Nonintrusive laser probing of unsteady combustion processes provides the capability for the remote, in—situ, spatially and temporally precise measurement of, among other parameters, gas temperature and species concentrations. The predominant spatially—precise laser diagnostics for temperature and species include spontaneous Raman scattering (RS) and coherent anti-Stokes Raman spectroscopy (CARS) for measurements of major, i.e., ≥~ 1%, constituents, and laser—induced fluorescence spectroscopy (LIFS) and degenerate four wave mixing (DFWM) for minor, ppm level, species. Rayleigh scattering can provide total density measurements. These techniques are reviewed, including two-dimensional implementations, their state of maturity assessed, and major remaining research issues identified.

Laser Diagnostics for Temperature and Species in Unsteady Combustion

Pattern formation and competition in nonlinear optics

Analytic and numerical investigations of a cavity containing a Kerr medium are reported. The mean field equation with plane-wave excitation and diffraction is assumed. Stable hexagons are dominant close to threshold for a self-focusing medium. Bistable switching frustrates pattern formation for a self-defocusing medium. Under appropriate parametric conditions that we identify, there is coexistence of a homogeneous stationary solution, of a hexagonal pattern solution and of a large (in principle infinite) number of localized structure solutions which connect the homogeneous and hexagonal state. Further above threshold, the hexagons show defects, and then break up with apparent turbulence. For Gaussian beam excitation, the different symmetry leads to polygon formation for narrow beams, but quasihexagonal structures appear for broader beams.

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Pattern formation in a passive Kerr cavity

Beams of light interacting with nonlinear-optical materials naturally form complex transverse profiles. Self-focusing, filamentation, modulational instabilities, spontaneous pattern formation and symmetry breaking, nonlinear waveguiding, and transverse mode excitation are but some of the many phenomena that are observed in lasers, in optical bistability, and in the free interaction of beams. As an introduction to a special issue of the Journal of the Optical Society of America B on transverse effects in nonlinear-optical systems, we provide this overview of history and current work.

Overview of transverse effects in nonlinear-optical systems

The detection of OH in a laminar premixed air–methane flame by degenerate four-wave mixing (DFWM) is The detection reported. The four-wave mixing spectrum was recorded in the region of the R1, R2 band heads of the (0, 0) 2∑–2Π transition, showing clearly resolved rotational structure. Applications of the DFWM technique to combustion are discussed. As an example, the distribution of sodium atoms in a sodium-seeded flame has been diagnostics measured using a 90° geometry to enhance spatial resolution.

Detection of OH in a flame by degenerate four-wave mixing

The first observation to our knowledge of degenerate four-wave mixing in a sodium-seeded flame is reported. In addition, the theoretical dependence of the phase-conjugate output on the sodium concentration is verified experimentally, demonstrating the potential of this procedure for use as a diagnostic tool to measure species concentration in combusting or noncombusting media.

Phase conjugation in a flame

Degenerate conical emissions that appear in laser-pumped atomic sodium have been reported. The cones appear for both single- and double-pump geometries. The nonlinear medium is a small, high-density sodium-seeded flame. Earlier work that relates the cones to the process of degenerate phase conjugation is extended. The analysis for the single-pump phenomenon is formally related to amplified spontaneous emission. The double-pump case is analogous to a distributed-feedback laser. Theory and experiment are presented and discussed.

Degenerate conical emissions in atomic-sodium vapor

Observations of a new type of conical emission in atomic sodium provide the motivation for a series of phase conjugation experiments. These experiments show large variations in the reflectivity as a function of probe angle, which are not explained by existing theory. Analysis which includes self-induced refractive index changes leads to numerical calculations, which agree well with the observations. >

Conical emissions and phase conjugation in atomic sodium vapor

We report on the use of a sodium-seeded flame as an amplifying phase conjugator. The environment is unique in that a number density of 1016 cm−3 is maintained despite a lateral dimension of 1 mm. We combine two of these optically pumped flames to form a phase-conjugate resonator. Finally, we observe oscillation that is due to the action of a single flame and two pump beams.

John Pender

Papers

Phase conjugation in a flame

Degenerate conical emissions in atomic-sodium vapor

Conical emissions and phase conjugation in atomic sodium vapor

Sodium phase-conjugate oscillator in a combusting environment.