
materials. In some cases, this focus may be scanned over a small distance using
the optical memory effect. However, in many cases of interest, the optical
memory effect has a limited range or is even too small to be measured. In such
cases, one often resorts to measuring the full transmission matrix (TM) of the
sample to completely control the light transmission. However, this process is
time-consuming and may not always be possible. We introduce a new method for
focusing and scanning the focus at any arbitrary position behind the medium by
measuring only a subset of the transmission matrix, called Sparse Field
Focusing (SFF). With SFF, the scan range is not limited to the memory effect
and there is no need to measure the full transmission matrix. Our experimental
results agree well with our theoretical model. We expect this method will find
applications in imaging through scattering media, especially when the optical
memory effect range is small.Comment: 5 pages, 3 figures, 1 movie(visualization 1
Model-based wavefront shaping microscopy
Wavefront shaping is increasingly being used in modern microscopy to obtain
distortion-free, high-resolution images deep inside inhomogeneous media.
Wavefront shaping methods typically rely on the presence of a 'guidestar' in
order to find the optimal wavefront to mitigate the scattering of light.
However, this condition cannot be satisfied in most biomedical applications.
Here, we introduce a novel, guidestar-free wavefront shaping method in which
the optimal wavefront is computed using a digital model of the sample. The
refractive index model of the sample, that serves as the input for the
computation, is constructed in-situ by the microscope itself. In a proof of
principle imaging experiment, we demonstrate a large improvement in the
two-photon fluorescence signal through a diffuse medium, outperforming the
state-of-the-art wavefront shaping techniques by a factor of 21
Optimizing field-of-view of deep-tissue scanning microscopy
For centuries, the optical microscope has been a crucial instrument for new biological findings, as microscopes were the first devices allowing to observe the internal processes of the cell. Unfortunately, this observation requires the use of thin samples, as biological tissue scatters the incoming light, resulting in a blurred image. An ever increasing number of deep-tissue imaging technique have pushed the penetration depth of the optical microscope. Methods such as adaptive optics [1] allow focusing inside biological tissue by correcting for scattering introduced by the sample. However, adaptive optics methods can only correct for image distortions caused by scattering over a single small area (i.e., field-of-view) within tissue.
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Physical key-protected one-time pad
We describe an encrypted communication principle that forms a secure link between two parties without
electronically saving either of their keys. Instead, random cryptographic bits are kept safe within the unique
mesoscopic randomness of two volumetric scattering materials. We demonstrate how a shared set of
patterned optical probes can generate 10 gigabits of statistically verified randomness between a pair of
unique 2 mm^3 scattering objects. This shared randomness is used to facilitate information-theoretically
secure communication following a modified one-time pad protocol. Benefits of volumetric physical storage
over electronic memory include the inability to probe, duplicate or selectively reset any bits without
fundamentally altering the entire key space. Our ability to securely couple the randomness contained within
two unique physical objects can extend to strengthen hardware required by a variety of cryptographic
protocols, which is currently a critically weak link in the security pipeline of our increasingly mobile
communication culture
Multimode Fibers for Quantum-Secure Communication
Multimode fibers support a multitude of transverse optical modes. These modes are mixed by the fiber. By complex wavefront shaping through the multimode fiber, we can undo this mixing, making it possible to communicate through the fiber even at very low light levels