Weather modelling, detection of clean air turbulence, detection of wind shear, and other applications benefit from precise wind speed measurements. This work reports on progress towards a direct detection lidar velocimetry method utilizing an adaptable vapor filter. The method uses spectral hole burning, in which a pump beam excites atoms in a vapor cell moving them from the ground state to a trapping state, creating deep “holes” in the inhomogenously transmission profile. A three-level atomic model was created and experimentally validated to characterize the position and magnitude of the hole in the transmission profile, as well as the time required to reach equilibrium, under different pump frequencies and powers. Work towards using the technique for tabletop velocimetry measurements of an under-expanded jet is described.

In this paper, we present the design, characterization, and calibration of an inverted pendulum thrust stand designed for ultra-sensitive measurement of pulsed, periodic thrust. Target applications include photonic propulsion, breakthrough propulsion, and other low-thrust propulsion mechanisms that can be modulated. High sensitivity is achieved by implementing optical heterodyne measurement of displacement, which we show achieves sub-nm resolution. In addition, the high quality factor of the rotational resonance in the stand is exploited to amplify the displacement response to pulsed, resonant thrust generation by several orders of magnitude. As a demonstration of the stand's high sensitivity, we perform a first calibration using light pressure generated by nine reflections of a continuous-wave laser producing an average power of up to 10Watts. The calibration is performed at a resonant frequency of f = 0.748 +/- 0.005 Hz and a quality factor of Q similar to 85. Using a modulated square wave forcing with 50% duty cycle, we determine a sensitivity slope of 464 +/- 20 nN/cycle and a detection limit of < 100 nN. To the authors' knowledge, this is the first use of light pressure for thrust stand calibration and establishes a sensitivity suitable for measuring modulated thrust levels for a variety of low-thrust applications.

The long-term objective of the Artemis program of a sustained lunar presence requires scalable and terrain adaptable power delivery, particularly for transmitting power to Permanently Shadowed Regions near the lunar south pole. These regions, shielded from direct sunlight due to lunar axial tilt and topography, are rich in volatiles but challenging for power delivery by physical cabling. A novel approach using self-guided optical power beaming - enabled by mutual coupling between a laser and a co-propagating cold atom beam- offers a compelling solution. The physical interaction between the laser and particle beams minimizes diffraction through refractive index gradients, while the laser's dipole forces confine neutral atoms. This works aims to analyze the potential for generating curved power beaming trajectories with this concept under lunar gravity. To assess system feasibility, optimal beam trajectories were computed using a symplectic integrator and altimetry data from the Lunar Reconnaissance Orbiter. The analysis identified the minimum radius of curvature and optimal velocity required to maintain terrain clearance. Optimum trajectories were then translated into beam design requirements, including required refractive index contrast, dipole trapping strength, and bending tolerances governed by the distortion sensitivity parameter. Results indicate that for allowable curvature corresponding to 10% of the lunar radius, over 53% of a 200 km x 200 km region near Shackleton Crater becomes accessible, or over 2.5x the reachable area of a comparable line-of-sight power beaming system. Gains are further amplified when considering only Permanently Shadowed Regions, where we observe a 6x increase in accessible area. These findings establish key design parameters for over-the-horizon power beaming and support its further investigation for power delivery to remote lunar sites.

This study investigates the use of combined femtosecond and nanosecond laser pulses to enhance laser-induced plasma discharges at high repetition rates and elevated pressures. For this purpose, synchronized and delayed laser pulses of 0.5 - 2 mJ energy were focused into a pressurized air chamber with visual diagnostics conducted using high-speed Schlieren imaging. While femtosecond pulses alone produced steady, localized heating, the addition of ns pulses produced brighter breakdown events accompanied by strong shock waves. While the breakdown frequency was observed to increase with chamber pressure, bright sparks remain relatively rare at approximately one per hundred femtosecond pulses. Imaging reveals that nanosecond breakdown events are localized near the femtosecond laser beam axis, although avalanche breakdown occurred most often between 0.14 - 0.20 ms after femtosecond pulses, rather than coinciding with the pulses. This was further established by the observation of no significant difference between synchronized and unsynchronized pulse trains, despite the requirement of femtosecond laser excitation to observe avalanche breakdown. These findings suggest that while the initial fs laser ionization is insufficient to seed most nanosecond breakdown events, the interaction between pulses may play a crucial role in producing species, such as negative ions, that stimulate breakdown on subsequent nanosecond pulses.

Density fluctuation measurements with high spatio-temporal resolution provide meaningful insights into the dynamics of turbulent structures in shear layers, boundary layers, and disturbances that influence the transition to turbulence. The current contribution presents a new multiple perspective digital holographic imaging technique that aims to resolve high-frequency density fluctuations with spatial discrimination along the line of sight. A key element of the proposed method is the generation of a beamlet array of varying view angles that is spatially overlapped within the measurement zone. We present the first experimental realization of this approach using a 7-element hexagonally packed fly’s eye lens array and analyze results for a pair of mutually perpendicular and axially offset sonic jets. The experimental apparatus also incorporates heterodyne detection with a heterodyne frequency set to 1/4 of the imaging frame rate. The optical system performance is evaluated for a crossing jet flow and key design considerations for the multi-perspective optical system are discussed.

This paper presents a comprehensive evaluation of unseeded Femtosecond Laser Electronic Excitation Tagging (FLEET) using a 1030 nm Yb:KGW femtosecond laser. In the first part of this work, fundamental studies were conducted in a vacuum chamber to assess the fluorescence process in dry air and nitrogen. Specifically, the effects of pressure, pulse energy, and focal length on the temporal decay of the nitrogen first and second positive band emission are quantified and reported. The results show similar trends to previous reports of FLEET emission behavior at other visible and near infra-red excitation wavelengths, indicating that the same fundamental fluorescence mechanism occurs after 1030 nm excitation. In the second part of this work, we explore the application of 1030 nm FLEET to continuous high-speed velocimetry in an under-expanded laboratory jet. Unseeded velocity measurements at 50 kHz in air were acquired at several points in the flow using burst gating of a high-speed intensifier. Frequency domain analysis of over 50,000 time-resolved measurements show strong periodic fluctuations near 8 kHz due to an acoustic resonance near the jet exit. Analysis of the streamwise velocity autocorrelation is shown to provide a means to separate uncorrelated random measurement error from true flow fluctuations. These results establish a first demonstration of continuous, high-speed FLEET measurements at 1030 nm and provides foundational data to inform future applications of this unseeded velocimetry technique in turbulent high-speed flows.

In this work, the authors discussed an experimental investigation of different design solutions and pumping arrangements for efficient ultraviolet pumping of an Optical Parametric Oscillator by a Pulse-Burst Laser. Cavity length, pump beam profile, and pump pulse duration were considered and their effects on the resulting conversion efficiency were quantified. Signal conversion efficiencies as high as 28 % were recorded for 250 kHz long burst pumping with 8.25 ns pulses. It was also determined that longer pump pulses are beneficial for pumping OPO cavities exceeding 70 mm in length, keeping the pulse length at 12 to 15 times the cavity round trip time or longer. Additionally, a compact design for a crystal mount with a synchronous counter-rotating mechanism was presented. Several cavities with such a crystal holder design were constructed with the lengths varying between 25 mm for a 532 nm pumped OPO to 40 mm for a 355 nm pumped OPO. Lastly, the replacement of a high reflectivity mirror in the cavity with a total-internal-reflection retroreflector prism was discussed with the primary benefits being the ease of alignment and a more uniform signal beam profile produced.

A key issue when designing hypersonic vehicles is the design of the control system. At hypersonic speeds, changes in the flow field can occur rapidly which necessitates fast response time control effectors. Traditional actuator-driven control surfaces can be bulky and may exhibit a relatively slow response time. Conversely, energy deposition-based control effectors have very fast response times which may be needed for high-speed systems. Various energy deposition techniques have been investigated historically, including laser-based approaches and electrical discharges, which have been explored separately. There are often challenges associated with these energy deposition techniques, including non-precise location control and low efficiency. The present effort is focused on combining a fs laser pulse to create a low resistance plasma filament between a pair of electrodes, followed by a higher-efficiency electrical discharge which supplies the bulk of the energy to the flow through this low resistance region. Static vacuum chamber tests were performed to calibrate the interaction between the laser guide and electrical discharge. A series of hypersonic wind tunnel tests were then performed over a canonical wedge and compression ramp geometry to demonstrate effectiveness of reducing the surface pressure downstream of the energy deposition region in a high-speed convective flow. This dual-mode energy deposition flow control configuration enables both precise location control and higher energy efficiency into the air, which may lead to more effective hypersonic flow control approaches in future systems.

Precise wind velocity measurements are of interest for a variety of applications including weather forecasting and detection of clean air turbulence and wind shear. This work investigates a concept known as spectral hole burning to create an actively tunable atomic vapor filter in order to improve the resolution of LIDAR wind velocity profiling. By adjusting the frequency and intensity of a pump beam in an alkali vapor cell, steep slopes in the transmission profile may be used to monitor shifts in frequency of a probe beam. A three-level atomic model to simulate the spectral hole is described and validated experimentally through pump-probe spectroscopy. The model then optimizes parameters including cell length, cell temperature, pump intensity and pump frequency to find the steepest transmission slope obtainable. At this location, frequency resolutions of 0.69 MHz are predicted per 1% change in transmission, corresponding to a 0.27 m/s velocity sensitivity. From this information, the feasibility and limitations of using spectral hole burning as a controllable atomic filter for LIDAR velocimetry measurements are discussed.

Femtosecond laser electronic excitation tagging (FLEET) velocimetry is applied in a hypersonic boundary layer behind an array of turbulence-inducing trips. One-dimensional mean velocity and root-mean-square (RMS) of velocity fluctuation profiles are extracted from FLEET emissions oriented across a 2.75. wedge and through a boundary layer above a flat plate in two test campaigns spanning 21 tunnel runs. The experiment was performed in the Texas A&M University Actively Controlled Expansion tunnel that operated near Mach 6.0 with a Reynolds number near 6 x 106 m(-1) and a working fluid of air at a density near 2.5 x 10(-2) kg m(-3). Detailed analysis of random and systematic errors was performed using synthetic curves for error in the mean velocity due to emission decay and the error in the RMS velocity fluctuation due to random error. The boundary layer behind an array of turbulence-inducing trips is documented to show the breakdown of coherent structures. FLEET velocimetry is compared to the tunnel Data Acquisition System, Vibrationally Excited Nitric Oxide Monitoring results, and Reynolds-Averaged Navier-Stokes computational fluid dynamics to verify results.

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as spectral/angular filters, have often been used as a point measurement technique, with spatial resolution as low as a few hundred μm, defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several μm resolution over a few millimeters along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented,i.e., for a focused probe beam waist with a diameter of ∼150 μm, the rejection ring can provide resolution up to several millimeter length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated, using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the tradeoff of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters, to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility, to accommodate various experimental conditions.

In weakly ionized plasmas such as those associated with hypersonic flows, Thomson scattering (TS) measurements often suffer from background interference from Rotational Raman scattering (RRS) and plasma luminosity. To overcome this background interference, we propose a new concept, Filtered low-angle Thomson scattering (FLATS), the principle of which is based on the frequency narrowing of the Thomson scattering collected in the forward direction in contrast to the collection angle independent Rotational Raman scattering and plasma luminosity. In this work, we explain the principles of FLATS and present experimental observations of TS and RRS at multiple collecting angles, which confirm the narrowing TS and the frozen RRS at such angles. Additionally, successful spectral separation of TS from background scattering at a low-angle of 22 deg observation, which leads to improved measurement accuracy, is demonstrated. Combined with an atomic vapor prism that can serve as a dispersion-based spectrometer, this technique is expected to enable Thomson measurement in low-temperature, partially ionized plasmas in air/hypersonic environments.

In this article, the authors present an experimental investigation of the thermochemical relaxation regions downstream of symmetric Mach reflections generated in Mach 8.5 flows with stagnation enthalpies from 7 to 10 MJ/kg. The experiment was performed in the Hypervelocity Expansion Tunnel at the National Aerothermochemistry and Hypersonics Laboratory of Texas A&M University. The investigation focused on studying the nitric oxide morphology in the vicinity of a strong normal shock wave and shear layers. Experimental planar laser-induced fluorescence measurements at 250 kHz repetition rate with a quasi-simultaneous natural emission photography demonstrate a sufficient rejection of natural emission and reveal intricate flow structures in the shock vicinity. Steady computational fluid dynamics (CFD) flow solutions obtained with the US3D software were used to simulate laser-induced fluorescence signal levels for quantitative comparison against the experimentally acquired data. The investigation accomplished four objectives: complex flowfield visualization in the vicinity of the Mach stem with 4 μs temporal resolution, evaluation of the level of high-temperature chemistry effects introduced to the freestream by the expansion tunnel, both qualitative and quantitative CFD simulation validation, and cross-checking or introduction of fundamental properties of nitric oxide laser-induced fluorescence including absorption cross section for vibrationally excited states of nitric oxide and a collisional quenching cross section of nitric oxide by atomic nitrogen.

In this article, the authors discuss an experimental demonstration of a seeded Optical Parametric Oscillator as a light source for Slow Light Imaging Spectroscopy applications. An advantage of such a system, namely narrow linewidth of less than 400 MHz, allows for precise profiling of slow light features of atomic rubidium vapor, produced by high refractive index gradients in the vicinity of resonant transitions. Time delays of up to 48 ns were observed with a clear definition of the delayed pulse from the parasitic Mie and Rayleigh scattering. A comparison between the experimental data and numerical simulation over a frequency detuning range from -1 GHz to 4 GHz shows a good agreement for both qualitative and quantitative results.