Laser-Based Optical Diagnostics

Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy
CARS has been first demonstrated by Maker and Terhune in 1965. The techniques involves a nonlinear four-wave mixing process generated by three collinear beams  illumination of a sample that enhances the weak Raman signals by several order of magnitude. The CARS signals carry important information on the dynamics of molecular vibrations and rotations and can be used for species identification, temperature and concentration measurement. Hybrid fs/ps CARS is an optimized CARS technique (Prince et at., 2006, Pestov et al., 2007) which uses wideband femtosecond pulses to induce vibrational and rotational transitions, and a time-delayed narrowband probe pulse to probe the molecular response, while suppressing the non-resonant background interference. The technique has enhanced chemical selectivity and it has been successfully used for species detection in liquid, solids and gaseous media, and temperature measurements in gases.

Femtosecond Laser Electronic Excitation Tagging (FLEET)
FLEET is a molecular tagging velocimetry technique invented and patented at Princeton University. This technique enables nonintrusive measurements of velocity of very high speed flows, inaccessible to traditional anemometer instruments. In the aerospace industry, high speed and turbulent flows are generated in wind tunnels to simulate and study the motion of hypersonic vehicles flying at high altitudes and in rarefied air conditions.
FLEET makes use of the high intensity and short pulse length of a femtosecond laser to turn the nitrogen molecules (the largest constituent of air) along the laser beam into long-time fluorescent tracers whose evolution can be monitored in time, as they move with the flow. The velocity of the flow can be precisely measured by gating the imaging of the tagged line at multiple successive times, following the line as it evolves with time. Since its invention, this technique has been widely applied in research laboratories and aerospace testing facilities.

Two-Photon-Absorption Laser Induced Fluorescence (TALIF)
TALIF is an optical diagnostic technique used for density measurement of atoms and molecules based on the absorption of laser radiation, their excitation and subsequent spontaneous fluorescent emission. In contrast to single-photon laser induced fluorescence (first reported by Zare and coworkers in 1968) which is based on using lasers in the UV range for atomic and molecular excitation, TALIF (first demonstrated in atomic hydrogen and deuterium by Bokor and coworkers in 1981) relies on a two-photon absorption process generated by deep UV lasers. With the advent of ultrafast lasers, femtosecond TALIF enables probing atoms and molecules at their fundamental time scale of vibrations. TALIF is a valuable tool for measurement of atomic density and temperature of reactive species (e.g., O, N, H) over a wide range of pressures. Nonintrusive and direct measurement of density, temperature and species concentration in aerospace ground test facilities are essential for validating the results of computational models.

Optical fiber-based hybrid fs/ps CARS System for Standoff Trace Detection of Chemicals

Standoff trace detection methods aim to detect traces of chemicals from a distance, without the operator. intervention. Optical spectroscopy techniques such as Raman and Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy can provide fingerprint detection for materials and trace-amount substances.

Speckodyne has designed and developed a standoff trace detection system based on the hybrid fs/ps CARS technique coupled with the enhanced capability of CARS (pump and Stokes) beams generation in an optical fiber. An all-fiber light source architecture enables generation of three CARS excitation beams from a single-unit, small footprint femtosecond fiber laser and a highly nonlinear optical fiber. This has enabled the development of a compact, transportable system. An X/Y stage system allows for micrometer scanning and CARS imaging. A software package for data acquisition, scanning detection and identification of target chemicals has demonstrated the fully operational capability of our system. Two versions of the prototype have been developed and delivered to NIITEK/Chemring Group, PLC.  System characterization and applications have been reported. 

Femtosecond Laser Electronic Excitation Tagging (FLEET) Velocimetry Measurement System

Speckodyne Corp. and Plasma TEC, Inc. have designed and developed a FLEET velocimetry measurement system for direct velocity measurements of flow velocity in large scale hypersonic wind tunnel facilities. 1 kHz rate FLEET velocimetry has been demonstrated in hypersonic Mach 10, 14 (2016-2017) and Mach 18 (2019-2020) freestream and boundary layer flow at the AEDC Hypersonic Wind Tunnel 9 in White Oak, MD. For the first time at this facility the theoretical predictions have been confronted with direct experimental measurements. The system development effort has been carried out within a Phase I&II AFOSR SBIR program awarded to Plasma TEC, Inc. Two enhancement programs have supported the effort of expanding the system capability with the addition of hybrid fs/ps CARS spectroscopy. Rotational and vibrational CARS temperature measurements at the AEDC Tunnel 9 enabled the first demonstration of nonequilibrium hypersonic flow conditions.

Dual Femtosecond Laser Electronic Excitation Tagging (FLEET) and Two-Photon Absorption Laser Induced Fluorescence (fsTALIF) system for measurements in arc-jet facilities

Arc-jet flows generated in ground testing facilities allows for testing materials and development and validation of thermal protection systems for hypersonic re-entry vehicles. Optical diagnostics are crucial in providing experimental data for testing and modeling.

Plasma TEC, Inc. in collaboration with Speckodyne has designed, developed and demonstrated a dual, fsTALIF and FLEET system for atomic species and, respectively flow velocimetry measurements in arc-jet facilities.

The system has been implemented at The University of Texas at Arlington's Aerodynamics Research Center (UTA ARC) in Oct. 2020, where, in collaboration with Princeton University, milestones measurements of both fsTALIF and FLEET have been achieved in the arc-jet plasma wind tunnel. By tailoring the FLEET detection and imaging strategy to address the specifics of the arc-jet environment, an enhanced signal- to- noise ratio was achieved. Atomic oxygen and velocity measurements in Mach 5 (2000 m/s) hypersonic flow were obtained. This provided kHz rate imaging of velocity profiles, and it marks the first time FLEET has been implemented in a high enthalpy hypersonic arc facility. 

Multifunctional Laser System for 1D and 2D Imaging Diagnostic for High-Enthalpy Test Facilities

Speckodyne in collaboration with Plasma TEC, Inc. and Princeton University have proposed and demonstrated the feasibility of a novel, multifunctional optical diagnostic platform for kilohertz rate, non-intrusive, quantitative 1D and 2D imaging of relevant gas parameters in arc driven and other high enthalpy ground testing facilities. The feasibility study has been carried out within a Phase I NASA SBIR program awarded to Speckodyne Corp.

Speckodyne was recently awarded a Phase II NASA SBIR contract for the prototype development, implementation, delivery and demonstration at NASA AMES's ground-based facilities.