1D interferometric Rayleigh scattering velocimetry and thermometry using VIPA.

The work introduces a VIPA-based interferometric Rayleigh scattering instrument for tracer-free, simultaneous temperature and velocity measurements along a 1D volume. A virtually imaged phased array (VIPA) replaces the Fabry-Perot etalon conventionally used in interferometric Rayleigh scattering, allowing the extension of the technique from 0D (point or multi-point) to 1D. The Rayleigh-Brillouin spectrum is a function of pressure and temperature and can be used for temperature diagnostics in isobaric flows.

One-dimensional interferometric Rayleigh scattering velocimetry using a virtually imaged phased array.

One-dimensional interferometric Rayleigh scattering velocimetry is demonstrated using a virtually imaged phased array (VIPA). A continuous-wave laser emitting at 532 nm is used as the source, and a low-noise CCD camera is used as the detector. The VIPA has a free spectral range of 15 GHz and is coated for the wavelength range 500-600 nm.

Mole fraction measurement through a transparent quarl burner using filtered Rayleigh scattering.

A filtered Rayleigh scattering system is developed and applied to measure the mole fraction of methane in a methane-air swirl flow through a transparent conical quartz quarl. Light scattering from the location where the laser beam is incident on the surface of the quarl is orders of magnitudes larger than Rayleigh scattering from the gas mixture of interest. This diffusive scattering is suppressed using molecular absorption by an iodine cell and using spatial filtering by an optical aperture.

High-speed Rayleigh-Raman measurements with subframe burst gating.

A 10-kHz one-dimensional Rayleigh-CH Raman instrument capable of achieving highly precise measurement of temperature and methane mole fraction is demonstrated. The system uses a pulse-burst laser as the light source and back-illuminated electron-multiplied CCD cameras as the detectors. The cameras are operated in the subframe burst gating mode, to combine a high sampling rate, low noise, and a slow readout.

Effect of noise-induced wavelength fluctuation in tunable diode lasers on narrow-linewidth absorption measurements.

Tunable diode laser absorption spectroscopy is being widely used to make sensors for diagnostic purposes in various engineering applications. Since the wavelength of many diode lasers used in such sensors is sensitive to the driving current, even noise as small as a few μA in the driving current can cause a wavelength fluctuation of ∼±0.5  pm, which is large enough to interfere with sensitive absorption measurements.

Temperature characterization of a radiating gas layer using digital-single-lens-reflex-camera-based two-color ratio pyrometry.

The two-color ratio pyrometry technique using a digital single-lens reflex camera has been used to measure the time-averaged and path-integrated temperature distribution in the radiating shock layer in a high-enthalpy flow. A 70 mm diameter cylindrical body with a 70 mm long spike was placed in a hypersonic shock tunnel, and the region behind the shock layer was investigated. The systematic error due to contributions from line emissions was corrected by monitoring the emission spectrum from this region using a spectrometer.

Baseline correction for stray light in log-ratio diode laser absorption measurements.

Log-ratio detection is a convenient technique for making temperature and concentration measurements using sensors based on tunable diode laser absorption spectroscopy. In many practical sensing applications, it is difficult to avoid stray light falling on the signal photodiode of the sensor. This stray light acts as noncommon-mode interference and introduces a systematic error in absorption measurements, which is not removed by baseline subtraction.