Megahertz-rate shock-wave distortion cancellation via phase conjugate digital in-line holography

Simple and low-cost microscopy setup for 3D particle field measurement using incoherent illumination and open-source hardware

The quantification of 3D particle field is of interest for a vast range of fields. While in-line particle holography (PH) can provide high-resolution measurements of particles, it suffers from speckle noise. Plenoptic imaging (PI) is less susceptible to speckle noises, but it involves a trade-off between spatial and angular resolution, rendering images with low resolution. Here, we report a simple microscopy setup with the goals of getting the strengths of both techniques. It is built with off-the-shelf and cost-effective components including a photographic lens, a diaphragm, and a CCD camera. The cost of the microscopy setup is affordable to small labs and individual researchers. The pupil plane of the proposed setup can be mechanically accessible, allowing us to implement pupil plane modulation and increase the depth of field (DOF) without requiring any additional relay lenses. It also allows us to understand the working principle of pupil plane modulation clearly, benefiting microscopy education. It illuminates the sample (particles) using diffuse white light, and thus avoids the problem of speckle noise. It captures multiple perspective images via pupil plane modulation, without requiring trading off angular and spatial resolution. We validate the setup with 2D and 3D particle samples. RESEARCH HIGHLIGHTS: We report a simple and cost-effective microscopy setup with the goals of getting the strengths of plenoptic imaging and in-line particle holography. It is built with off-the-shelf and cost-effective components. The cost of the microscopy setup is affordable to small labs and individual researchers. The pupil plane of the proposed setup can be mechanically accessible, allowing us to implement pupil plane modulation and increase the DOF without requiring any additional relay lenses. It also allows us to understand the working principle of pupil plane modulation clearly, benefiting microscopy education. It illuminates the sample (particles) using diffuse white light, and thus avoids the problem of speckle noise. It captures multiple perspective images via pupil plane modulation, without requiring trading off angular and spatial resolution. We validate the setup with 2D and 3D particle samples.

Ultraviolet digital holographic microscopy (DHM) of micron-scale particles from shocked Sn ejecta

A cloud of very fast, O(km/s), and very fine, O(µm), particles may be ejected when a strong shock impacts and possibly melts the free surface of a solid metal. To quantify these dynamics, this work develops an ultraviolet, long-working distance, two-pulse Digital Holographic Microscopy (DHM) configuration and is the first to replace film recording with digital sensors for this challenging application. A proposed multi-iteration DHM processing algorithm is demonstrated for automated measures of the sizes, velocities, and three-dimensional positions of non-spherical particles. Ejecta as small as 2 µm diameter are successfully tracked, while uncertainty simulations indicate that particle size distributions are accurately quantified for diameters ≥4 µm. These techniques are demonstrated on three explosively driven experiments. Measured ejecta size and velocity statistics are shown to be consistent with prior film-based recording, while also revealing spatial variations in velocities and 3D positions that have yet to be widely investigated. Having eliminated time-consuming analog film processing, the methodologies proposed here are expected to significantly accelerate future experimental investigation of ejecta physics.

The Texas A&M University Hypervelocity Impact Laboratory: A modern aeroballistic range facility

Novel engineering materials and structures are increasingly designed for use in severe environments involving extreme transient variations in temperature and loading rates, chemically reactive flows, and other conditions. The Texas A&M University Hypervelocity Impact Laboratory (HVIL) enables unique ultrahigh-rate materials characterization, testing, and modeling capabilities by tightly integrating expertise in high-rate materials behavior, computational and polymer chemistry, and multi-physics multiscale numerical algorithm development, validation, and implementation. The HVIL provides a high-throughput test bed for development and tailoring of novel materials and structures to mitigate hypervelocity impacts (HVIs). A conventional, 12.7 mm, smooth bore, two-stage light gas gun (2SLGG) is being used as the aeroballistic range launcher to accelerate single and simultaneously launched projectiles to velocities in the range 1.5-7.0 km/s. The aeroballistic range is combined with conventional and innovative experimental, diagnostic, and modeling capabilities to create a unique HVI and hypersonic test bed. Ultrahigh-speed imaging (10M fps), ultrahigh-speed schlieren imaging, multi-angle imaging, digital particle tracking, flash x-ray radiography, nondestructive/destructive inspection, optical and scanning electron microscopy, and other techniques are being used to characterize HVIs and study interactions between hypersonic projectiles and suspended aerosolized particles. Additionally, an overview of 65 2SLGG facilities operational worldwide since 1990 is provided, which is the most comprehensive survey published to date. The HVIL aims to (i) couple recent theoretical developments in shock physics with advances in numerical methods to perform HVI risk assessments of materials and structures, (ii) characterize environmental effects (water, ice, dust, etc.) on hypersonic vehicles, and (iii) address key high-rate materials and hypersonics research problems.

Acousto-optically driven lensless single-shot ultrafast optical imaging

Driven by many applications in a wide span of scientific fields, a myriad of advanced ultrafast imaging techniques have emerged in the last decade, featuring record-high imaging speeds above a trillion-frame-per-second with long sequence depths. Although bringing remarkable insights into various ultrafast phenomena, their application out of a laboratory environment is however limited in most cases, either by the cost, complexity of the operation or by heavy data processing. We then report a versatile single-shot imaging technique combining sequentially timed all-optical mapping photography (STAMP) with acousto-optics programmable dispersive filtering (AOPDF) and digital in-line holography (DIH). On the one hand, a high degree of simplicity is reached through the AOPDF, which enables full control over the acquisition parameters via an electrically driven phase and amplitude spectro-temporal tailoring of the imaging pulses. Here, contrary to most single-shot techniques, the frame rate, exposure time, and frame intensities can be independently adjusted in a wide range of pulse durations and chirp values without resorting to complex shaping stages, making the system remarkably agile and user-friendly. On the other hand, the use of DIH, which does not require any reference beam, allows to achieve an even higher technical simplicity by allowing its lensless operation but also for reconstructing the object on a wide depth of field, contrary to classical techniques that only provide images in a single plane. The imaging speed of the system as well as its flexibility are demonstrated by visualizing ultrashort events on both the picosecond and nanosecond timescales. The virtues and limitations as well as the potential improvements of this on-demand ultrafast imaging method are critically discussed.

History and metrology applications of a game-changing technology: digital holography [Invited]

In digital holography (DH), information in the hologram is recorded and stored in digital format in discrete bits. Like its parent, holography, DH evolved over many years with periods of dormancy and revival. Almost abandoned, multiple times, unanticipated events or developments in separate industries revived it with explosive, quantum jumps, making it useful and popular to a wide audience. Although its history has been treated in many papers and books, the field is dynamic and constantly providing new opportunities. Having been born long before low-cost, fast, powerful digital computers and digital detectors were available, DH was confined to the academic world, where practical applications and commercial opportunities were few if any. Consumer demand that led to low-cost personal computers, high-resolution digital cameras, supporting software, and related products changed the situation drastically by providing every potential researcher affordable, powerful hardware and software needed to apply image processing algorithms and move DH to new practical application levels. In this paper, as part of the sixtieth anniversary of off-axis holography, we include a brief introduction to the fundamentals of DH and examine the history and evolution of DH during its periods of rise and fall. We summarize many new emerging techniques, applications, and potential future applications along with additional details for metrological examples from the authors' research.

0.1-5 MHz ultrahigh-speed gas density distributions using digital holographic interferometry

Gas density distributions for an underexpanded jet at several different pressure ratios were measured at ultrahigh speeds in this work using digital holographic interferometry (DHI). DHI measurements have generally been performed on the order of several Hz in the literature, although some recent groups report measurements at 10 and 100 kHz. We demonstrate 2D imaging of gas density distributions at imaging rates up to 5 MHz, which is an increase by a factor of 50 compared to the previous DHI literature. A narrow-linewidth, continuous-wave laser was used in a Mach-Zehnder configuration, and the holograms were recorded using one of two different CMOS cameras. The interferograms were analyzed using the Fourier method, and a phase unwrapping was performed. Axisymmetric flow was assumed for the region near the nozzle exit, and an Abel inversion was performed to generate a planar-slice gas density distribution from the line-of-sight unwrapped phase. The challenges and opportunities associated with performing DHI measurements at ultrahigh speeds are discussed.

High-energy laser pulses for extended duration megahertz-rate flow diagnostics

Optical diagnostics of highly dynamic supersonic and hypersonic flows requires laser sources with a combination of high pulse intensities and fast repetition rates. A burst-mode Nd:YAG laser system is presented for increasing the overall energy of 532 nm pulse trains by ∼100× and the number of high-energy pulses by 30× for extended duration megahertz-rate flow diagnostics. At a lower repetition rate of 100 kHz, unprecedented energies near 1 J/pulse are achieved at 532 nm over a 1.1 ms burst. The laser performance is characterized and demonstrated for megahertz-rate laser-induced breakdown spectroscopy in a Mach 2 turbulent jet.