Ultrafast Optical Diagnostics

CARS was initially demonstrated by Maker and Terhune in 1965. The technique enables non-intrusive measurements of molecular vibrations with high spatial resolution with a wide range of applications due to its chemically specific and high resolution measurement capability. CARS involves a nonlinear four-wave mixing process, in which three collinear laser beams illuminate a sample to amplify otherwise weak Raman signals by several order of magnitude. CARS spectral signatures provide insight into molecular vibrational and rotational dynamics, enabling accurate species identification, temperature and concentration measurement. Hybrid fs-ps CARS is an optimized CARS technique (Prince et al., 2006, Pestov et at., 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 measurement in gaseous flows. Leveraging a patented innovation (Dogariu, US9163988B2) for all-fiber based CARS beams generation, Speckodyne has developed prototype systems that integrate the fs-ps CARS technique into a functional technology for trace-species chemical detection. This prototype features a compact and portable design, operating with a small-footprint femtosecond laser source. 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. Trace detection can be achieved in less than 100 ms. System characterization and applications have been reported.

Continued R&D efforts led to the creation of an add-on module for CARS spectroscopy-based vibrational and rotational temperature measurements in hypersonic and high-speed flows, plasma diagnosis and combustion research. The add-on module has been successfully integrated into multiple multifunctional platforms, enabling CARS thermometry measurements and enhancing the optical diagnostic and measurement capabilities of several hypersonic testing facilities.

FLEET is a molecular tagging velocimetry technique invented and patented at Princeton University (US9863975B2). This technique enables non-intrusive measurements of velocity of high-speed gaseous 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 employs a high-intensity femtosecond laser with short pulses to excite nitrogen molecules – the dominant component of air along the laser beam -into long-duration fluorescent tracers, allowing their temporal evolution to be monitored as they they are carried by the flow. The velocity of the flow can be precisely measured by gating the imaging of the tagged line at successive time intervals, allowing its temporal displacement to be followed. Since its invention, this technique has been widely applied in research laboratories and aerospace testing facility. Speckodyne and Plasma TEC, Inc. have pioneered the design a FLEET velocimetry measurement system for a large scale hypersonic wind tunnel facility. 1 kHz rate FLEET velocimetry has been demonstrated in hypersonic Mach 10, 14 and 18 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.

Supported by SBIR funding, Speckodyne has integrated FLEET velocimetry and CARS thermometry into a multiparameter measurement system which has equipped academic research and hypersonic wind testing facilities ( e.g., UTA ARC center, NASA AMES) with the state-of-the art technology.

TALIF is an optical diagnostic technique used for density measurements 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 laser 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, H, N) over a wide range of pressures. Non-intrusive and direct measurement of density, temperature and species concentration in aerospace ground testing facilities are essential for validating the results of computational models.

Supported by SBIR funding, Speckodyne has integrated the TALIF capability into a multifunctional system (FLEET, CARS, TALIF) and delivered it for applications in hypersonic wind testing (e.g., UTA ARC center, NASA AMES).