Methods to extend the spectral tuning range and signal-to-noise ratio via waveform optimization were examined using diplexed RF-modulation with continuous-wave distributed-feedback lasers, with relevance to MHz-rate laser absorption spectroscopy. With a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold. The effect of square waveform duty cycle and leading edge ramp rate are examined. Scan depths on the order of 1 cm −1 at a rate of 1 MHz are achieved with continuous-wave DFB quantum cascade lasers. The attenuation of the high-frequency laser signals due to detector bandwidth are also examined, and limitations are noted. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth, with a representative optimized waveform determined for a 200 MHz system. This waveform is validated for accuracy across the scan range and for scan-to-scan repeatability using room temperature laser absorption measurements. The method is then deployed in a detonation tube to measure temperature, pressure, and CO concentration at MHz rates.

A mid-infrared laser absorption technique is developed for sensing of temperature and carbon monoxide (CO) number density from 2000 K to above 9000 K. To resolve multiple rovibrational lines, a distributedfeedback quantum cascade laser (DFB-QCL) is modulated across 80% of its current range using a trapezoidal waveform via a bias-tee circuit. The laser attains a spectral scan depth of 1 cm −1 , at a scan frequency of 1 MHz, which allows for simultaneous measurements of four isolated CO transitions near 2011 cm −1 (4.97 µm) with lower-state energies spanning 3,000 to 42,000 cm −1 . The number density and temperature are calculated using a Boltzmann regression of the four population fractions. This method leverages the information contained in each transition and yields a lower uncertainty than using a single line pair. The sensor is validated in shock tube experiments over a wide range of temperatures and pressures (2300-8100 K, 0.3-3 atm). Measurements behind reflected shock waves are compared to a kinetic model of CO dissociation up to 9310 K and are shown to recover equilibrium conditions. The high temperature range of the sensor is able to resolve rapid species and temperature evolution at near electronvolt conditions making it suitable for investigations of high-speed flows, plasma applications, and high-pressure detonations.

Variations in injection-current waveform are examined using diplexed RF-modulation with continuous-wave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-to-noise ratio for MHz-rate laser absorption spectroscopy. Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leading-edge ramp rate are further examined. A spectral scan depth on the order of 1 cm-1 at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 µm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via laser absorption spectroscopy at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work.

An optical method for high-speed line-of-sight pressure measurements using infrared laser absorption spectroscopy is presented with detailed uncertainty analysis related to thermodynamic and compositional variation in combustion environments. The technique exploits simultaneous sub-microsecond sensing of temperature and mole fraction to extract pressure from collisional line broadening at MHz rates. A distributed-feedback quantum-cascade laser near 5 $$\upmu $$ μ m in a bias-tee circuit is used to spectrally resolve multiple rovibrational transitions in the P-branch of the fundamental band of carbon monoxide, which may be seeded or nascent to reacting flows. A comprehensive approach for estimating collisional line broadening in complex combustion gas mixtures is presented. Uncertainty is quantified for a wide range of conditions, reflecting different fuels, equivalence ratios, reaction progress, and combustion modes (deflagration and detonation), which influence gas composition and temperature. The sensor was evaluated for accuracy and precision in both a high-enthalpy shock-tube facility in hydrocarbon–air ignition experiments and behind ethylene–oxygen detonation waves in a detonation-impulse tube facility at temperatures from 1500 to 3000 K and pressures from 0.5 to 10 bar. Pressure measurements were compared to measurements by piezoelectric pressure transducers and to theoretical estimates from normal-shock and Chapman-Jouguet simulations. The high-speed path-integrated optical pressure measurement offers an alternative to traditional electromechanical transducers that are constrained to values at wall boundaries and have proven unreliable in some harsh reacting flows.

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