Investigator: Boris S. Leonov
In the fields of hypersonics, combustion, and high-enthalpy flows, the time scales of transient processes and boundary layer structures are from 100th µs down to nanosecond scale [TJL13]. Hypersonics, in particular, is striving for quantitative, non-intrusive high speed and repetition rate diagnostic tools and imaging techniques. In this field, some of the phenomena of special interest for researchers are the laminar-turbulent boundary layer transition as well as Shock Wave Boundary Layer Interactions (SWBLI). Both of these phenomena are critically important for practical applications, including hypersonic flight, because of the strong impact on aerodynamic forces, heat transfer and ablation as well as potential plasma shielding that a vehicle might experience. In experimental investigations, it was shown that the turbulent modes in a tripped, cold, turbulent hypersonic boundary layer have spectral signatures at 50, 100 and 200 kHz [SB17]. A general dependence of spatial scale and speed of an object on the temporal resolution of an imaging system is shown in Fig. 1. Using an example of a turbulent boundary layer, the spatial scale of interest is on the order of millimeters with speeds of . Additionally, the Nyquist-Shannon sampling theory should be considered. This theory indicates that to measure a phenomenon, diagnostics should be performed at least at a double acquisition rate . This results in a MHz acquisition rate requirement for hypersonic diagnostic systems [MGG16; TJL13].Hypersonic ground testing facilities, such as the Hypervelocity Expansion Tunnel (HXT) at Texas A&M University, greatly benefit from high speed and non-intrusive diagnostic tools, which first became available with the invention of lasers in 1960’s. In such diagnostic approaches, a relatively small amount of laser energy couples into the internal modes of atoms and molecules via a nano-, pico- or femtosecond laser pulse; gas properties are then determined by the resulting behaviour which is observed through absorption, scattering or fluorescence. With current advances in technology, MHz repetition rate Nd:YAG based fully solid state Pulse-Burst Lasers (PBL) are becoming commercially available, which brings laser diagnostic capabilities to the desired repetition rates [WM00; Jia+08].
Out of a variety of laser applications in the field of diagnostics ranging from Laser-Induced Breakdown Spectroscopy (LIBS) to Laser Induced Fluorescence (LIF), there are several of particular interest for high enthalpy hypersonic experimentation. Such techniques are Molecular Tagging Velocimetry (MTV) for velocity field measurements; filtered Rayleigh and Thomson scattering for free stream measurement of number densities of molecules and electrons respectively [Bak+00].
In general, MTV, being a pump-probe technique, requires two laser beams of a particular wavelength. Traditionally, Nd:YAG pumped dye lasers have been used in order to achieve a desired wavelength output. The disadvantages of such method are its broad linewidth (order of GHz) as well as the requirement of changing the entire dye used in the laser to make adjustments to the output wavelength. Additionally, conventional dye lasers are not suitable for high repetition rates of PBLs [Jia+08]. Another solution to making a variable wavelength output laser is use of Optical Parametric Oscillators (OPO) that utilize non-linear crystals instead of a dye. In this work, a two-wavelength pumped dual-OPO scheme will be combined with an Optical Parametric Amplifier (OPA) stage to achieve the desired wavelengths at 1 MHz repetition rate. The required system efficiency and improved performance will be achieved by implementing injection seeding with active cavity locking, which has not yet been demonstrated on a burst-mode-laser-pumped OPO.
An Optical Parametric Oscillator is a device that uses non-linear gain media, represented by a solid-state crystal, and parametric amplification by a cavity to transform a given beam into a beam of a desired frequency [Hau+91]. Usually, BBO crystals are used as gain media for signal and idler beams (output) that are generated by splitting a high-energy pump beam photon into two of lower energies. Such photon energy splitting allows OPOs to output tunable radiation (depending on an angle with respect to internal crystal lattice) from the Ultra-Violet (UV) to Near-Infrared (NIR) with a total conversion efficiency of up to 40%. A general schematic of an OPO cavity is shown in Fig. 2 with a High Reflectivity Mirror (HRM) and Output Coupler (OC) forming a cavity that provides feedback for the gain media.
Although general OPOs are advantageous, a basic or “free-running” OPO has several drawbacks that led to the use of a conventional Nd:YAG plus dye laser combination. It has been observed that if a single-mode pump beam power is exceeding the lasing threshold of the crystal, signal and idler are spectrally broadened and usually multi-mode [AW04]. This significantly reduces usability of such beam for diagnostic techniques requiring molecular tagging or exciting of a specific molecular transition. Improvement in spectral purity of the signal can be achieved by injection-seeding with a narrow band Continuous Wave (CW) laser at either signal or idler wavelength [Fix+93].Injection-seeding introduces a strong initial condition for the parametric amplification process which eliminates unwanted spectral modes as well as provides reduction of a lasing threshold and 20% increase in power [Whi+04; Hau+95]. Resulting linewidths as low as 135 MHz have been previously demonstrated [Bax+00]. Modern implementation of injection-seeding requires matching the cavity resonance and injection seeding frequency, also known as, cavity locking. This technique allows use of medium and high finesse cavities without sacrifice in spatial beam quality [Kul+05]. In the Ref. [Kul+05], it was shown that a high finesse cavity can have 4 times greater output power in comparison to a low finesse cavity. In this work, we design and build such improved OPOs for use with a Pulse-Burst Laser at MHz operation rate, first time to the best of our knowledge.
Advances in hypersonic ground testing set the new requirement for diagnostic systems – MHz acquisition rates. With the development of Pulse-Burst Lasers, laser diagnostics have become the most versatile tool for studying dynamic phenomena with high temporal and spatial resolution. Currently, we are testing performance of a commercial OPO with Injection Seeding and Active Cavity Locking developed in house. A custom Ultra High-Resolution Spectrometer combined with a high-speed camera (2 MHz) and a beam profiler is used for monitoring the beam quality. Based on the results of preliminary work and extensive literature review we have designed and are currently building a pair of custom OPOs (pumped by 3rd and 2nd harmonics). In this work, we plan to develop a Combined Optical Parametric System to be used with a Pulse-Burst Laser to bring MTV in general and Krypton Tagging Velocimetry in particular to MHz acquisition rates. This new capability will allow nonintrusive investigation of velocity fields in high enthalpy flows, which will be verified through boundary layer velocity measurements in the Texas A&M Hypervelocity Expansion Tunnel (HXT).
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