arXiv subjects

Alexander Jesacher

Publications and source records attributed to Alexander Jesacher.

14 recordsLinked to original sources

arXiv2026

Fast dynamic wavefront correction for multi-photon microscopy with a high resolution MEMS phase-only modulator

Multi-photon microscopy is a powerful technique for deep-tissue imaging, providing high spatial resolution at increased penetration depth. Nevertheless, imaging remains largely restricted to superficial tissue layers well below 1 mm. Adaptive optics based on indirect wavefront sensing can significantly extend the accessible imaging depth, but their iterative operation is typically time-consuming and therefore limits the correction of strong, spatially complex aberrations. Until recently, this limitation was partly due to the lack of high-speed phase modulators offering sufficiently large numbers of actuators. Recent technological advances have addressed this bottleneck with the emergence of megapixel phase modulators operating at kilohertz rates. Here, we demonstrate wavefront correction in multi-photon microscopy using a high-speed phase-only MEMS modulator combined with a rapidly converging scatter-compensation algorithm. We experimentally correct complex aberrations comprising 144 spatial modes in less than one second, resulting in signal enhancements exceeding a factor of two. Benchmarking against a fast liquid-crystal spatial light modulator reveals a sixfold increase in correction speed. We further demonstrate adaptive optics imaging in mouse brain tissue using both two-photon and three-photon excitation fluorescence microscopy. These results indicate that high-resolution MEMS-based spatial light modulators enable efficient indirect wavefront sensing and represent a promising platform for high-speed wavefront shaping in non-linear microscopy.

physics.optics↗

arXiv2024

Output beam shaping of a multimode fiber amplifier

Multimode fibers provide a promising platform for realizing high-power laser amplifiers with suppressed nonlinearities and instabilities. The potential degradation of optical beam quality has been a major concern for highly multimode fiber amplifiers. We show numerically that the beam propagation factor M2 of a single-frequency multimode fiber amplifier can be reduced to nearly unity by shaping the input or output beam profile with spatial phase-masks. Our method works for narrowband multimode fiber amplifiers with strong gain saturation, pump depletion, random mode coupling and polarization mixing. The numerical results validate our approach of utilizing highly multimode excitation to mitigate nonlinear effects in high-power fiber amplifiers and performing input wavefront shaping to control output beam profile and polarization state.

physics.optics↗

arXiv2024

Complex-valued scatter compensation in nonlinear microscopy

Nonlinear, i.e., multi-photon microscopy is a powerful technique for imaging deep into biological tissues. Its penetration depth can be increased further using adaptive optics. In this work, we present a fast, feedback-based adaptive-optics algorithm, termed C-DASH, for multi-photon imaging through multiply-scattering media. C-DASH utilises complex-valued light shaping (i.e., joint shaping of amplitude and phase), which offers several advantages over phase-only techniques: it converges faster, it delivers higher image quality enhancement, it shows a robust performance largely insensitive to the axial position of the correction plane, and it has a higher ability to cope with, on the one hand, scattering media that vary over time and, on the other hand, scattering media that are partially absorbing. Furthermore, our method is practically self-aligning. We also present a simple way to implement C-DASH using a single reflection off a phase-only spatial light modulator. We provide a thorough characterisation of our method, presenting results of numerical simulations as well as two-photon excited fluorescence imaging experiments.

physics.optics↗

arXiv2023

Interactive simulation and visualization of point spread functions in single molecule imaging

The point spread function (PSF) is fundamental to any type of microscopy, most importantly so for single-molecule localization techniques, where the exact PSF shape is crucial for precise molecule localization at the nanoscale. However, optical aberrations and fixed fluorophore dipoles can lead to non-isotropic and distorted PSFs, thereby complicating and biasing conventional fitting approaches. In addition, some researchers deliberately modify the PSF by introducing specific phase shifts in order to provide improved sensitivity, e.g., for localizing molecules in 3D, or for determining the dipole orientation. For devising an experimental approach, but also for interpreting obtained data it would be helpful to have a simple visualization tool which calculates the expected PSF for the experiment in mind. To address this need, we have developed a comprehensive and accessible computer application that allows for the simulation of realistic PSFs based on the full vectorial PSF model. It incorporates a wide range of microscope and fluorophore parameters, enabling an accurate representation of various imaging conditions. Further, our app directly provides the Cramer-Rao bound for assessing the best achievable localization precision under given conditions. In addition to facilitating the simulation of PSFs of isotropic emitters, our application provides simulations of fixed dipole orientations as encountered, e.g., in cryogenic single-molecule localization microscopy applications. Moreover, it supports the incorporation of optical aberrations and phase manipulations for PSF engineering, as well as the simulation of crowded environments with overlapping molecules. Importantly, our software allows for the fitting of custom aberrations directly from experimental data, effectively bridging the gap between simulated and experimental scenarios, and enhancing experimental design and result validation.

physics.optics↗

arXiv2023

High Speed Precise Refractive Index Modification for Photonic Chips through Phase Aberrated Pulsed Lasers

Integrated photonic chips have significant potential in telecommunications, classic computing, quantum systems, and topological photonics. Direct laser writing offers unique capability for creating three-dimensional photonic devices in an optical glass chip with quick prototyping. However, existing laser writing schemes cannot create index-modified structures in glass that precisely match the laser focal shape while also achieving high scanning speed and high refractive index contrast. Here, we introduce the theory of a refractive index modification scheme that combines the advantages of both traditional non-thermal and thermal regime fabrication methods. We also propose a model of waveguide formation that was verified through a thorough study on the effects of phase aberrations on the laser focus. The presented new photonic chip fabrication scheme uses a novel focal intensity distribution, where pulse energy is relocated to the bottom of a laser focus by manipulating primary and higher order spherical aberrations. The technique can produce index modifications with high scanning speed (20 mm/s or higher), high index contrast (16 x 10-3), and high precision to fabricate with arbitrary cross-sections. This method has potential to expand the capabilities of photonic chips in applications that require small-scale, high precision, or high contrast refractive index control.

physics.optics↗

arXiv2023

Optofluidic adaptive optics in multi-photon microscopy

Adaptive optics in combination with multi-photon techniques is a powerful approach to image deep into a specimen. Remarkably, virtually all adaptive optics schemes today rely on wavefront modulators which are reflective, diffractive, or both. This, however, can pose a severe limitation for applications. Here, we present a fast and robust sensorless adaptive optics scheme adapted for transmissive wavefront modulators. We study our scheme in numerical simulations and in experiments with a novel, optofluidic wavefront shaping device which is transmissive, refractive, polarisation-independent and broadband. We demonstrate scatter correction of two-photon-excited fluorescence images of microbeads as well as brain cells and benchmark our device against a liquid-crystal spatial light modulator. Our method and technology could open new routes for adaptive optics in scenarios where previously the restriction to reflective and diffractive devices may have staggered innovation and progress.

physics.optics↗

arXiv2022

Direct laser written aperiodic photonic volume elements for complex light shaping with high efficiency: inverse design and fabrication

Light plays the central role in many applications. The key to unlocking its versatility lies in shaping it into the most appropriate form for the task at hand. Specifically tailored refractive index modifications, directly manufactured inside glass using a short pulsed laser, enable an almost arbitrary control of the light flow. However, the stringent requirements for quantitative knowledge of these modifications, as well as for fabrication precision, have so far prevented the fabrication of light-efficient aperiodic photonic volume elements (APVEs). Here we present a powerful approach to the design and manufacturing of light-efficient APVEs. We optimize application-specific 3D arrangements of hundred thousands of microscopic voxels and manufacture them using femtosecond direct laser writing inside millimeter-sized glass volumes. We experimentally achieve unprecedented diffraction efficiencies up to 80%, which is enabled by precise voxel characterization and adaptive optics during fabrication. We demonstrate APVEs with various functionalities, including a spatial mode converter and combined intensity shaping and wavelength-multiplexing. Our elements can be freely designed and are efficient, compact and robust. Our approach is not limited to borosilicate glass, but is potentially extendable to other substrates, including birefringent and nonlinear materials, giving a preview of even broader functionalities including polarization modulation and dynamic elements.

physics.optics↗

arXiv2022

On-chip beam rotators, adiabatic mode converters, and waveplates through low-loss waveguides with variable cross-sections

Photonics integrated circuitry would benefit considerably from the ability to arbitrarily control waveguide cross-sections with high precision and low loss, in order to provide more degrees of freedom in manipulating propagating light. Here, we report a new method for femtosecond laser writing of optical-fibre-compatible glass waveguides, namely spherical phase induced multi-core waveguide (SPIM-WG), which addresses this challenging task with three dimensional on-chip light control. Fabricating in the heating regime with high scanning speed, precise deformation of cross-sections is still achievable along the waveguide, with shapes and sizes finely controllable of high resolution in both horizontal and vertical transversal directions. We observed that these waveguides have high refractive index contrast of 0.017, low propagation loss of 0.14 dB/cm, and very low coupling loss of 0.19 dB coupled from a single mode fibre. SPIM-WG devices were easily fabricated that were able to perform on-chip beam rotation through varying angles, or manipulate polarization state of propagating light for target wavelengths. We also demonstrated SPIM-WG mode converters that provide arbitrary adiabatic mode conversion with high efficiency between symmetric and asymmetric non-uniform modes; examples include circular, elliptical modes and asymmetric modes from ppKTP (periodically-poled potassium titanyl phosphate) waveguides which are generally applied in frequency conversion and quantum light sources. Created inside optical glass, these waveguides and devices have the capability to operate across ultra-broad bands from visible to infrared wavelengths. The compatibility with optical fibre also paves the way toward packaged photonic integrated circuitry, which usually needs input and output fibre connections.

physics.optics↗

arXiv2022

Sensorless wavefront correction in two-photon microscopy across different turbidity scales

Adaptive optics (AO) is a powerful tool to increase the imaging depth of multiphoton scanning microscopes. For highly scattering tissues, sensorless wavefront correction techniques exhibit robust performance and present a straight-forward implementation of AO. However, for many applications such as live-tissue imaging, the speed of aberration correction remains a critical bottleneck. Dynamic Adaptive Scattering compensation Holography (DASH) -- a fast-converging sensorless AO technique introduced recently for scatter compensation in nonlinear scanning microscopy -- addresses this issue. DASH has been targeted at highly turbid media, but to-date it has remained an open question how it performs for mild turbidity, where limitations imposed by phase-only wavefront shaping are expected to impede its convergence. In this work, we study the performance of DASH across different turbidity regimes, in simulation as well as experiments. We further provide a direct comparison between DASH and a novel, modified version of the Continuous Sequential Algorithm (CSA) which we call Amplified CSA (a-CSA).

physics.optics↗

arXiv2021

Inverse design of gradient-index volume multimode converters

Graded-index optical elements are capable of shaping light precisely and in very specific ways. While classical freeform optics uses only a two-dimensional domain such as the surface of a lens, recent technological advances in laser manufacturing offer promising prospects for the realization of arbitrary three-dimensional graded-index volumes, i.e. transparent dielectric substrates with voxel-wise modified refractive index distributions. Such elements would be able to perform complex light transformations on compact scales. Here we present an algorithmic approach for computing 3D graded-index devices, which utilizes numerical beam propagation and error reduction based on gradient descent. We present solutions for millimeter-sized elements addressing important tasks in photonics: a mode sorter, a photonic lantern and a multimode intensity beam shaper. We further discuss suitable cost functions for all designs to be used in the algorithm. The 3D graded-index designs are spatially smooth and require a relatively small refractive index range in the order of $10^{-2}$, which is within the reach of direct laser writing manufacturing processes such as two-photon polymerization.

physics.optics↗

arXiv2021

Holographic beam shaping of partially coherent light

We present an algorithm for holographic shaping of partially coherent light, bridging the gap between traditional coherent and geometric optical approaches. The description of partially coherent light relies on a mode expansion formalism, with possibly thousands of individual modes, and the inverse-design optimization algorithm is based on gradient descent with the help of algorithmic differentiation. We demonstrate numerically and experimentally that an optical system consisting of two phase patterns can potentially achieve any intensity profile transformation with good accuracy.

physics.optics↗

arXiv2021

Robust and bias-free localization of individual fixed dipole emitters achieving the Cramér Rao bound

Single molecule localization microscopy has the potential to resolve structural details of biological samples at the nanometer length scale. However, to fully exploit the resolution it is crucial to account for the anisotropic emission characteristics of fluorescence dipole emitters. In case of slight residual defocus, localization estimates may well be biased by tens of nanometers. We show here that astigmatic imaging in combination with information about the dipole orientation allows to extract the position of the dipole emitters without localization bias and down to a precision of ~1nm, thereby reaching the corresponding Cramér Rao bound. The approach is showcased with simulated data for various dipole orientations, and parameter settings realistic for real life experiments.

physics.optics↗

arXiv2017

Four-dimensional light shaping: manipulating ultrafast spatio-temporal foci in space and time

Spectral dispersion of ultrashort pulses allows simultaneous focusing of light in both space and time creating so-called spatio-temporal foci. Such space-time coupling may be combined with existing holographic techniques to give a further dimension of control when generating focal light fields. It is shown that a phase-only hologram placed in the pupil plane of an objective and illuminated by a spatially chirped ultrashort pulse can be used to generate three dimensional arrays of spatio-temporally focused spots. Exploiting the pulse front tilt generated at focus when applying simultaneous spatial and temporal focusing (SSTF), it is possible to overlap neighbouring foci in time to create a smooth intensity distribution. The resulting light field displays a high level of axial confinement, with experimental demonstrations given through two-photon microscopy and non-linear laser fabrication of glass.

physics.optics↗

arXiv2010

Three-dimensional imaging of direct-written photonic structures

Third harmonic generation microscopy has been used to analyze the morphology of photonic structures created using the femtosecond laser direct-write technique. Three dimensional waveguide arrays and waveguide-Bragg gratings written in fused-silica and doped phosphate glass were investigated. A sensorless adaptive optical system was used to correct the optical aberrations occurring in the sample and microscope system, which had a lateral resolution of less than 500 nm. This non-destructive testing method creates volume reconstructions of photonic devices and reveals details invisible to other linear microscopy and index profilometry techniques.

physics.optics↗