ABSTRACT
The degenerate four-wave-mixing spectral profile of the R(1)(9) transition in the A(2)Σ(+) ? X(2)∏(0, 0) band of OH has been measured for various combinations of saturating pump beams. With increasing pumpbeam intensity the degenerate four-wave-mixing line shape changes dramatically near line center. In phase-conjugate geometry, three distinct spectral line shapes were observed for the cases of (1) equally intense pump beams, (2) a strong forward pump and a weak backward pump, and (3) a weak forward pump and an intense backward pump. A significant saturation dip appears in the spectrum near line center for case (3). The measured spectra have been modeled by the use of nonperturbative numerical solutions of the density matrix equations, and agreement between the calculations and the experimental results is excellent. The differences in the saturated line shapes for cases (2) and (3) are explored theoretically, and the calculated results are compared with previous theoretical work [Bloch and Ducloy, J. Opt. Soc.Am. 73, 635 (1985)] in which the line shapes were calculated in the limit of infinite Doppler broadening.
ABSTRACT
The polarization properties of backward-box, degenerate four-wave mixing from OH molecules are measured in a flame and compared with theory. Ratios of signal intensities for different laser polarizations are shown to exceed perturbation-theory predictions for a closed two-level system when optical intensity gratings are formed in the interaction volume. These results are shown to be inconsistent with population-nonconserving effects and are attributed to scattering from quenching-induced thermal density gratings.
ABSTRACT
Light scattering from a laser-induced thermal grating produced in an atmospheric-pressure H(2)/O(2) flame is observed with a phase-matching geometry commonly used in resonant four-wave mixing and laser-induced grating spectroscopy. The presence of thermal gratings is confirmed in both the time and the frequency domains in two distinct experiments. Diluting the flame with helium decreases the thermal grating signal intensity. Experimental results agree well with calculations based on a solution of the linearized hydrodynamic equations.
ABSTRACT
High-resolution degenerate four-wave mixing spectra of the R(1)(9) A(2)Sigma(+) ? X(2)II(0,0) transition of OH have been measured in a hydrogen/oxygen flame at atmospheric pressure where the Doppler and homogeneous linewidths are of comparable magnitude. The observed sub-Lorentzian Doppler-free line profile is fit well by existing perturbative models by using homogeneous broadening consistent with two-photon absorption measurements. Power broadening of the spectral profile with increasing pump-beam intensity was observed and is compared with predictions from a nonperturbative two-level model.
ABSTRACT
Using diagrammatic perturbation theory, we calculate the higher-order susceptibility for the n-wave process, omega(0) = (n/2 - 1)(omega(1) - omega(2)) + omega(1), in a phase-matched n-wave-mixing geometry. We include the 16 Zeeman and hyperfine levels of the sodium ground (3S((1/2))) and excited (3P((1/2))) states, finding resonances at subharmonics [+/-(1/2), +/-?, ..., +/-1(n/2 - 1)] of the ground-level transition frequencies. The computed spectrum for eight-wave mixing is in satisfactory agreement with experiment. In addition, a theoretical twelve-wave-mixing spectrum predicts a new higher-order selection rule.
ABSTRACT
We describe Doppler-free 205-nm two-photon excitation studies of atomic hydrogen performed using a single-mode pulsed dye-laser system in low-pressure flames. Excitation spectra confirm calculated line-strength ratios for transitions to the 3S and 3D states. Collisional broadening rates are obtained in several flames and compared with quenching rates measured in the same flames, using time-resolved fluorescence measurements.
ABSTRACT
Using four-, six-, and eight-wave-mixing geometries, we observe subharmonic resonances in collision-enhanced spectra in a sodium-seeded hydrogen-air flame. Appearing at high intensity, these resonances occur at +/-1/2, +/-1/3, +/-1/4, and +/-1/5 times the frequency of the ground-state hyperfine splitting omega(hfs). We argue that these resonances result from higher-order processes, specifically chi((n)), where n >/= 5. In particular, to describe the resonances at +/-omega(hfs)/5, at least chi((13)) is required. These high-order collision-enhanced effects are true perturbative nonlinearities and not sequential four-wave mixing processes.
ABSTRACT
We have designed and tested a compact injection seeding system consisting of a diode-laser-pumped Nd:YAG master oscillator and a permanent-magnet Faraday isolator. With active resonator frequency stabilization, this system permits highly reliable single-axial-mode operation of a Q-switched Nd:YAG laser over a period of hours. The system is capable of injection seeding both stable and unstable resonator designs and is suitable for injection seeding commercial lasers with only minor modifications.
ABSTRACT
We report the use of gain switching to obtain 60 mW of single-longitudinal-mode peak output power from a laser-diode-excited monolithic Nd:YAG laser. The device is demonstrated to operate at repetition rates in excess of 1 kHz and exhibits a spectral linewidth of less than 8 MHz. This oscillator provides an ideal source for injection seeding of laboratory Nd:YAG laser systems.
ABSTRACT
The effects of intensity fluctuations in the frequency-doubled output of a multimode Nd:YAG pump laser on coheren anti-Stokes Raman spectroscopy (CARS) signal generation have been investigated in a crossed-beam, twocolor CARS experiment. Measurements of signal intensity as a function of time delay (small compared with pulse length) in one pump beam show a maximum at zero delay. In measurements on a single unresolved transition, however, the nonresonant signal is enhanced by almost a factor of 2 more than the resonant signal. Our results demonstrate the non-Gaussian nature of the laser field statistics and introduce new considerations into the analysis of experimental CARS spectra.
ABSTRACT
A technique for obtaining sensitive, highly reproducible, time-resolved inverse Raman measurements is reported. Experimental results are presented for the nitrogen vibrational Q branch at a pressure of 10 atm. For these measurements the signal, normalized to the pump-laser energy, exhibits fluctuations of 1.4% (rms) about the average of 500 measurements; these deviations are within a factor of 2 of the quantum noise limit.
ABSTRACT
Coherent anti-Stokes Raman spectroscopy (CARS) has been used to obtain radial temperature profiles in an axisymmetric methane diffusion flame. Temperatures were obtained from analysis of background-free nitrogen Q- and O-branch spectra. The spectra were analyzed with a nonlinear least-squares CARS fitting program and compared to measurements from radiation- and conduction-corrected thermocouples. Excellent agreement was obtained in regions of relatively constant temperature, whereas improved CARS spatial resolution was required to obtain agreement near steep temperature gradients.
ABSTRACT
A new technique for obtaining point measurements in gases by infrared absorption is described. The method is based on a recently discovered quadratic Stark effect on vibrational and rotational molecular transitions that is induced by high-intensity optical fields. Detectivity calculations for CO, an important combustion gas, are presented together with a proof-of-principle demonstration of Stark-induced absorption in ammonia.
