MHz laser absorption spectroscopy via diplexed RF modulation for pressure, temperature, and species in rotating detonation rocket flows

Abstract

A mid-infrared laser absorption sensing method has been developed to quantify gas properties (temperature, pressure, and species density) at MHz measurement rates, with application to annular rotating detonation rocket flows. Bias-tee circuitry is integrated with distributed feedback quantum cascade and interband cascade lasers in the 4−5 m range enabling diplexed radio frequency (RF) wavelength modulation on the order of several MHz while yielding sufficient scan depth to capture multiple rovibrational transitions in the fundamental vibrational bands of CO and CO 2. Sub-microsecond spectrally-resolved CO absorption lineshapes provide for inference of temperature and species from a two-line area ratio and pressure from collision line-width. CO 2 column density is inferred from peak-to-valley differential absorption at the bandhead near 4.19 m. A field demonstration on a methane-oxygen rotating detonation rocket engine was performed utilizing an in situ single-ended retro-reflection optical configuration aligned at the exhaust plane. The target gas properties are temporally-resolved at up to 3 MHz across rotating detonations with up to 20 kHz cycle frequency.

Ultrafast Laser Absorption Spectroscopy in the Mid-Infrared for Measuring Temperature and Species in Combustion Gases

“…(2) This accuracy was shown to be repeatable after rebuilding the optical setup. (3) The scaling factor on ∆ν C returned by the MSFR was the same for both of the grating resolutions which was, in turn, consistent with broadening uncertainties reported in the literature [1]. (4) Last, the IRF FWHM agrees relatively well with values which were measured directly using a QCL.…”

“…Laser-absorption spectroscopy (LAS) is widely used to provide non-invasive measurements of temperature, chemical species, pressure, and velocity. The demand for LAS measurements in high pressure and highly transient combustion and propulsion environments (e.g., detonation engines, rocket motors) continues to propel innovations in LAS diagnostics [1,2,3,4,5] and data-processing techniques [6,7,8]. Several of these innovations have focused on increasing the usable spectral bandwidth of LAS diagnostics while maintaining or improving temporal resolution.…”

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

“…Leak rates are in the range 0.1 -1 mtorr/min and are measured with a Baratron manometer (627D) before each test. In this work, the laser chip was modulated using the RF-diplexed system described in [17] with a square waveform. This type of modulation maximizes the signal intensity, and hence optimizes the signal to noise ratio (SNR) during the spectral scan.…”

RF-waveform optimization for MHz-rate DFB laser absorption spectroscopy in dynamic combustion environments

“…The measurement of time-resolved thermodynamic and thermochemical properties can aid in the improvement of the physical models of these systems and in the diagnosis of performance shortcomings. Laser absorption spectroscopy (LAS) has proven useful in measuring gas properties in detonation environments [1][2][3][4][5][6][7][8], namely temperature and species. Initial near-MHz rate LAS approaches for detonation engines utilized fixed-wavelength laser absorption [7,9,10].…”

“…Unfortunately, most scanned-wavelength LAS techniques have been limited to measurement rates on the order of 10-100 kHz, which are insufficient to fully resolve the flow transients between detonation cycles. To address this shortcoming, our group has recently developed a method to extend scanned-wavelength laser absorption spectroscopy to MHz rates and the mid-wave infrared by utilizing diplexed radio-frequency modulation with distributed feedback (DFB) lasers enabled by the usage of a bias-tee circuit [2,[12][13][14][15]. We demonstrated this method for measurement of infrared spectral transitions of carbon monoxide and carbon dioxide in a rotating detonation engine using two continuous-wave DFB lasers modulated at MHz rates [2].…”