Time-resolved absorption spectroscopy in electron beam melting with blue vertical-cavity surface-emitting lasers.

This paper addresses the challenge of understanding the dynamics of the interaction between partially evaporated metal and the liquid metal melt pool in electron beam melting (EBM), an additive manufacturing technology. Few contactless, time-resolved sensing strategies have been applied in this environment. We used tunable diode-laser absorption spectroscopy (TDLAS) to measure vanadium vapor in the EBM of a Ti-6Al-4V alloy at 20 kHz. Our study includes, to our knowledge, the first-time use of a blue GaN vertical cavity surface emitting laser (VCSEL) for spectroscopy. Our results reveal a plume that is roughly symmetrical with a uniform temperature. Moreover, we believe this work presents the first application of TDLAS for time-resolved thermometry of a minor alloying element in EBM.

Tomographic absorption spectroscopy based on dictionary learning.

Tomographic absorption spectroscopy (TAS) has an advantage over other optical imaging methods for practical combustor diagnostics: optical access is needed in a single plane only, and the access can be limited. However, practical TAS often suffers from limited projection data. In these cases, priors such as smoothness and sparseness can be incorporated to mitigate the ill-posedness of the inversion problem. This work investigates use of dictionary learning (DL) to effectively extract useful a priori information from the existing dataset and incorporate it in the reconstruction process to improve accuracy. We developed two DL algorithms; our numerical results suggest that they can outperform classical Tikhonov reconstruction under moderate noise conditions. Further testing with experimental data indicates that they can effectively suppress reconstruction artifacts and obtain more physically plausible solutions compared with the inverse Radon transform.

Multispecies absorption spectroscopy of detonation events at 100 kHz using a fiber-coupled, time-division-multiplexed quantum-cascade-laser system.

A mid-infrared fiber-coupled laser system constructed around three time-division-multiplexed quantum-cascade lasers capable of measuring the absorption spectra of CO, CO2, and N2O at 100 kHz over a wide range of operating pressures and temperatures is demonstrated. This system is first demonstrated in a laboratory burner and then used to measure temperature, pressure, and concentrations of CO, CO2, and N2O as a function of time in a detonated mixture of N2O and C3H8. Both fuel-rich and fuel-lean detonation cases are outlined. High-temperature fluctuations during the blowdown are observed. Concentrations of CO are shown to decrease with time for fuel-lean conditions and increase for fuel-rich conditions.

Simultaneous optimization method for absorption spectroscopy postprocessing.

A simultaneous optimization method is proposed for absorption spectroscopy postprocessing. This method is particularly useful for thermometry measurements based on congested spectra, as commonly encountered in combustion applications of H2O absorption spectroscopy. A comparison test demonstrated that the simultaneous optimization method had greater accuracy, greater precision, and was more user-independent than the common step-wise postprocessing method previously used by the authors. The simultaneous optimization method was also used to process experimental data from an environmental chamber and a constant volume combustion chamber, producing results with errors on the order of only 1%.

Demonstration of temperature imaging by H2O absorption spectroscopy using compressed sensing tomography.

This paper introduces temperature imaging by total-variation-based compressed sensing (CS) tomography of H2O vapor absorption spectroscopy. A controlled laboratory setup is used to generate a constant two-dimensional temperature distribution in air (a roughly Gaussian temperature profile with a central temperature of 677 K). A wavelength-tunable laser beam is directed through the known distribution; the beam is translated and rotated using motorized stages to acquire complete absorption spectra in the 1330-1365 nm range at each of 64 beam locations and 60 view angles. Temperature reconstructions are compared to independent thermocouple measurements. Although the distribution studied is approximately axisymmetric, axisymmetry is not assumed and simulations show similar performance for arbitrary temperature distributions. We study the measurement error as a function of number of beams and view angles used in reconstruction to gauge the potential for application of CS in practical test articles where optical access is limited.

Comparison of Frequency-Domain and Time-Domain Baseline Correction Approaches for Infrared Absorption Spectroscopy of Mixtures Containing Up to 464 Components.

Many baseline correction approaches have been developed to address baseline artifacts observed in measured infrared (IR) absorption spectra during post-processing. These approaches offer distinct advantages and disadvantages, and the choice of which one to employ depends on the complexity of baseline artifacts present in a particular application. In this paper, we compare the performance of two baseline correction approaches: a frequency-domain polynomial fitting approach and a time-domain modified free induction decay approach, under various baseline scenarios, spectral resolutions, and noise levels for mixtures containing up to 464 species. Our results showed that the frequency-domain approach outperformed the time-domain approach by a factor of up to 16 when the baseline was represented by a sine wave with fewer than two cycles over the full spectral range. On the other hand, the time-domain approach performed up to 12 times better when the baseline featured two cycles of a sine wave. Additionally, we observed that the time-domain approach exhibited higher sensitivity to spectral resolution and underperformed when the noise level was high. The findings of this study emphasize the importance of numerically testing a few candidate approaches for a given application, taking into consideration baseline characteristics, as well as the spectral resolution and noise constraints of the application.

50-kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomography.

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.

Measurements of multiple gas parameters in a pulsed-detonation combustor using time-division-multiplexed Fourier-domain mode-locked lasers.

Hyperspectral absorption spectroscopy is being used to monitor gas temperature, velocity, pressure, and H(2)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(2)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.

Validation of temperature imaging by H2O absorption spectroscopy using hyperspectral tomography in controlled experiments.

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.

Fourier-transform absorption spectroscopy in reciprocating engines.

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.

Application of time-division-multiplexed lasers for measurements of gas temperature and CH4 and H2O concentrations at 30 kHz in a high-pressure combustor.

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.

Tomographic imaging of temperature and chemical species based on hyperspectral absorption spectroscopy.

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.

Optical beating of polychromatic light and its impact on time-resolved spectroscopy. Part II: Strategies for spectroscopic sensing in the presence of optical beating.

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.

Optical beating of polychromatic light and its impact on time-resolved spectroscopy. Part I: Theory.

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.

High speed engine gas thermometry by Fourier-domain mode-locked laser absorption spectroscopy.

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.

Assessment of multiphoton absorption in inert gases for the measurement of gas temperatures.

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.

Optimized wavelength selection for molecular absorption thermometry.

A differential evolution (DE) algorithm is applied to a recently developed spectroscopic objective function to select wavelengths that optimize the temperature precision of water absorption thermometry. DE reliably finds optima even when many-wavelength sets are chosen from large populations of wavelengths (here 120 000 wavelengths from a spectrum with 0.002 cm(-1) resolution calculated by 16 856 transitions). Here, we study sets of fixed wavelengths in the 7280-7520 cm(-1) range. When optimizing the thermometer for performance within a narrow temperature range, the results confirm that the best temperature precision is obtained if all the available measurement time is split judiciously between the two most temperature-sensitive wavelengths. In the wide temperature range case (thermometer must perform throughout 280-2800 K), we find (1) the best four-wavelength set outperforms the best two-wavelength set by an average factor of 2, and (2) a complete spectrum (all 120 000 wavelengths from 16 856 transitions) is 4.3 times worse than the best two-wavelength set. Key implications for sensor designers include: (1) from the perspective of spectroscopic temperature sensitivity, it is usually sufficient to monitor two or three wavelengths, depending on the sensor's anticipated operating temperature range; and (2) although there is a temperature precision penalty to monitoring a complete spectrum, that penalty may be small enough, particularly at elevated pressure, to justify the complete-spectrum approach in many applications.

Selection of multiple optimal absorption transitions for nonuniform temperature sensing.

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.

Simple multiwavelength time-division multiplexed light source for sensing applications.

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.

Robust method for calculating temperature, pressure, and absorber mole fraction from broadband spectra.

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.