Injection Locked, High Pulse Energy Alexandrite Laser System for Atomic Filter Based Diagnostics of
Abstract
Abstract Publically ReleasableThere is a compelling need to develop advanced diagnostic methods for understandinghypersonic flow f,ields and associated nonequilibrium interactions with ablated materials andvehicle surfaces in order to identify critical physical p,rocesses, validate computational models, andfacilitate the design of advanced and robust hypersonic platforms. Methods that utilize,lasers areparticularly attractive since they can acquire data remotely by accessing spectral features ofmolecular and atomic species,. In contrast to Laser Induced Fluorescence, Raman and Thomsonscattering are not affected by quenching, ionization or predissociatio,n, so they can measurelocalized true gas, electron and ion properties and are thus of primary interest for diagnostics.Raman scatter,ing provides direct measures of molecular state populations, temperature andspecies. Atomic oxygen also has a Raman spectrum and thu,s it can also be directly measured.Thomson scattering provides a measure ofelectron and ion density and temperature. Raman andThomso,n scattering are weak, but the use of atomic filtering methods for diagnostics is extremelypromising since atomic filters placed in,front of cameras allow suppression of out of bandbackground light while providing exceptionally high spectral resolution and high co,llectionefficiency. This approach requires high pulse energy, frequency agile, narrow line width lasers.The proposed injection locke,d Alexandrite laser is unique in its capabilities and well suitedfor the atomic filter diagnostic approaches. It differs significant,ly from the more standard injectionlocked Nd:YAG laser in that it is frequency tunable over a wide range in the near infrared (720nm, to 800 nm), it has a long pulse length (~70 nsec - so it is less limited by optical breakdown thusincreasing the maximum Raman and,Thomson scattering by a factor of ten over pulses from theNd:YAG lasers), it has ultra narrow linewidth for high resolution spectros,copy, and it hasexceptionally low out-of-band Amplified Spontaneous Emission. There are no atomic filters thatoverlap the Nd:YAG las,er spectrum, but many that overlap the Alexandrite laser spectrum.Ti:sapphire lasers, Optical Parametric Oscillators and dye lasers,can be tuned to relevantwavelengths, but they need other lasers such as the Nd:YAG laser to pump them and they havefundamentally bro,ader linewidths, short pulse lengths, and significant out of band background. Inall these cases the complexity of the system is grea,tly increased compared to the Alexandrite laser,the output pulse energy greatly decreased, and out of band light becomes problematic,.The Alexandrite laser will enable methods that include Filtered Low Angle ThomsonScattering (FLATS) for the measurements of electro,n density and temperature, RotationalRaman by Atomic Prism Spectroscopy (RRAPS) for simultaneous multicomponent onedimensional imagi,ng of species and nonequilibrium states, atomic re-fluorescence imaging fortwo dimensional Raman imaging of selected molecular rotat,ional states, and ultraviolet FilteredRayleigh Scattering (FRS) imaging of gas density, temperature and pressure.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 05, 2022
- Source ID
- N000142212241
Entities
People
- Richard B. Miles
Organizations
- Office of Naval Research
- Texas Engineering Experiment Station
- United States Navy