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ABSTRACT: Hyperspectral absorption spectroscopy is being used to monitor gas temperature, velocity, pressure, and H<sub>2</sub>O mole fraction in a research-grade pulsed-detonation combustor (PDC) at the Air Force Research Laboratory. The hyperspectral source employed is termed the TDM 3-FDML because it consists of three time-division-multiplexed (TDM) Fourier-domain mode-locked (FDML) lasers. This optical-fiber-based source monitors sufficient spectral information in the H<sub>2</sub>O absorption spectrum near 1350 nm to permit measurements over the wide range of conditions encountered throughout the PDC cycle. Doppler velocimetry based on absorption features is accomplished using a counterpropagating beam approach that is designed to minimize common-mode flow noise. The PDC in this study is operated in two configurations: one in which the combustion tube exhausts directly to the ambient environment and another in which it feeds an automotive-style turbocharger to assess the performance of a detonation-driven turbine. Because the enthalpy flow [kilojoule/second] is important in assessing the performance of the PDC in various configurations, it is calculated from the measured gas properties.
Applied Optics 04/2013; 52(12):2893-904. · 1.41 Impact Factor
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ABSTRACT: This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows.
Optics Express 01/2013; 21(1):1152-62. · 3.59 Impact Factor
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ABSTRACT: This paper describes a preliminary demonstration and validation of temperature imaging using hyperspectral H2O absorption tomography in controlled experiments. Fifteen wavelengths are monitored on each of 30 laser beams to reconstruct the temperature image in a 381 mm × 381 mm square room-temperature plane that contains a 102 mm × 102 mm square zone of lower or higher temperature. The hyperspectral tomography technique attempts to leverage multispectral information to enhance measurement fidelity. The experimental temperature images exhibit average accuracies of 2.3% or better, with pixel-by-pixel standard deviations of less than 1%. In addition, even when the internal zone is only 4 K cooler than the surroundings, its presence is still detectable; statistical analysis of the associated experimental image reveals a 98% confidence that the internal zone is in fact cooler than the surroundings.
Applied Optics 02/2011; 50(4):A29-37. · 1.41 Impact Factor
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ABSTRACT: A crucial aspect in the design of sensors based on absorption spectroscopy involves selecting the optimal transitions. Therefore, the goal of this paper is to develop a method of selecting multiple optimal transitions for the measurement of nonuniform temperature distributions based on absorption spectroscopy. Previously developed methods are largely restricted to the relatively simple case of selecting two transitions for uniform distributions. Our new method addresses the restrictions of previous methods and is applicable to more general cases. The method was validated using both numerical tests and experimental results and is expected to be useful in the design of sensors based on multispectral absorption spectroscopy.
Applied Spectroscopy 11/2010; 64(11):1274-82. · 1.66 Impact Factor
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ABSTRACT: Two time-division-multiplexed (TDM) sources based on fiber Bragg gratings were applied to monitor gas temperature, H(2)O mole fraction, and CH(4) mole fraction using line-of-sight absorption spectroscopy in a practical high-pressure gas turbine combustor test article. Collectively, the two sources cycle through 14 wavelengths in the 1329-1667 nm range every 33 μs. Although it is based on absorption spectroscopy, this sensing technology is fundamentally different from typical diode-laser-based absorption sensors and has many advantages. Specifically, the TDM lasers allow efficient, flexible acquisition of discrete-wavelength information over a wide spectral range at very high speeds (typically 30 kHz) and thereby provide a multiplicity of precise data at high speeds. For the present gas turbine application, the TDM source wavelengths were chosen using simulated temperature-difference spectra. This approach is used to select TDM wavelengths that are near the optimum values for precise temperature and species-concentration measurements. The application of TDM lasers for other measurements in high-pressure, turbulent reacting flows and for two-dimensional tomographic reconstruction of the temperature and species-concentration fields is also forecast.
Applied Optics 09/2010; 49(26):4963-72. · 1.41 Impact Factor
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ABSTRACT: We have adapted our in-cylinder Fourier-transform spectroscopy technique to measure absorption spectra in a reciprocating engine. Previously, we had used the technique for emission spectroscopy; the upgrade to absorption spectroscopy mode is important because it allows for more quantitative analysis of gas properties than is possible with emission spectroscopy. Here, we discuss fuel, H(2)O, and CO(2) spectra measured in an engine using a spark-plug-based probe for optical access and use the water portion of the spectra to determine in-cylinder gas temperature. The temperature results show that heat transfer effects can significantly bias thermometry when fiber-coupled engine probes are used.
Applied Optics 09/2010; 49(25):4728-34. · 1.41 Impact Factor
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ABSTRACT: A novel technique has been developed to obtain simultaneous tomographic images of temperature and species concentration based on hyperspectral absorption spectroscopy. The hyperspectral information enables several key advantages when compared to traditional tomography techniques based on limited spectral information. These advantages include a significant reduction in the number of required projection measurements, and an enhanced insensitivity to measurements/inversion uncertainties. These advantages greatly facilitate the practical implementation and application of the tomography technique. This paper reports the development of the technique, and the experimental demonstration of a prototype sensor in a near-adiabatic, atmospheric-pressure laboratory Hencken burner. The spatial and temporal resolution enabled by this new sensing technique is expected to resolve several key issues in practical combustion devices.
Optics Express 06/2009; 17(10):8602-13. · 3.59 Impact Factor
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ABSTRACT: We present a novel multiwavelength, time-division multiplexed laser design that continuously cycles through N spectrally narrow wavelengths, spending a specified, fixed time on each one. The design is based on a matched compressor/stretcher and a custom waveform generator applying modulation preferably to the gain medium. The realization discussed here utilizes a pulsed semiconductor optical amplifier in an all-fiber cavity containing fiber Bragg gratings. The laser cycles through 19 wavelengths in a 44 nm wide spectral band (1333-1377 nm) every 15 micros. The source contains no moving parts, offers high repetition rates, narrow spectral linewidths, and custom spectral profiling of the output.
Optics Letters 05/2008; 33(7):738-40. · 3.40 Impact Factor
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ABSTRACT: Optical beating of polychromatic light is reviewed and its potential impact on time-resolved spectroscopy is analyzed. In particular, the dependence of the quasi-random beating of thermal light on quantities including average power, spectral shape, and spectral width are reviewed.
Applied Spectroscopy 03/2008; 62(2):220-9. · 1.66 Impact Factor
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ABSTRACT: Time-resolved spectroscopy can be compromised by optical beating, which is inherent to polychromatic light sources and signals. For incoherent light sources, the random interference can partially or completely mask the spectroscopic signature of interest if the time dynamics of the interference are similar to or faster than that of the signature. Part I of this review focused on the theory of this process with an emphasis on thermal light sources, and in this part, four methods to mitigate or circumnavigate the detrimental impact of interference on time-resolved spectroscopy are reviewed: use of light with a controlled, non-stochastic phase, use of narrow-bandwidth light, averaging, and pulse referencing.
Applied Spectroscopy 03/2008; 62(2):230-7. · 1.66 Impact Factor
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ABSTRACT: We have adapted a standard Fourier-transform infrared (FTIR) spectrometer (Thermo-Scientific Nexus 670) for in-cylinder measurements of gas spectra. During engine operation, the engine shaft encoder signal is logged continuously along with the FTIR's He–Ne laser signal and infrared interferogram signal. The engine piston and FTIR mirror move in an uncorrelated fashion, so that after many minutes of engine operation, a complete interferogram is populated at each piston position. Afterward, the data are compiled into a series of spectra versus crank-angle degree. Here, we present a near-infrared H2O thermal emission spectrum measured through a fiber-optic spark plug connected to an engine (Briggs 128603-OHV).
Measurement Science and Technology 02/2008; 19(4):043001. · 1.49 Impact Factor
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ABSTRACT: We demonstrate measurements of OH absorption spectra in the post-flame zone of a McKenna burner using spatial heterodyne spectroscopy (SHS). SHS permits high-resolution, high-throughput measurements. In this case the spectra span approximately 308-310 nm with a resolution of 0.03 nm, even though an extended source (extent of approximately 2x10(-7) m(2) rad(2)) was used. The high spectral resolution is important for interpreting spectra when multiple absorbers are present for inferring accurate gas temperatures from measured spectra and for monitoring weak absorbers. The present measurement paves the way for absorption spectroscopy by SHS in practical combustion devices, such as reciprocating and gas-turbine engines.
Applied Optics 01/2008; 46(36):8635-40. · 1.41 Impact Factor
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ABSTRACT: We present a novel method for low noise, high-speed, real-time spectroscopy to monitor molecular absorption spectra. The system is based on a rapidly swept, narrowband CW Fourier-domain mode-locked (FDML) laser source for spectral encoding in time and an optically time-multiplexed split-pulse data acquisition system for improved noise performance and sensitivity. An acquisition speed of ~100 kHz, a spectral resolution better than 0.1 nm over a wavelength range of ~1335-1373 nm and a relative noise level of ~5 mOD (~1% minimum detectable base-e absorbance) are achieved. The system is applied for crank-angle-resolved gas thermometry by H(2)O absorption spectroscopy in an engine motoring at 600 and 900 rpm with a precision of ~1%. Influences of various noise sources such as laser phase and intensity noise, trigger and synchronization jitter in the electronic detection system, and the accuracy of available H(2)O absorption databases are discussed.
Optics Express 12/2007; 15(23):15115-28. · 3.59 Impact Factor
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ABSTRACT: A robust method is described for calculating temperature, mole fraction, and pressure from measured absorption spectra (absorption coefficients versus optical frequency). The key components to the method are smoothing, differentiation, spectral axis warping, and linear least-squares fitting. The method works best when spectra span a full rotational branch of the target molecule, but in principle it works for any spectral span. The examples presented assume a measured spectrum over the 7246.4-7518.8 cm(-1) range, which encompasses the R branch of the v(1)+v(3) band of H(2)O; however, the techniques should work for most measured spectra.
Applied Optics 08/2007; 46(19):4117-24. · 1.41 Impact Factor
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ABSTRACT: A dispersive grating compressor was included in a fiber ring laser to generate an unequally spaced frequency comb spanning approximately 1549-1552 nm. Beating of nearby modes in the comb naturally assigns unique amplitude modulation frequencies to each spectral component emitted. The source contains no moving parts. The single-mode fiber-coupled output is directed through hydrogen cyanide gas and detected by a photodiode. A Fourier transform of a 1 ms record yields a spectrum that agrees with results from a grating spectrometer at 0.06 nm resolution. By engineering stable, broadband combs, the technique could result in a universal and simple approach for spectroscopy at almost arbitrary measurement speeds and spectral resolutions limited only by Fourier principles.
Optics Letters 12/2006; 31(21):3179-81. · 3.40 Impact Factor
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ABSTRACT: Hyper-spectral sensing strategies are surveyed and novel developments based on Fourier-domain techniques are presented.
Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 06/2006
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ABSTRACT: A spatially resolved optical technique to measure gas temperature was assessed. The technique relies on multiphoton absorption in inert gases. In contrast to laser-induced fluorescence, absorption is insensitive to collisional deactivation, and, in contrast to one-photon absorption, multiphoton absorption only occurs around the focus point of a typical laser beam. Multiphoton absorption features both the merits of being insensitive to quenching and of being a spatially resolved technique. In a case study we assessed two-photon absorption in xenon upon exciting the 5p6 1S0-->5p56p[5/2]2 transition in xenon at a wavelength of 256 nm. The amount of light absorbed by xenon is related to the number density of the gas, and if the gas pressure is known then the gas temperature can be inferred from the number density. Two-photon absorbance was measured as a function of xenon number density and was used to validate a theoretical model of the absorption process. We discuss the circumnavigation of experimental challenges in applying this technique and analyze its precision in terms of the inferred gas temperature.
Applied Spectroscopy 03/2006; 60(3):246-53. · 1.66 Impact Factor
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ABSTRACT: Optical sensors applied to practical devices often encounter beam steering: the wander and/or diffusion of laser light. Here we provide a framework for minimizing the sensitivity of transmission-based sensors to beam steering without quantitative prediction of the severity of the beam-steering field. Typical goals are increased transmission and/or minimized fluctuations in transmission; such features can improve optical sensor performance (e.g., improved signal-to-noise ratio, response time, or spectral resolution). In our framework, we introduce a parameter for characterizing beam-steering severity. We then compare two approaches for absorption spectroscopy and show that the preferred approach depends on the total spectral range monitored, the spectral resolution desired, and the severity of the beam steering.
Applied Optics 12/2005; 44(31):6762-72. · 1.41 Impact Factor
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ABSTRACT: Differential laser absorption was investigated for its merit in liquid oxygen (LOX) sensing. Whereas previous researchers have used differential absorption to detect trace concentrations of a substance, we use differential absorption to monitor small changes in large amounts of a substance. Two lasers of different wavelengths were intensity modulated 180 deg out of phase from each other and multiplexed into a single beam. After probing the LOX, the total transmitted signal was demodulated by a lock-in amplifier. Our experiment simulated rapid changes in LOX number density by varying the length of an approximately 73 mm path through pure LOX. In this experiment, we demonstrated the ability to monitor LOX number density with an uncertainty of approximately 1% with a time constant of 3 micros. The uncertainty could be halved by doubling the path length, and this improvement could be repeated as long as the relative intensity noise of the lasers is the dominating factor. We discuss the benefits of differential absorption for problems requiring an extended dynamic range.
Applied Optics 11/2005; 44(28):6058-66. · 1.41 Impact Factor
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ABSTRACT: Rapid excitation scans of laser-induced fluorescence (LIF) have been demonstrated. Broadband light was generated in a photonic crystal fiber and transmitted through a long fiber. Due to group-velocity dispersion in the long fiber, a wavelength scan emerged from the fiber in time. The wavelength was swept over approximately one octave in approximately 150 ns. The generated light was used to excite LD 700 Perchlorate diluted in methanol. The LIF excitation scan had a spectral resolution of approximately 15 nm, and the integrated fluorescence spectrum was found to be within 7% of the integrated absorption spectrum of the dye molecule. The method presented makes possible spatially and spectrally resolved LIF excitation scans with scanning speeds up to the limits set by the excited-state lifetime of the dye molecule.
Optics Letters 10/2005; 30(18):2394-6. · 3.40 Impact Factor
