Diode Laser Sensors for Ground Testing

Abstract

Tunable diode laser (TDL) absorption has proven highly effective for time-resolved in situ sensing of gaseous flows, and as a result such sensors are utilized increasingly in the development and control of advanced propulsion and combustion systems. Sensors using TDL absorption along a line-of-sight have the capability of fast or even continuous measurements of multiple flowfield quantities. Recent advances in TDL absorption-based sensors suggest excellent potential for development into routine diagnostic tools for a wide variety of ground-test applications. From research in our laboratory we select three aspects: (1) the development of wavelength-multiplexed absorption concepts, allowing multiple measurements along each line-of-sight; (2) ground-test applications of TDL absorption to pulse detonation engines, gas turbine sector rigs, and scramjet combustor research; and (3) extension of wavelength-multiplexed TDL sensing to hydrocarbon fuels using new wavelength-tunable mid-IR lasers.

Monitoring temperature in high enthalpy arc-heated plasma flows using tunable diode laser absorption spectroscopy

“…Tunable diode laser absorption spectroscopy (TDLAS) is a powerful, non-intrusive optical diagnostics technique that has been widely used for sensing gas species in a variety of harsh environments. 7,8 For air plasma diagnostics, TDLAS is particularly attractive since strong isolated electronic transitions of excited O, N, and Ar atoms exist in the visible and near-infrared (NIR) regions of the spectrum where diode lasers are readily available. In addition, diode laser based sensors are compact, rugged, and can be easily fiber-coupled for remote operation.…”

Diode Laser Absorption Spectroscopy Measurements in a Gaseous Hydrogen / Oxygen Rocket

“…At present, tunable diode laser sensors have been developed that operate from 1 kHz up to 50 kHz. [19][20][21][22][23][24][25][26][27][28] The time resolution of scanned wavelength absorption can be improved significantly if one uses the peak height only of the absorption feature to determine the temperature. In doing so, the time resolution of the measurement is limited only by the responsiveness of the photodetector and data acquisition system.…”

“…Here, an external modulation is superimposed over the laser drive-current, which itself can be either scanned or fixed. These methods have been extensively developed 19,22,[28][29][30][31][32][33][34][35][36][37][38] for sensing of trace-species, and can greatly reduce the noise encountered in direct absorption measurements. Detection of the second harmonic (2f) is typical (detection at twice the modulation frequency).…”

“…Absorption spectroscopy has received a great deal of attention since the advent of tunable diode lasers, particularly diode lasers in the near infrared region of the spectra. 19,22,28,36,46 Most studies have been performed at atmospheric pressure conditions or at pressures of a few atmospheres, which is much lower than the pressures inside typical rocket chambers. A major challenge for application of absorption spectroscopy to high pressure rocket chambers is pressure broadening, due to molecular collisions, of the spectral lineshape.…”

Simultaneous measurements of multiple flow parameters for scramjet characterization using tunable diode-laser sensors

“…For instance, a wellestablished variation (WMS-2f ) involves imposing a high-frequency modulation on top of the wavelength tuning, measuring the second harmonic detection of the absorption signal, and using the change in the absorption signal (instead of the absorption signal itself, as in the direct absorption strategy) to infer temperature and concentration [14,28]. This WMS2f variation of TDLAS has been demonstrated to enhance the SNR of the measurements under weak absorbance [29]. However, in this work, we preferred the direct absorption strategy for several reasons.…”