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Nonlinear Optics

Scattering noise reduction in phase conjugators via photo-induced density redistribution in atomic vapors

Masaaki Yokota, P. Yeh

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Investigates the noise due to Rayleigh scattering and its reduction via the spatial redistribution of atoms due to the standing wave pattern of the pump beams. For operation near the resonance, the interaction of the induced atomic dipole with the optical field leads to a periodic potential for the atoms due to the interference standing wave pattern. An assumption of the Boltzmann distribution yields a periodic distribution of the number density of atoms. In the paper, we investigate the effect of detuning and saturation on the noise due to Rayleigh scattering and address the issue of signal to noise ratio in nonlinear optical phase conjugation. We also investigate the minimum incident intensity needed to achieve a signal to noise ratio of one (S/N=1).

DOI
10.1109/nlo.1998.710319

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Investigates the noise due to Rayleigh scattering and its reduction via the spatial redistribution of atoms due to the standing wave pattern of the pump beams. For operation near the resonance, the interaction of the induced atomic dipole with the optical field leads to a periodic potential for the atoms due to the interference standing wave pattern. An assumption of the Boltzmann distribution yields a periodic distribution of the number density of atoms. In the paper, we investigate the effect of detuning and saturation on the noise due to Rayleigh scattering and address the issue of signal to noise ratio in nonlinear optical phase conjugation. We also investigate the minimum incident intensity needed to achieve a signal to noise ratio of one (S/N=1).
We investigate the reduction of Rayleigh scattering in four-wave mixing due to photo-induced spatial redistribution of atoms in atomic vapors. The Rayleigh scattering from the pump beams in optical phase conjugation is a major source of noise, which affects the fidelity of the phase-conjugated image. In the blue-shifted operation, the atoms are driven toward the low-intensity regions of the standing-wave pattern, leading to a reduction of Rayleigh scattering. We also investigate the effect of saturation on the differential scattering cross section.
An experimental investigation was made of the influence of the state of optically polished surfaces of various materials on the scattering coefficient which governs the power of the intrinsic noise of phase-conjugate mirrors based on a degenerate four-wave interaction. The angular dependence of this coefficient was also investigated. The noise power of a phase-conjugate mirror made of InAs, operating at the optimal pump wave intensities 1— 2 MW cm -2 , may be reduced to ~2 × 10 -7 W per one spatial mode of the radiation to be phase-conjugated. This was achieved by increasing the angle between the axes of the signal and the pump beams up to ~1 rad without a significant reduction of the reflection efficiency, which can be ~500%.
We present the results of a comprehensive investigation of the quantum-noise properties of a continuous-wave phase-conjugate mirror (PCM) formed using backward-four-wave mixing in potassium vapor. We characterize the quantum-noise properties of the PCM as functions of the vapor density, pump detuning from resonance, and relative frequency detuning of the signal beam from the probe beam. We compare the noise measurements of the PCM with the predicted noise of an ideal quantum-noise-limited PCM and determine the value of the minimum signal that can be used to perform phase conjugation with unity signal-to-noise ratio. For the range of vapor densities studied, we find that the PCM operates nearest the quantum-noise limit and that the value of the minimum signal is lowest under conditions in which the reflectivity is maximized. These results demonstrate that it is possible to perform phase conjugation with signals as weak as 14 fW with near-unity reflectivity. Our measurements are in qualitative agreement with the predictions of a quantum theory of phase conjugation via nearly degenerate four-wave mixing in a two-level system.
We observe optical phase conjugation using the surface nonlinearity of a dense atomic potassium vapor. For an atomic number density of 1.5\ifmmode\times\else\texttimes\fi{}1017{10}^{17}1017 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, phase-conjugate reflectivities of 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ are measured for pump intensities approaching the saturation intensity of 10 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. We also demonstrate aberration correction using surface phase conjugation. The measured properties of the surface-phase-conjugate signal are in excellent agreement with the predictions of a theory of surface phase conjugation by degenerate four-wave mixing, in which the origin of the nonlinear response is saturated absorption.
We investigate the transformation of Rayleigh speckle patterns by an atomic cesium vapor. After propagation through the atomic vapor, the incident exponentially decaying intensity distribution of the Rayleigh speckles is transformed with enhancement of high intensities probabilities (extreme events). The measured intensity distributions are fitted by Weibull distributions. The nonlinear interaction between the atomic vapor and the optical field leads to intensity-dependent refraction and absorption, resulting in changes of the speckle spots' intensities with propagation. We measure the inverse of the shape parameter of the Weibull distributions as a function of laser detuning and interpret the results by modeling the speckle propagation.
A model is formulated to investigate quantitatively how near-resonant degenerate four-wave mixing (DFWM) in a vapor of homogeneously broadened two-state atoms is affected by radiation scattered from the pump beams. The spectral intensity distribution of the scattered radiation in the interaction volume is obtained by solving the equation of radiative transfer and a detailed treatment of the modification this fluctuating field causes on the optical dipole induced by the coherent laser fields is presented. For a broad parameter regime, this modification can be expressed via a single quantity that incorporates the energy of the fluctuating field together with bandwidths and frequency offsets from the atomic resonance. Numerical solutions are given which show that the scattered field causes the DFWM reflectivity to be depressed by a signficant amount, depending most strongly on a characteristic transverse length of the interaction volume. Approximate analytic expressions for the reflectivity are also given which assist in physically understanding the underlying mechanisms.