Optical Beam Path Research Articles

Electrically tunable infrared optics enabled by flexible ion-permeable conducting polymer-cellulose paper

Materials that provide dynamically tunable infrared (IR) response are important for many applications, including active camouflage and thermal management. However, current IR-tunable systems often exhibit limitations in mechanical properties or practicality of their tuning modalities, or require complex and costly fabrication methods. An additional challenge relates to providing compatibility between different spectral channels, such as allowing an object to be reversibly concealed in the IR without making it appear in the visible range. Here, we demonstrate that conducting polymer-cellulose papers, fabricated through a simple and cheap approach, can overcome such challenges. The papers exhibit IR properties that can be electrochemically tuned with large modulation (absolute emissivity modulation of 0.4) while maintaining largely constant response in the visible range. Owing to high ionic and electrical conductivity, the tuning of the top surface can be performed electrochemically from the other side of the paper even at tens of micrometer thicknesses, removing the need for overlaying electrode and electrolyte in the optical beam path. These features enabled a series of electrically tunable IR devices, where we focus on demonstrating dynamic radiative coolers, thermal camouflage, anti-counterfeiting tags, and grayscale IR displays. The conducting polymer-cellulose papers are sustainable, cheap, flexible and mechanically robust, providing a versatile materials platform for active and adaptive IR optoelectronic devices.

A motorized rotation mount for the switching of an optical beam path in under 20ms using polarization control.

We present a simple motorized rotation mount for a half-wave plate that can be used to rapidly change the polarization of light. We use the device to switch a high power laser beam between different optical dipole traps in an ultracold atom experiment. The device uses a stepper motor with a hollow shaft, which allows a beam to propagate along the axis of the motor shaft, minimizing inertia and mechanical complexity. A simple machined adapter is used to mount the wave plate. We characterize the performance of the device, focusing on its capability to switch a beam between the output ports of a polarizing beam splitter cube. We demonstrate a switching time of 15.9(3) ms, limited by the torque of the motor. The mount has a reaction time of 0.52(3) ms and a rotational resolution of 0.45(4)°. The rotation is highly reproducible, with the stepper motor not missing a step in 2000 repeated tests over 11h.

Two-Color-Thermography for Temperature Determination in Laser Beam Welding of Low-Melting Materials.

Spatial and temporal knowledge of temperature evolution is crucial in laser beam welding of low-melting materials such as aluminum alloys. Current temperature measurements are restricted to (i) one-dimensional temperature information (e.g., ratio-pyrometers), (ii) a priori knowledge of emissivity (e.g., thermography), and (iii) high-temperature regions (e.g., two-color-thermography). This study presents a ratio-based two-color-thermography system that enables acquiring spatially and temporally resolved temperature information for low-melting temperature ranges (<1200 K). The study demonstrates that temperature can be accurately determined despite variations in signal intensity and emissivity for objects emitting constant thermal radiation. The two-color-thermography system is further transferred into a commercial laser beam welding set-up. Experiments with varying process parameters are conducted, and the ability of the thermal imaging method to measure dynamic temperature behavior is assessed. Image artifacts presumably caused by internal reflections inside the optical beam path limit the direct application of the developed two-color-thermography system during dynamic temperature evolution.

Performance of a chirped-pulse Fourier transform millimeter wave spectrometer in the range of 75-110 GHz.

We present a home-built chirped-pulse Fourier transform millimeter wave (CP-FTMMW) spectrometer. The setup is devoted to the sensitive recording of high-resolution molecular spectroscopy in the W band between 75 and 110GHz. We describe the experimental setup in detail, including a characterization of the chirp excitation source, the optical beam path, and the receiver. The receiver is a further development of our 100GHz emission spectrometer. The spectrometer is equipped with a pulsed jet expansion and a DC discharge. Spectra of methyl cyanide as well as hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) products from the DC discharge of this molecule are recorded to characterize the performance of the CP-FTMMW instrument. The formation of the HCN isomer is favored by a factor of 63 with respect to HNC. Hot/cold calibration measurements enable a direct comparison of the signal and noise levels of the CP-FTMMW spectra to those of the emission spectrometer. For the CP-FTMMW instrument, we find many orders of magnitude of signal enhancement and a much stronger noise reduction due to the coherent detection scheme.

Quasi-Collinear AOTF Spectral Transmission Under Temperature Gradients Aroused by Ultrasound Power Absotption.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that provides an extremely large AO interaction length for achieving extremely high spectral resolution of AO tunable filters (AOTFs). Large AO interaction length also makes it possible to implement multifrequency diffraction in quasi-collinear AOTFs, which has found multiple applications in modern optoelectronics. The most widespread of them being ultrashort laser pulse shaping when the pulse shape and spectral composition is controlled by the spectral composition of the ultrasound pulse aroused in the AO crystal. The operation of quasi-collinear AOTFs is accompanied by the appearance of the temperature gradients in the AO device mainly due to the acoustic power absorption by the material. We experimentally assessed the influence of these gradients on the AOTF spectral characteristics. A theoretical model based on the Raman-Nath equations was proposed, which allows to consider the influence of ultrasound attenuation and temperature gradients on the AOTF transmission. This model is valid for transmission simulations both in single- and modulated-frequency AOTF operation modes. The study includes the effects of acoustic wave attenuation and AO phase matching shift caused by inhomogeneous crystal temperature along the optical beam path. The compensation strategy based on ultrasound frequency and magnitude adjustment is proposed for minimizing the effect of temperature gradients and acoustic field attenuation on AOTF spectral transmission for broadband operation in ultrashort laser pulse shaping.

Measurement of airborne ultrasound using laser Doppler vibrometry

The speed of light in air is dependent on the air's instantaneous density. Since air density is modulated by sound, sound in the air can be observed and measured using optical methods. One such optical method is Laser Doppler Vibrometry (LDV). Most commonly, LDVs measure the mechanical velocity of a surface. However, by placing a rigid reflector beneath a sound beam in air, it is possible to measure the time rate-of-change of optical refractive index and thus to measure dynamic changes in air density, or sound. In prior demonstrations, this method has been used to visualize sound fields in the audible frequency range and ultrasonic range underwater. Here, we present the first measurements of high-intensity airborne ultrasound beams in the frequency range spanning 100 kHz–300 kHz. We observe accumulated distortion, wave steepening, and weak shock formation as high intensity sound beams propagate. LDV measurements are compared against numerical simulations of the sound field. Advantages of the LDV technique are discussed, and we also attempt to quantify limitations of the technique which include spatial averaging of the measurand along and normal to the optical beam path.

Towards robust dynamic beam shaping for laser cutting applications

Dynamic beam shaping brings a large potential for the laser cutting process by increasing the cutting speed and improving the cut edge quality. However, laser beam oscillation is almost exclusively realized with 2-mirror galvanometer scanners that limit the industrial implementation of the technology. Within this work, a new concept platform based on a single-mirror scanning system with a 4 kW fiber laser is tested for cutting of 12 mm thick plates in different materials. Technology knowledge transfer from existing datasets with common 2-mirror scanners has been used to speed up the optimization of the novel system. The proposed solution is able to substitute conventional deflection mirrors and can be easily integrated into the optical beam path of existing industrial machines. This paper introduces the advanced laser cutting process possibilities and experimentally demonstrates the dynamic beam shaping performance compared to the static beam reference system.

Digital holographic polarimeter using dual reference beam interferometry.

An off-axis digital holographic imaging polarimeter was developed to estimate the Jones matrices of an object. The Jones vector image of the electric field returned from the object is determined from a single holographic recording using the interference between the dual, nearly orthogonal, reference beams. The technique compensates for phase variations in the optical beam paths between the recorded holograms and relaxes the need to generate orthogonal illumination polarization states. A minimization algorithm was developed to compute an estimation of the Jones matrix image of an object based on a set of measured Jones vector images. A proof-of-concept demonstration was performed to compute an estimated Jones matrix image of a polarimetrically complex object using digital holograms recorded with 6 different illumination polarizations.

Depth-of-field extension in optical imaging for rapid crystal screening.

The depth of field (DoF) was extended 2.8-fold to achieve rapid crystal screening by retrofitting a custom-designed micro-retarder array (µRA) in the optical beam path of a nonlinear optical microscope. The merits of the proposed strategy for DoF enhancement were assessed in applications of second-harmonic generation imaging of protein crystals. It was found that DoF extension increased the number of crystals detected while simultaneously reducing the number of `z-slices' required for screening. Experimental measurements of the wavelength-dependence of the extended DoF were in excellent agreement with theoretical predictions. These results provide a simple and broadly applicable approach to increase the throughput of existing nonlinear optical imaging methods for protein crystal screening.

Implementation of a 4Pi-SMS super-resolution microscope.

The development of single-molecule switching (SMS) fluorescence microscopy (also called single-molecule localization microscopy) over the last decade has enabled researchers to image cell biological structures at unprecedented resolution. Using two opposing objectives in a so-called 4Pi geometry doubles the available numerical aperture, and coupling this with interferometric detection has demonstrated 3D resolution down to 10 nm over entire cellular volumes. The aim of this protocol is to enable interested researchers to establish 4Pi-SMS super-resolution microscopy in their laboratories. We describe in detail how to assemble the optomechanical components of a 4Pi-SMS instrument, align its optical beampath and test its performance. The protocol further provides instructions on how to prepare test samples of fluorescent beads, operate this instrument to acquire images of whole cells and analyze the raw image data to reconstruct super-resolution 3D data sets. Furthermore, we provide a troubleshooting guide and present examples of anticipated results. An experienced optical instrument builder will require ~12 months from the start of ordering hardware components to acquiring high-quality biological images.

Design and Performance of the Solid-State Laser Trigger System for HERMES III

The HERMES III accelerator is an 18–20-MeV linear induction accelerator constructed at Sandia National Laboratories in the late 1980s and which continues operation to this day. As part of recent modernization efforts, the laser triggering system on the accelerator has been replaced with a newly designed solid-state system. This system consists of ten Nd:YAG lasers, each having a nominal output energy of 40–45 mJ at a wavelength of 266 nm. The beam from each laser is split such that it triggers two of the Rimfire gas switches on the accelerator. Compared to the previous laser triggering system, this arrangement makes it possible to more readily tailor the final output pulse shape, and the overall reliability of this key accelerator system is now improved. The design of the new laser triggering system is presented here, along with details pertaining to the energy budgeting, optical beam paths, and electrical triggering of the lasers. The initial operational data from the HERMES III accelerator using this new triggering system are also presented.

Adaption of a topographical measurement system for beam welding processes with high temporal and spatial resolution

Both typical beam welding processes laser beam welding LBW and electron beam welding EBW feature the deep penetration welding process. Theses processes rely on a vapor capillary with a delicate, highly dynamic equilibrium of various opposing forces. The direct observation of the vapor capillary's inner surface is hindered with growing capillary depth by the thermal optical emissions with high intensities due to the high temperatures inside the capillary. Even with the use of high intensity laser based illumination and band pass filters adjusted for the laser wavelength, it is merely possible to observe only the upper most part of the capillaries opening. The lack of observability, especially with high temporal resolution to assess the highly dynamic effects contributes to the state of the capillary models that actually are describing quasi-static modes or implement the less dynamic effects that can be observed with state of the art methods.The topographical measurement system, developed by the Institut für Strahlwerkzeuge Universität Stuttgart IFSW, uses the process inherent thermal emission of the capillary governed by the complex refractive index of the base material to reconstruct the inner geometry of the visible capillary surface. This is achieved by using the dependence of the emissivity between emission angle and polarization that is described by the Fresnel equations (Sawannia et al., 2018). The measurement system uses the fact that the quotient of two or more polarization components of the emission from a surface increment is depending on the angle of surface inclination but are almost independent of the surface temperature (Weberpals et al., 2011). Based on the results of the application of the measurement principle with laser beam welding, an adaption to electron beam welding is developed to assess the observability of capillaries with large depth and to contribute in the improvement of capillary models in the wake of the development. To allow the use of the improved measurement system to capture the EBW process capillary with unmatched spatial and temporal resolution, a complete new optical observation beam path was developed and fitted insight a custom-made pressurized chamber inside an electron beam welding machine.

In situ 3D reconfigurable ultrasonically sculpted optical beam paths.

We demonstrate that optical beams can be spatially and temporally shaped in situ by forming 3D reconfigurable interference patterns of ultrasound waves in the medium. In this technique, ultrasonic pressure waves induce a modulated refractive index pattern that shapes the optical beam as it propagates through the medium. Using custom-designed cylindrical ultrasonic arrays, we demonstrate that complex patterns of light can be sculpted in the medium, including dipole and quadrupole shapes. Additionally, through a combination of theory and experiment, we demonstrate that these optical patterns can be scanned in radial and azimuthal directions. Moreover, we show that light can be selectively confined to different extrema of the spatial ultrasound pressure profile by temporally synchronizing lightwave and ultrasound. Finally, we demonstrate that this technique can also be used to define spatial patterns of light in turbid media. The notion of in situ 3D sculpting of optical beam paths using ultrasound interference patterns can find intriguing applications in biological imaging and manipulation, holography, and microscopy.

Slim coherent backlight unit for holographic display using full color holographic optical elements.

The coherent backlight unit (BLU) using a holographic optical element (HOE) for full-color flat-panel holographic display is proposed. The HOE BLU consists of two reflection type HOEs that change the optical beam path and shape by diffraction. The illumination area of backlight is 150 mm x 90 mm and the thickness is 10 mm, which is slim compared to other conventional coherent backlight units for holographic display systems. This backlight unit exhibits a total efficiency of 8.0% at red (660 nm), 7.7% at green (532 nm), and 3.2% at blue (460 nm) using optimized recording conditions for each wavelength. As a result, we could get a bright full color hologram image.

A protocol for registration and correction of multicolour STED superresolution images.

Multicolour fluorescence imaging by STimulated Emission Depletion (STED) superresolution microscopy with doughnut-shaped STED laser beams based on different wavelengths for each colour channel requires precise image registration. This is especially important when STED imaging is used for co-localisation studies of two or more native proteins in biological specimens to analyse nanometric subcellular spatial arrangements. We developed a robust postprocessing image registration protocol, with the aim to verify and ultimately optimise multicolour STED image quality. Importantly, this protocol will support any subsequent quantitative localisation analysis at nanometric scales. Henceforth, using an approach that registers each colour channel present during STED imaging individually, this protocol reliably corrects for optical aberrations and inadvertent sample drift. To achieve the latter goal, the protocol combines the experimental sample information, from corresponding STED and confocal images using the same optical beam path and setup, with that of an independent calibration sample. As a result, image registration is based on a strategy that maximises the cross-correlation between sequentially acquired images of the experimental sample, which are strategically combined by the protocol. We demonstrate the general applicability of the image registration protocol by co-staining of the ryanodine receptor calcium release channel in primary mouse cardiomyocytes. To validate this new approach, we identify user-friendly criteria, which - if fulfilled - support optimal image registration. In summary, we introduce a new method for image registration and rationally based postprocessing steps through a highly standardised protocol for multicolour STED imaging, which directly supports the reproducibility of protein co-localisation analyses. Although the reference protocol is discussed exemplarily for two-colour STED imaging, it can be readily expanded to three or more colours and STED channels.

Photoconductive LT-GaAs Terahertz Antennas: Correlation Between Surface Quality and Emission Strength

We investigate the influence of the surface properties of a low-temperature-grown GaAs photoconductive antenna on the terahertz (THz) emission strength, using a specially designed THz time-domain spectroscopy system. The system allows us to excite six different positions along the 10 μm gap of a coplanar stripline antenna with a length of 10 mm without changing the alignment of the optical or THz beam path. A comparison to the surface roughness and the grain size which are extracted from an atomic force and a scanning electron microscope is given.

Ghost reflections of Gaussian beams in anamorphic optical systems with an application to Michelson interferometer.

In this paper, a methodology is developed to model and analyze the effect of undesired (ghost) reflections of Gaussian beams that are produced by anamorphic optical systems. The superposition of these beams with the nominal beam modulates the nominal power distribution at the recording plane. This modulation may cause contrast reduction, veiling parts of the nominal image, and/or the formation of spurious interference fringes. The developed methodology is based on synthesizing the beam optical paths into nominal and ghost optical beam paths. Similar to the nominal beam, we present the concept that each ghost beam is characterized by a beam size, wavefront radius of curvature, and Gouy phase in the paraxial regime. The nominal and ghost beams are sequentially traced through the system and formulas for estimating the electric field magnitude and phase of each ghost beam at the recording plane are presented. The effective electric field is the addition of the individual nominal and ghost electric fields. Formulas for estimating Gouy phase, the shape of the interference fringes, and the central interference order are introduced. As an application, the theory of the formation of the interference fringes by Michelson interferometer is presented. This theory takes into consideration the ghost reflections that are formed by the beam splitter. To illustrate the theory and to show its wide applicability, simulation examples that include a Mangin mirror, a Michelson interferometer, and a black box optical system are provided.

Anomalous Bloch oscillations in arrays of coupled waveguides

Anomalous Bloch oscillations in arrays of coupled waveguides have been investigated using two different approaches: multiple scattering formalism and coupled-mode model. Both approaches are shown to yield identical results. Formulas for the parameters of the coupled-mode model are derived. An analytical expression for the optical beam path is obtained, and the condition for the existence of anomalous Bloch oscillations is found.

Anomalous optical Bloch oscillations

Abstract Anomalous optical Bloch oscillation in waveguide arrays are investigated within the generalized Coupling Mode Model (CMM). This model takes into account the next neighbor interaction between the waveguides along with the nearest neighbor interaction. The exact analytical solution for the optical beam path is found. A criterion of the appearance of the anomalous Bloch oscillations against the standard Bloch oscillations background is discovered. A comparison between the analytical result and the numerical simulation reveals a good agreement.

Fluorescence lifetime imaging with near-infrared dyes

AbstractNear-infrared (NIR) dyes are used as fluorescence markers in small animal imaging and in diffuse optical tomography. In these applications it is important to know whether the dyes bind to proteins or to other tissue constituents, and whether their fluorescence lifetimes depend on the targets they bind to. Unfortunately, neither the optical beam paths nor the detectors of commonly used in confocal and multiphoton laser scanning microscopes (LSMs) directly allow for excitation and detection of NIR fluorescence. This paper presents three ways of adapting existing LSMs with time-correlated single photon counting (TCSPC) fluorescence lifetime imaging (FLIM) systems for NIR FLIM: 1) confocal systems with wideband beamsplitters and diode laser excitation, 2) confocal systems with wideband beamsplitters and one-photon excitation by titanium-sapphire lasers, and 3) two-photon systems with optical parametric oscillator (OPO) excitation and non-descanned detection. A number of NIR dyes are tested in biological tissue. All of them show clear lifetime changes depending on the tissue structures they are bound to. We therefore believe that NIR FLIM can deliver supplementary information about the tissue composition and on local biochemical parameters.