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Physical Review A

Excess noise acquired by a laser beam after propagating through an atomic-potassium vapor

William V. Davis, Martti Olavi Kauranen, Elna M. Nagasako, R. J. Gehr, Alexander L. Gaeta, Robert W. Boyd, G. S. Agarwal

Published May 1, 1995• 23 citations
Publisher

Abstract

We have found that an intense shot-noise-limited laser beam tuned near the 4 $^{2}$${\mathit{S}}_{1/2}$\ensuremath{\leftrightarrows}4 $^{2}$${\mathit{P}}_{3/2}$ potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell. The noise is maximum for laser detunings of approximately \ifmmode\pm\else\textpm\fi{}1 GHz and falls to nearly the shot-noise limit for detunings greater than \ifmmode\pm\else\textpm\fi{}3 GHz. We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state. We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam. The predictions of this theory are in good agreement with the experimental data.

DOI
10.1103/physreva.51.4152

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Physical Review A•May 1, 1996•5 citations
We have found that an intense shot-noise-limited laser beam tuned near the 4 ^{2}$${\mathit{S}}_{1/2}\ensuremath{\leftrightarrows}4 ^{2}$${\mathit{P}}_{3/2} potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell. The noise is maximum for laser detunings of approximately \ifmmode\pm\else\textpm\fi{}1 GHz and falls to nearly the shot-noise limit for detunings greater than \ifmmode\pm\else\textpm\fi{}3 GHz. We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state. We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam. The predictions of this theory are in good agreement with the experimental data.
We have observed that a single laser beam that has propagated through an atomic potassium vapor can acquire excess noise. In particular, we find that even if the incident laser beam is shot noise limited, the transmitted laser beam contains noise in excess of its shot noise limit. We interpret this observation in terms of the amplification of vacuum sidemodes of the incident laser beam through forward four-wave mixing processes. These amplified sidebands then beat with the incident laser beam at the photodetector to create excess noise. The excess noise is maximum when the laser frequency is detuned from the potassium resonance by approximately one linewidth on either side of the resonance. We are presently developing a quantitative theoretical model to describe the process that produces this excess noise. We believe that this excess noise can degrade the noise properties of any nonlinear optical interaction that utilizes atomic vapors.
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 propose and show experimental evidence of a mechanism that is important in reducing the amount of noise reduction that can be achieved at low (<100 MHz) frequencies utilizing atomic vapors as the nonlinear medium.1 The mechanism is based on the two-beam-coupling gain2 experienced by the vacuum sidebands of a weak probe wave as it interacts with a strong degenerate pump wave in an atomic vapor. The mechanism leads to the amplification of intensity fluctuations of an initially shot-noise-limited probe field.
We have observed optical phase-sensitive noise in homodyne detection of a cw light beam which has propagated through sodium vapor near the D lines. Although at present the minimum noise is at or above the vacuum-state quantum limit due to zero-point fluctuations, nevertheless its phase behavior is like that of a squeezed state. Our observations are consistent both with the predictions of the quantum-mechanical theory of degenerate four-wave mixing and the theory of multiatom resonance fluorescence in the forward direction.
We have measured the gain and statistical-noise properties of a weak probe beam amplified through two-beam coupling in atomic-potassium vapor for both the Rabi and stimulated Rayleigh gain features. The probe-beam gain was observed to be as large as 125 for the Rabi feature, whereas the gain was typically less than 2 for the Rayleigh feature. The rms noise at the Rabi gain feature was approximately three times greater than the ideal amplifier quantum-noise limit. For the Rayleigh gain feature, the rms noise was 12 to 15 times greater than the ideal amplifier quantum-noise limit. We present a fully quantum-mechanical theory of two-beam coupling in a system of two-level atoms which includes the effects of atomic motion and pump-beam absorption. The predictions of this theory are in good agreement with the experimental data. \textcopyright{} 1996 The American Physical Society.