Phase-conjugate Geometry Research Articles

Degenerate four-wave mixing spectroscopy and spectral simulation of C2 in an atmospheric pressure oxy-acetylene flame

The d 3Πg↔a 3Πu Swan bands of C2 have been recorded with high resolution using DFWM in the nearly Doppler free, phase conjugate geometry. C2 was probed in a standard oxy-acetylene welding flame with excellent signal-to-noise ratio and spectral resolution. Theoretical spectra were simulated and fitted directly to the complex overlapping spectra. The good agreement obtained shows that DFWM holds promise to become a robust and reliable tool for flame thermometry. Current theories of DFWM are reviewed in context of the present work and advantages and disadvantages of the technique are discussed.

Degenerate four-wave mixing spectroscopy with short-pulse lasers: theoretical analysis

The use of picosecond lasers for degenerate four-wave mixing (DFWM) diagnostics of atmospheric pressure flames has been suggested as a means of overcoming the complicated collision-rate dependence of the DFWM signal in the nanosecond-laser pulse-length regime. In this paper DFWM spectroscopy in the regime in which the laser pulse length (τL) is less than the characteristic collisional time (τC) is investigated theoretically. DFWM signal levels for the phase-conjugate geometry (counterpropagating pump beams) are calculated by integration of the time-dependent density-matrix equations at numerous grid points along the phase-matching axis and by summing of the polarization contribution from each of these grid points. Both purely homogeneously broadened resonances and resonances that are both collision broadened and Doppler broadened are considered. DFWM signal generation for these two types of resonances in the fully transient regime (τL≪τC) is strikingly different. For a homogeneously broadened resonance, the DFWM signal persists long after the laser beam has passed through the medium. The decay rate of the DFWM signal is determined by the collision rate in the medium, and the integrated DFWM signal is still dependent on the collision rate even in this fully transient regime. However, when the Doppler broadening of the resonance is much greater than the collisional broadening, the DFWM signal decays with a fall time close to that of the laser pulse. For τL≪τC, we find that it is the laser spectral width, rather than the collisional width, that dictates the range of velocity groups with which the pulses effectively interact. The signal decays with a fall time close to that of the laser pulse because the excited velocity groups radiate out of phase with one another. In a medium in which the Doppler width is much greater than the collisional width, the DFWM reflectivity is independent of collision rate for τL≪τC, but the signal level is dependent on the Doppler width. Finally, we investigate the effect of different laser pulse lengths on reflectivity saturation curves for resonances with different levels of Doppler broadening.

Detection of methyl radicals in a flat flame by degenerate four-wave mixing

We report the spatially resolved detection of methyl radicals in a methane-air flat flame, using degenerate four-wave mixing (DFWM). A frequency-tripled dye laser pumped with a frequency-doubled Nd:YAG laser was used to access the Herzberg beta(1) band of methyl near 216 nm. Using a nearly phase-conjugate geometry, we detected methyl with high spatial resolution [0.2 mm (0.3 mm) vertical (horizontal) and ~6 mm longitudinal] and with good signal-to-noise ratio in a rich (ø = 1.55) flame. Compared with laser absorption spectra, DFWM spectra were much less influenced by a broad featureless background. From the absorption data, we measured the peak methyl concentration to be 650 parts in 10(6), resulting in an estimated DFWM detection limit of 65 parts in 10(6).

Degenerate four-wave-mixing line shapes of hydroxyl at high pump intensities

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.

Doppler-free ultrafast modulation spectroscopy with phase-conjugation geometry.

We have studied theoretically a phase-conjugate ultrafast modulation spectroscopy (PCUMS) in a Doppler-broadened system. The pump beams consist of two frequency components which drive two different transitions. It is found that the four-wave-mixing (FWM) signal exhibits damping oscillation as the relative time delay between two pump beams is varied, from which the energy-level splitting can be deduced. We have considered the cases that pump beams have either narrow band or broadband linewidth and found that for both cases a Doppler-free precision in the measurement of the energy-level splitting can be achieved. The advantage of PCUMS is that the energy-level splitting between states can be widely separated. Finally, the relationship between PCUMS and other Doppler-free techniques in frequency and time domains has also been discussed.

Rotational line strengths in degenerate four wave mixing

In this paper we present a derivation of the rotational line strengths involved in fully resonant degenerate four wave mixing processes (DFWM). General expressions are given for diatomic molecules with different Hund's coupling cases. More precisely, we tabulated the rotational line strengths for the electronic transition 2Σ+ -2Π of OH and NO. For the ground state, Hund's cases (b), (a) and the intermediate case (a-b) are considered. We also stress the dependence of the line strengths with respect to the geometry and to the polarization of the incident beams. So, we consider the optical phase conjugation (OPC) geometry and the forward (FDFWM) one. We also give expressions for the line strengths in different polarization arrangements, i.e. parallel or crossed polarized beams. A comparison is made between these cases. Finally, theoretical simulations, using our line strengths calculations, are compared with experimental spectra of OH obtained in a methane-air flame using OPC and FDFWM.

Light-induced second-harmonic generation in azo-dye polymers

The permanent all-optical poling of an azo-aromatic acrylic copolymer is experimentally demonstrated by seeding preparation in a backward phase-conjugation geometry. The microscopic mechanism involves an orientational hole burning followed by orientational redistribution caused by trans-to-cis isomerization of the azo-dye chromophores. The characteristic kinetics of monitored by second-harmonic generation.

Real-time image transmission and interferometry through a distorting medium using two phase conjugators

A phase-conjugate geometry with two photorefractive crystals is demonstrated. It can be used for real-time one- or two-way image transmission through a distorting medium and for real-time holographic interferometry.

Orientational dependence of degenerate four-wave mixing in multiple-quantum-well structures

The transient grating technique is used to demonstrate how the temporal response of four-wave mixing in multiple-quantum-well structures depends on sample orientation. Variation of the decay times associated with the short-period grating in a phase-conjugate geometry between 40 and 500 psec is demonstrated in a GaAs/AlGaAs multiple-quantum-well structure and interpreted in terms of the large anisotropy in the carrier diffusion.

Nonlinear mixing and phase conjugation in broad-area diode lasers

Four-wave mixing is investigated in broad-area diode lasers. Two external fields are injected into the device using a phase conjugation geometry and the nonlinear four-wave mixing signal observed by performing spectrally resolved measurement of the far field. By varying the injection geometry, the excited mode of the broad-area diode can be controlled. The gain nonlinearity generates a conjugate wave within the device, thereby resulting in far-field emission in the conjugate direction. Nonlinear conjugate signal efficiency levels of ∼−21 dB have been observed.

Generation of non classical states of light by phase conjugation and parametric conversion

It is shown that oscillators using nearly degenerate parametric conversion or four-wave mixing in the phase conjugation geometry generate pairs of highly correlated photons. The intensity difference of the two corresponding light beams is then expected to be below the shot noise limit. This property can be used to produce intensity stabilized light beams below the quantum limit.

Measuring the phase of slow kerr-type nonlinearities: The role of phase modulation

We show theoretically and verify experimentally how the phase modulation of the laser pulse affects the phase of the various contributions to the nonlinear polarization in the optical phase conjugation geometry. This provides an explanation for the apparent sum rule violation observed by Jain and Lind in semiconductor-doped glasses. It also allows to define the conditions which must be fulfilled when one wants to measure the phase of a slow Kerr-type nonlinearity.

Determination of temporal correlation of ultrafast laser pulses using phase conjugation

A new time domain single shot conjugation autocorrelator is described. Three beams derived from the same initial laser pulse interact in a nonlinear medium to produce a backward signal wave in a 90° phase conjugate geometry. The pulse duration is determined from the spatial width of the phase conjugate beam emerging from the interaction region. Pulse duration measurements of pulses selected from a YAG laser train show that shorter pulses are generated toward the tail of the pulse train.

Raman-induced phase conjugation spectroscopy

Complete Raman spectra of transparent media are obtained with a single 10-nsec, 10-mJ, laser pulse by a new spectroscopic technique which exploits resonant behavior of nondegenerate four-wave mixing in the phase-conjugate geometry.