[ { "id": "authors:hpdn2-mwg73", "collection": "authors", "collection_id": "hpdn2-mwg73", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SCHLao00", "type": "article", "title": "Accuracy and uncertainty of single-shot, nonresonant laser-induced thermal acoustics", "author": [ { "family_name": "Schlamp", "given_name": "Stefan", "clpid": "Schlamp-S" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Sobota", "given_name": "Thomas H.", "clpid": "Sobota-T-H" }, { "family_name": "Cummings", "given_name": "Eric B.", "clpid": "Cummings-E-B" } ], "abstract": "We study the accuracy and uncertainty of single-shot nonresonant laser-induced thermal acoustics measurements of the speed of sound and the thermal diffusivity in unseeded atmospheric air from electrostrictive gratings as a function of the laser power settings. For low pump energies, the measured speed of sound is too low, which is due to the influence of noise on the numerical data analysis scheme. For pump energies comparable to and higher than the breakdown energy of the gas, the measured speed of sound is too high. This is an effect of leaving the acoustic limit, and instead creating finite-amplitude density perturbations. The measured thermal diffusivity is too large for high noise levels but it decreases below the predicted value for high pump energies. The pump energy where the error is minimal coincides for the speed of sound and for the thermal diffusivity measurements. The errors at this minimum are 0.03% and 1%, respectively. The uncertainties for the speed of sound and the thermal diffusivity decrease monotonically with signal intensity to 0.25% and 5%, respectively.", "issn": "0003-6935", "publisher": "Optical Society of America", "publication": "Applied Optics", "publication_date": "2000-10-20", "series_number": "30", "volume": "39", "issue": "30", "pages": "5477-5481" }, { "id": "authors:gcj8j-6xd71", "collection": "authors", "collection_id": "gcj8j-6xd71", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SCHLmst00", "type": "article", "title": "Neural network data analysis for laser-induced thermal acoustics", "author": [ { "family_name": "Schlamp", "given_name": "Stefan", "clpid": "Schlamp-S" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Cummings", "given_name": "Eric B.", "clpid": "Cummings-E-B" } ], "abstract": "A general, analytical closed-form solution for laser-induced thermal acoustic (LITA) signals using homodyne or heterodyne detection and using electrostrictive and thermal gratings is derived. A one-hidden-layer feed-forward neural network is trained using back-propagation learning and a steepest descent learning rule to extract the speed of sound and flow velocity from a heterodyne LITA signal. The effect of the network size on the performance is demonstrated. The accuracy is determined with a second set of LITA signals that were not used during the training phase. The accuracy is found to be better than that of a conventional frequency decomposition technique while being computationally as efficient. This data analysis method is robust with respect to noise, numerically stable and fast enough for real-time data analysis.", "doi": "10.1088/0957-0233/11/6/323", "issn": "0957-0233", "publisher": "Measurement Science and Technology", "publication": "Measurement Science and Technology", "publication_date": "2000-06-01", "series_number": "6", "volume": "11", "issue": "6", "pages": "784-794" }, { "id": "authors:772fy-23x08", "collection": "authors", "collection_id": "772fy-23x08", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SCHLol00", "type": "article", "title": "Laser-induced thermal-acoustic velocimetry with heterodyne detection", "author": [ { "family_name": "Schlamp", "given_name": "Stefan", "clpid": "Schlamp-S" }, { "family_name": "Cummings", "given_name": "Eric B.", "clpid": "Cummings-E-B" }, { "family_name": "Sobota", "given_name": "Thomas H.", "clpid": "Sobota-T-H" } ], "abstract": "Laser-induced thermal acoustics (LITA) was used with heterodyne detection to measure simultaneously and in a single laser pulse the sound speed and flow velocity of NO2-seeded air in a low-speed wind tunnel up to Mach number M = 0.1. The uncertainties of the velocity and the sound speed measurements were ~0.2 m/s and 0.5%, respectively. Measurements were obtained through a nonlinear least-squares fit to a general, analytic closed-form solution for heterodyne-detected LITA signals from thermal gratings. Agreement between theory and experiment is exceptionally good.", "issn": "0146-9592", "publisher": "Optics Letters", "publication": "Optics Letters", "publication_date": "2000-02-15", "series_number": "4", "volume": "25", "issue": "4", "pages": "224-226" }, { "id": "authors:0cbzs-ad586", "collection": "authors", "collection_id": "0cbzs-ad586", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:SCHLao99", "type": "article", "title": "Beam Misalignments and Fluid Velocities in Laser-Induced Thermal Acoustics", "author": [ { "family_name": "Schlamp", "given_name": "Stefan", "clpid": "Schlamp-S" }, { "family_name": "Cummings", "given_name": "Eric B.", "clpid": "Cummings-E-B" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "abstract": "Beam misalignments and bulk fluid velocities can influence the time history and intensity of laser-induced thermal acoustics (LITA) signals. A closed-form analytic expression for LITA signals incorporating these effects is derived, allowing the magnitude of beam misalignment and velocity to be inferred from the signal shape. It is demonstrated how instantaneous, nonintrusive, and remote measurement of sound speed and velocity (Mach number) can be inferred simultaneously from homodyne-detected LITA signals. The effects of different forms of beam misalignment are explored experimentally and compared with theory, with good agreement, allowing the amount of misalignment to be measured from the LITA signal. This capability could be used to correct experimental misalignments and account for the effects of misalignment in other LITA measurements. It is shown that small beam misalignments have no influence on the accuracy or repeatability of sound speed measurements with LITA.", "issn": "0003-6935", "publisher": "Optical Society of America", "publication": "Applied Optics", "publication_date": "1999-09-20", "series_number": "27", "volume": "38", "issue": "27", "pages": "5724-5733" }, { "id": "authors:t3gqb-th135", "collection": "authors", "collection_id": "t3gqb-th135", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:CUMol95", "type": "article", "title": "Measurement of gas-phase sound speedand thermal diffusivity over a broad pressure range using laser-induced thermal acoustics", "author": [ { "family_name": "Cummings", "given_name": "Eric B.", "clpid": "Cummings-E-B" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Brown", "given_name": "Michael S.", "clpid": "Brown-M-S" }, { "family_name": "DeBarber", "given_name": "Peter A.", "clpid": "DeBarber-P-A" } ], "abstract": "We report on the detection and analysis of signals generated from gas-phase laser-induced gratings over a large range of static pressure (0.04\u2013100 atm). We employed the experimental technique of laser-induced thermal acoustics and performed measurements on mixtures of NO2 in air and CO2 as a function of pressure. Accurate analysis of the acquired data was obtained from a full theory, including beam size effects. The theory fully reproduces the observed data for a ratio of molecular mean free path to grating wavelength extending from 1 to 4 x 10^-4. Nonlinear, least-squares fits between modeled and experimental signals provided accurate values of the sound speed and thermal diffusivity.", "issn": "0146-9592", "publisher": "Optics Letters", "publication": "Optics Letters", "publication_date": "1995-07-15", "series_number": "14", "volume": "20", "issue": "14", "pages": "1577-1579" }, { "id": "authors:j1wcw-n9119", "collection": "authors", "collection_id": "j1wcw-n9119", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:CUMao95", "type": "article", "title": "Laser-induced thermal acoustics (LITA) signals from finite beams", "author": [ { "family_name": "Cummings", "given_name": "E. B.", "clpid": "Cummings-E-B" }, { "family_name": "Leyva", "given_name": "I. A.", "clpid": "Leyva-I-A" }, { "family_name": "Hornung", "given_name": "H. G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "abstract": "Laser-induced thermal acoustics (LITA) is a four-wave mixing technique that may be employed to measure sound speeds, transport properties, velocities, and susceptibilities of fluids. It is particularly effective in high-pressure gases (>1 bar). An analytical expression for LITA signals is derived by the use of linearized equations of hydrodynamics and light scattering. This analysis, which includes full finite-beam-size effects and the optoacoustic effects of thermalization and electrostriction, predicts the amplitude and the time history of narrow-band time-resolved LITA and broadband spectrally resolved (multiplex) LITA signals. The time behavior of the detected LITA signal depends significantly on the detection solid angle, with implications for the measurement of diffusivities by the use of LITA and the proper physical picture of LITA scattering. This and other elements of the physics of LITA that emerge from the analysis are discussed. Theoretical signals are compared with experimental LITA data.", "issn": "0003-6935", "publisher": "Optical Society of America", "publication": "Applied Optics", "publication_date": "1995-06-20", "series_number": "15", "volume": "34", "issue": "15", "pages": "3290-3302" }, { "id": "authors:f6aym-qxe61", "collection": "authors", "collection_id": "f6aym-qxe61", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:CUMol94", "type": "article", "title": "Laser-induced thermal acoustics: simple accurate gas measurements", "author": [ { "family_name": "Cummings", "given_name": "E. B.", "clpid": "Cummings-E-B" } ], "abstract": "Laser-induced thermal acoustics (LITA), an optical four-wave mixing technique, has been used for sensitive measurement of the sound speed, thermal diffusivity, acoustic damping rate, and complex susceptibility of a gas. In LITA, laser-induced acoustic waves scatter laser light into a coherent, modulated signal beam. A simple expression accurately describes the signal. Atmospheric sound speeds accurate to 0.5% and transport properties accurate to 30% have been measured in a single shot without calibration. LITA spectra have been taken of weak spectral lines of NO2 in concentrations of less than 50 parts in 10^9. Signal reflectivities up to 10^4 are estimated.", "issn": "0146-9592", "publisher": "Optics Letters", "publication": "Optics Letters", "publication_date": "1994-09-01", "series_number": "17", "volume": "19", "issue": "17", "pages": "1361-1363" }, { "id": "authors:yb6s9-42m44", "collection": "authors", "collection_id": "yb6s9-42m44", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20141106-154701486", "type": "monograph", "title": "Techniques of single-shot thermometry by degenerate four-wave mixing", "author": [ { "family_name": "Cummings", "given_name": "Eric", "clpid": "Cummings-E-B" } ], "abstract": "Proper evaluation of high-enthalpy and chemically reacting flows often requires unconventional diagnostics.\nTemperatures in reentry flows may exceed 10000 K within a few mean-free-paths of a shock front. This temperature\nrapidly drops as gas molecules dissociate into nonequilibrium species and radicals (Vincenti and Kruger, 1965). Even in flows without such harsh conditions, conventional thermometry and mass spectrometry measurements may have insufficient time and space resolution. Furthermore, these probes are intrusive, affecting the local flow field and chemical composition, e.g.,through catalysis. \n\nThese problems have led to the application of laser spectroscopy and nonlinear optical techniques for the noninvasive probing of reacting fluid flows. Two of these techniques in particular have reached a fair level of maturity: coherent anti-Stokes Raman scattering (CARS) and laser-induced fluorescence spectroscopy (LIFS) (Eckbreth, 1988). CARS is a coherent optical technique which allows accurate point measurements of species concentration and temperature.Its use is limited by signal-to-noise to probing majority species. LIFS, on the other hand, is an incoherent technique which is much more sensitive than CARS and allows planar measurements, but is much less amenable to quantitative analysis.\n\nDegenerate four-wave mixing (DFWM) is a coherent nonlinear optical technique that has only recently been\napplied to flows of gasdynamic interest. This technique offers the promise of the quantitative accuracy of CARS\n(Dreier, 1990), the sensitivity of LIFS, and the capability for excellent flow-imaging (Rakestraw, 1990; Ewart, 1989).\nIt can be applied in various ways to provide spatially resolved measurements of species concentration, vibrational\npopulations, rotational populations (and thereby temperature), molecular diffusion rates, and other transport properties of a flowing inert or chemically reacting fluid (Fourkas, 1991).\n\nDFWM and CARS may be applied for thermometry in narrowband laser scanning experiments by measuring\nthe ground-state rotational population distribution of a probed species to a Boltzmann distribution (Bervas, 1992). In CARS, rotational spectra are \"washed out\" at very high temperatures by Doppler broadening, complicating thermometry. Narrowband DFWM spectra are \"Doppler-free\" (Dreier, 1990), allowing laser-scanning thermometry of high\ntemperature gases encountered, for example, in reentry-type flows.\n\nDFWM has several advantages over LIFS. The DFWM signal is coherent (laser-like) allowing efficient signal\ncollection with very high f-number optics. This feature is especially important in flows where optical access is limited or where luminosity is high. In addition, DFWM is not inherently susceptible to quenching (Dreier. 1990), unlike LIFS, allowing the use of DFWM in many cases where LIFS cannot be used or cannot be quantitatively interpreted\nbecause of quenching.\n \nDegenerate four-wave mixing has the advantage of relative experimental simplicity. Unlike CARS, DFWM has\ntrivial phase-matching requirements. It also requires only one laser and can be applied to atomic species. On the other\nhand, the interpretation of CARS signals is simpler and much more developed than that of DFWM. In some cases\nthe choice of DFWM over CARS is a trade-off between experimental and computational difficulty. Because the cost\nof computing power is decreasing while the cost of lasers is relatively constant, this trade-off will continue to shift towards DFWM.\n\nThis paper provides an overview of two \"single-shot\" resonant degenerate four-wave mixing thermometry techniques\nsuitable for making measurements in unsteady flows or in pulsed facilities such as the T5 free piston shock\ntunnel. The first technique involves measuring rotational population distributions and fitting them to Boltzmann\ndistributions to obtain rotational temperatures, and has been the conventional technique used in CARS, LIFS, and now\nDFWM. Single-shot thermometry by this technique requires a broad bandwidth laser to probe a number of rotational\nlines simultaneously (Ewart, 1990; Yip, 1992). This requirement complicates both the experimental apparatus and\nsignal interpretation. The second technique involves time resolution of the DFWM signal. Using this novel technique,\nlocal sound speeds and transport coefficients can be inferred. The physics of this technique are discussed along with a derivation of an analytical expression for the signal generated by this technique and its experimental advantages and challenges. Other thermometry techniques, such as measuring translational temperatures via DFWM lineshapes, etc., are not included in this paper.\n\nThe physical processes underlying resonant DFWM in gases are first discussed generically in order to provide\ninsight into this phenomenon. The two techniques of thermometry are then detailed in light of these processes. Complications and experimental considerations are presented. An expression for the time-resolved DFWM thermometry signal is derived. A brief history of the analytical use of DFWM is outlined, concluding with suggestions for future directions of research.", "doi": "10.7907/9hs4-w091", "publisher": "California Institute of Technology", "publication_date": "1992" } ]