Over last decade, ultrafast lasers became industrially viable tool for high precision material processing. Ability to modify in the bulk of transparent materials is one of unique attributes of this technology. This have been successfully used for glass cutting, implementation of photonic circuits and microfluidic chips, local engineering of optical fibre properties.
This talk will explore how ultrafast lasers can be used to engineer optical scattering systems. The exploitation of this process for developing low loss distributed sensing systems and compact optical spectrometer will be discussed and demonstrated.
With applications from photonics to seismology, wave scattering is ubiquitous in physics. Yet, to study scattering in highly heterogeneous materials, evidence must be obtained from theoretical approximations and surface measurements. Numerical approaches can offer an insight into the wave behavior deep within a complex structure; however, the large scale, with respect to the short wavelength of light, of most systems of interest makes photonic simulations some of the most challenging numerical problems. Memory and time constraints typically limit coherent light scattering calculations to the micrometer scale in 2D and to the nanoscale in 3D. The study of large photonic structures, or scattering in biological samples larger than a few cells, remains out of reach of conventional computational methods. Here, we highlight a connection between the wave equation that governs light-scattering and the structure of a recurrent network. A one-to-one correspondence enables us to leverage efficient machine learning infrastructure and address coherent scattering problems on an unprecedented scale.
Its subcellular resolution and minimal sample exposure make light-sheet microscopy the ideal tool to study biological specimen during their early development. A light-sheet microscope scans the sample with a plane of light and collects fluorescence with an objective orthogonal to the illumination. However, tightly focused Gaussian light-sheets suffer from a shallow depth of focus and are susceptible to scattering-induced aberrations. Light-sheets created by non-diffractive Airy beams can overcome this to yield isotropic sub-cellular resolution over a ten-fold larger field-of-view. Airy beam light-sheets have a characteristically curved structure and a broad transverse structure with side-lobes. Digital deconvolution of the raw data is thus essential to obtain high-fidelity images. Provided that the scan is along the direction of the detection axis, and all recorded data fits within working memory, a simple and efficient Wiener filter can recover accurate 3D images. However, multi-millimeter sized samples must be scanned with a light-sheet that is diagonal to the sample surface. The diagonal movement prevents the use of standard Wiener filtering. Moreover, the associated data sets can become too large to fit within the working memory of a consumer-grade GPU. This demands slow off-line processing, thus breaking a rapid experimental feedback-loop. Here, we investigate the potential of on-the-fly deconvolution of diagonally-scanned Airy light-sheet microscopy.
Light-sheet microscopy enables rapid 3D imaging of biological samples. Its field-of-view can be extended ten-fold by relying on propagation-invariant Airy beams. However, such beams propagate on a parabolic trajectory. By consequence, a light-sheet formed by Airy beams is not planar, thus warping the images. Here, we demonstrate a planar Airy light-sheet that does not rely on digital image restoration techniques for two-photon microscopy.
We demonstrate a polarization analyser based on processing of speckle patterns generated by a scattering medium. Each speckle pattern at a given wavelength and polarization state is unique and deterministic, and thus the polarization angle alters the speckle pattern motif. The polarization state of a given input light is obtained using reconstructive linear algebra methods. The system consists of a femtosecond laser written scattering chip and a CMOS sensor and contains no moving parts, making the proposed solution is low-cost and compact. The linear polarization angle was accurately reconstructed over a 0-20° test range, with 6 arcminutes (1/10° ) standard error. To demonstrate an application as a polarimeter, we used the system to measure Faraday rotation in a SF59 lead silicate glass within an electromagnet. The magnetic field was successfully traced by determining the induced changes in the input beam’s linear polarization angle in the range 0-80 mT with 10 mT standard error.
Fiber Bragg gratings are the most popular type of optical fiber sensor. However, its commercial use is frequently limited by high cost and complexity of the interrogation unit. Here, an interrogator based on a femtosecond laser written silica scattering chip is designed and implemented. Such device can directly reconstruct strain, from the scattering speckle patterns, with a resolution of 70 μϵ (microstrain) within the range of 180-700 μϵ, limited by the slippage of the fiber coating, with the potential to be reduced with the system improvements.
The common challenge for reconstructive spectrometers is achieving high spectral resolution without sacrificing device stability, size and costs. Here a fully integrated scattering chip-based spectrometer build on Raspberry Pi platform is designed and implemented. It exhibits no dependence on temperature and humidity (22.7-23.8 °C and 39.5-41 %), is confined in small space (box 50x35x35 mm) and it can reconstruct spectra with resolution up to 0.05 nm (50 pm). The only instability – gravitational mechanical micromovements were compensated by applying pixel binning and device can still reconstruct spectra from binned pictures as small as 32 x 24 pixels.
Femtosecond laser irradiation allows to modify the optical properties of transparent materials with high accuracy. In many applications, optical scattering produced by the laser irradiation is one of the major limiting factors. However, there are situations when the scattering is responsible for the basic principle of operation of the optical element. This report reviews two research directions where laser-induced scattering can be successfully exploited. First, spectroscopic measurements can be performed by analyzing the speckle patterns created by the scattering medium. The measurements are made possible by the strong dependence of the speckle pattern on the wavelength of light. A scattering chip created thanks to a femtosecond laser makes allows addressing the stability problem faced by many scattering spectrometers. The volumetric scattering centers are induced in silica substrate via micro-explosions caused by the focused laser beam. Such a spectrometer can be successfully used for interrogating fiber Bragg gratings or interferometers. Second example is found in optical reflectometry. This technology allows turning an optical fiber into a distributed microphone or thermometer. A single optical fiber can monitor a stretch of several tens of kilometers with an accuracy of several meters. Such systems have wide range of applications in civil engineering, geosciences and other fields. Reflectometry measurements are performed by observing the back-scattered light produced by the glass medium of the optical fiber. Femtosecond laser writing allows effectively increase backscattered light whilst introducing minimal additional losses. In this way, the sensitivity of reflectometric systems can be increased or their range can be extended.
Light-sheet fluorescence microscopy allows minimally invasive 3D imaging of biological samples. Unlike other microscopes, a light-sheet illuminates the focal plane from the side. To uniformly illuminate a wide field-of-view, propagation-invariant light-fields have been proposed. While the propagation-invariant Airy beam is particularly advantageous for single-photon excitation, its multi-photon performance is limited by its intricate transversal structure and curvature. Here we demonstrate the first planar Airy beam light-sheet. Its uniform and symmetric illumination enables rapid two-photon excitation across a wide field-of-view. Moreover, it eliminates the need for deconvolution and it can significantly simplify a dual-use, single and two-photon, imaging instrument.
Fluorescence light-sheet microscopy is gaining rapid adoption in developmental biology. With irradiation levels well below that of confocal and multi-photon microscopy, it enables the study of intact organs and organisms for prolonged time periods during development. Minimal sample exposure is achieved by selectively illuminating the focal plane with a second objective orthogonal to the detection axis. The light-sheet microscope’s ability to study intact biological samples as and when they grow highlights the importance of imaging deeper into biological samples. Yet, deep-tissue microscopy is hampered by autofluorescence and the scattering of light. Direct observations are therefore limited to highly transparent and thin samples. Here, we show how autofluorescence can be eliminated effectively by relying on reversible photoswitching fluorescence while we propose a way forward to study and control light propagation in optically-thick tissues.
We demonstrate the transfer of orbital angular momentum (OAM) to optically levitated microparticles in vacuum. We create two-dimensional (2D) and three-dimensional (3D) optical potentials possessing OAM. In the former case the microparticle is placed within a Laguerre-Gaussian (LG) beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. Our results reveal that there is a fundamental limit to the OAM that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. Whilst a LG beam scales in size with azimuthal index, recently we have created a “perfect vortex” beam whose radial intensity profile and radius are both independent of topological charge. As the Fourier transform of a “perfect vortex” yields a Bessel beam, imaging a “perfect vortex”, with its subsequent propagation thus realises a complex three-dimensional optical field. In this scenario we load individual silica microparticles into this field where the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle. As a result the trapped microparticle exhibits a complex three-dimensional motion that includes a periodic orbital motion between the Bessel and the “perfect vortex” beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex 3D optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement, and optical binding.
We demonstrate the transfer of orbital angular momentum to optically levitated
microparticles in vacuum [1]. We prepare two-dimensional and three-dimensional optical
potentials. In the former case the microparticle is placed within a Laguerre-Gaussian beam
and orbits the annular beam profile with increasing angular velocity as the air drag coefficient
is reduced. We explore the particle dynamics as a function of the topological charge
of the levitating beam. Our results reveal that there is a fundamental limit to the orbital angular
momentum that may be transferred to a trapped particle, dependent upon the beam
parameters and inertial forces present. This effect was predicted theoretically [2] and can be
understood considering the underlying dynamics arising from the link between the magnitude
of the azimuthal index and the beam radius [3].
Whilst a Laguerre-Gaussian beam scales in size with azimuthal index `, recently we
have created a “perfect” vortex beam whose radial intensity profile and radius are both
independent of topological charge [4, 5]. As the Fourier transform of a perfect vortex yields
a Bessel beam. Imaging a perfect vortex, with its subsequent propagation thus realises a
complex three dimensional optical field. In this scenario we load individual silica microparticles
into this field and observe their trajectories. The optical gradient and scattering forces
interplay with the inertial and gravitational forces acting on the trapped particle, including
the rotational degrees of freedom. As a result the trapped microparticle exhibits a complex
three dimensional motion that includes a periodic orbital motion between the Bessel and
the perfect vortex beam. We are able to determine the three dimensional optical potential
in situ by tracking the particle. This first demonstration of trapping microparticles within
a complex three dimensional optical potential in vacuum opens up new possibilities for
fundamental studies of many-body dynamics, mesoscopic entanglement [6, 7], and optical
binding [8, 9].
Light sheet microscopy has seen a resurgence as it facilitates rapid, high contrast, volumetric imaging with minimal sample exposure. Initially developed for imaging scattered light, this application of light sheet microscopy has largely been overlooked but provides an endogenous contrast mechanism which can complement fluorescence imaging and requires very little or no modification to an existing light sheet fluorescence microscope. Fluorescence imaging and scattered light imaging differ in terms of image formation. In the former the detected light is incoherent and weak whereas in the latter the coherence properties of the illumination source, typically a laser, dictate the coherence of detected light, but both are dependent on the quality of the illuminating light sheet. Image formation in both schemes can be understood as the convolution of the light sheet with the specimen distribution. In this paper we explore wavefront shaping for the enhancement of light sheet microscopy with scattered light. We show experimental verification of this result, demonstrating the use of the propagation invariant Bessel beam to extend the field of view of a high resolution scattered light, light sheet microscope and its application to imaging of biological super-cellular structures with sub-cellular resolution. Additionally, complementary scattering and fluorescence imaging is used to characterize the enhancement, and to develop a deeper understanding of the differences of image formation between contrast mechanisms in light sheet microscopy.
Propagation invariant, ‘non-diffracting’ beams have found numerous applications in areas as diverse as filamentation,
trapping, and photoporation. However, the prominent transverse structure of Bessel beams prevents
localized illumination, thus hampering its use for high resolution imaging with an extended focus. We investigate
the relationship between axial resolution, contrast, and propagation invariance for single and two-photon
fluorescence light sheet microscopy.
Single plane illumination microscopy (SPIM) allows rapid imaging of large, three-dimensional, samples of living
tissue. The thin light sheet ensures high contrast whilst photo-bleaching and damage are kept to a minimum.
However, many specimen of interest have a significant thickness. To date, high axial resolution in such specimen
has only been achieved by compromising these key advantages and adding considerable technical complexity.
Although the light sheet can propagate several hundreds of micrometers into the tissue, its width can be several
orders of magnitude larger than it would be in a homogeneous sample. In this paper we explore the use of pupil-phase
modulation to overcome such sample-induced aberrations and produce diffraction-limited deep inside
turbid samples.
We report the design of binary-amplitude masks that in conjunction with digital restoration enable mitigation of optical
aberrations. Essentially, the design process aims to maintain high modulation-transfer functions by reducing destructive
interference of optical-transfer-function phasors. Two optimization techniques are described: so-called contour masks
and the use of multiple pixelated masks. In general the resultant modulation-transfer function is 20% of a diffractionlimited
imaging system and due to the absence of nulls recorded images can be restored to a high-contrast diffractionlimited
image. Example applications are presented for correcting ocular aberrations and for conformal imaging.
A significantly increased defocus tolerance can be obtained by combining pupil phase-modulation with digital demodulation
in a hybrid imaging system. Designing the optimal pupil phase-modulation is however not a trivial task. We
show how hybrid imaging fidelity can be predicted and used to compare arbitrary phase-modulations. The evaluations of
two anti-symmetric and a symmetric phase-modulation yield initial design values that can be used for the optimization of
specific hybrid designs.
The use of hybrid optical-digital techniques facilitates improved optimisation of imaging systems. It involves the
combined use of optical coding of an image using pupil-plane phase-modulation of the transmitted wavefront and postdetection
digital decoding. Previous research in hybrid imaging tends to emphasize constancy of the modulation transfer
function with aberrations and ignore the significant variations in the phase transfer function. We show that the
restoration artefacts introduced by phase mismatch effects can also be used to deduce the defocus PSF, and when this is
achieved, an overall improvement in image quality can be attained. Both numerical simulations and experimental
images of hybrid imaging systems are presented.
A phase mask at the aperture stop of a hybrid digital-optical imaging system can improve its tolerance to aberrations.
The choice of the introduced phase modulation is crucial in the design of such systems. Several successful phase masks
have been described in the literature. These masks are typically derived by searching for optical-transfer-functions that
retain restorability under aberrations such as defocus. Instead of optimizing the optical-transfer-function for some desired
characteristics, we calculate the expected imaging error of the joint design directly. This was used to compare thirddegree
polynomial phase masks, including the cubic phase profile and a commonly used generalization. The analysis
shows how the optimal phase profile depth is always limited by noise and more importantly, numerical simulations show
that only a finite range of the third-degree polynomial profiles yield optimal performance.
The design of modern imaging systems is intricately concerned with the control of optical aberrations in systems that can
be manufactured at acceptable cost and with acceptable manufacturing tolerances. Traditionally this involves a multi-parameter
optimisation of the lens optics to achieve acceptable image quality at the detector. There is increasing interest
in a more generalised approach whereby digital image processing is incorporated into the design process and the
performance metric to be optimised is quality of the image at the output of the image processor. This introduces the
possibility of manipulating the optical transfer function of the optics such that the overall sensitivity of the imaging
system to optical aberrations is reduced. Although these hybrid optical/digital techniques, sometimes referred as
wavefront coding, have on occasion been presented as a panacea, it is more realistic to consider them as an additional
parameter in the optimisation process. We will discuss the trade-offs involved in the application of wavefront coding to
low-cost imaging systems for use in the thermal infrared and visible imaging systems, showing how very useful
performance enhancements can be achieved in practical systems.
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