High-speed interband cascade laser absorption sensor for multiple temperatures in CO2 rovibrational non-equilibrium

Abstract

An RF-diplexed laser absorption technique is employed to resolve spectral transitions of CO 2 in the mid-infrared at MHz rates to infer non-equilibrium temperatures of rotation and vibration. A distributed feedback interband cascade laser (DFB-ICL) resolves 7 transitions within the CO 2 fundamental asymmetric stretch band near 4.19 µm, representing a range of rotational and vibrational energies. Non-equilibrium spectra are resolved at MHz rates and fit with a two-temperature model for line intensities. The sensor is demonstrated on a high enthalpy shock tube, targeting temperatures between 1250 -3500 K and sub-atmospheric pressures in various CO 2 -Ar gas mixtures. Tests have shown that this sensor provides sensitive temperature measurements with 1-µs resolution over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times on the order of 10 µs are resolved and compared to existing models.

Multi-line Boltzmann regression for near-electronvolt temperature and CO sensing via MHz-rate infrared laser absorption spectroscopy

“…15 shows that, at 2000 K, a 10% error in the allocation of A over leads to a negligible increase in temperature and number density uncertainty, 0.01% and 0.05%, respectively. This confirms that the uncertainty arising due to the P(3,14) line was negligible in previous work [20,29]. However, at temperatures higher than 3500 K and 10% CO mole fraction, the temperature and number density uncertainty increase by several percents, which further motivates the use of the new line selection at those temperatures.…”

“…7, increased the scan-depth by one order of magnitude, sufficient to capture multiple rovibrational transitions of CO and CO 2 at several MHz [20]. This type of biastee circuit, modulated at high speeds, has also been employed in several entry-related studies where µs time resolution was needed [27][28][29].…”

“…The multi-line fit, in green, presents lower uncertainty than individual line pairs. The line pair used in previous works [20,29] is shown in black at 2% and 10% CO mole fraction to highlight the different range of application of each line pair.…”

“…15, when the P(3,14) line has a peak absorbance similar to P(0,31). The contribution of the P(3,14) line can be subtracted using iterative Voigt fitting but it is difficult to estimate how the uncertainty of this correction propagates to the temperature and number density measurements [20,29]. In this subsection, we numerically calculate the impact of the uncertainty generated by the subtraction of a neighboring line, taking as an example the line selection at 2008 cm −1 .…”

Extended tuning of distributed-feedback lasers in a bias-tee circuit via waveform optimization for MHz-rate absorption spectroscopy

“…Equation (43) is useful in that it reveals the functional form of the wavelength modulation due to sinusoidal current modulation. When fitting the relative wavenumber/chirp profile for a sinusoidal injection-current scan, it is thus more appropriate to fit the profile to a linear combination of a sine wave and another sine wave at double frequency, as has been performed in various works [8,48]. This second harmonic has been noted in analysis of wavelength-modulation spectroscopy, where sinusoidal injection-current modulation is used and modeled [49].…”