ABSTRACT
Coherent anti-Stokes Raman scattering is demonstrated as a quantitative diagnostic in low-density flows by mapping H(2) velocity and translational temperature inside and outside the nozzle of a resistojet. A spatial resolution of better than 35 mum along the flow direction and 350 mum transverse to it was attained in a density as low as 5 x 10(15) cm(-3). The accuracy of the velocity, inferred from the Doppler shift of the Q (1) Raman resonance, was limited by the scan linearity of the laser to +/-0.2 km/s. Translational temperatures, inferred from linewidths and complicated by saturation and ac Stark effects, had an accuracy of ~20%. A discussion of applicability to molecular nitrogen is presented.
ABSTRACT
An absorption tomography instrument that is capable of acquiring 100 projections of 100 elements each in less than 200 ns is described. The instrument uses time-multiplexed, fiber-optic fan-beam sources that are sequentially activated in groups to reduce greatly the total number of detectors required for achieving a given resolution. The quantitative details required to tailor this instrument to a particular application are presented. A single-fiber prototype was used to verify the design and establish its sensitivity. The sensitivity is limited by laser-speckle noise. The fiber-optic fan-beam generator can produce an interdetector correlation of the projection noise, reducing the effect of this noise on the reconstruction. The noise is measured as a function of optic-fiber stability and size, laser bandwidth and mode stability, and detector size.
ABSTRACT
The design for a fast absorption tomography instrument is presented. The instrument is capable of generating 100 projections of 100 elements each in less than 200 ns. It comprises temporally multiplexed fiber-optic fanbeam sources and large-area fast photodiodes. The spectral operating range is 200 to 1100 nm when silicon photodiodes are used. The sensitivity of the instrument is limited by laser speckle noise created by the fiber optic. The performance was evaluated by using a prototype system.
ABSTRACT
A method for accurately inferring temperatures and concentrations from simultaneous multiplex CARS spectra of N(2), CO, and H(2) is introduced. Only a single dye laser is employed in these measurements. Temperatures inferred from the nitrogen portion of spectra taken in a furnace of preanalyzed samples of these gases show agreement with temperatures measured by thermocouple to well within the estimated accuracies of the thermocouple (1%). Carbon monoxide and hydrogen concentrations may be inferred from the remainder of the spectrum with accuracies of 2 and 1/2%, respectively. Spectra taken downstream from a combustor burning diesel fuel and preheated air indicated that simultaneous measurements of temperature and CO concentration with mole fractions from below 0.01 may be made in turbulent reacting flows. The S(9) rotational line of hydrogen is employed to infer concentrations of H(2). Minimum detectable mole fractions of hydrogen are presented as functions of temperature and CO mole fraction. The effects of temporal instabilities of the dye laser and interspecies coupling during analysis on the accuracy of the inferred values are discussed.
