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Annual Meeting Optical Society of America

Excess noise imparted on a laser beam after it propagates through atomic-potassium vapor

William V. Davis, Martti Olavi Kauranen, Russell J. Gehr, Elna M. Nagasako, Robert W. Boyd

Published Jan 1, 1993• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1993.wxx.3

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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 have found that an intense shot-noise-limited laser beam tuned near the 4 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\leftrightarrows}4 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ 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 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 show both theoretically and experimentally that fluctuations of a weak probe beam are increased above the coherent-state shot-noise level as the beam interacts with a strong degenerate pump beam in an atomic vapor. The origin of this noise amplification process is the two-beam-coupling process between the pump beam and the vacuum sidebands of the probe beam. The major contribution to this excess noise arises from the quantum fluctuations of the atomic medium that are induced by the pump beam. We believe that this mechanism can be important in preventing the reduction of noise below the quantum-noise limit in experiments that utilize atomic vapors.
Saturated absorption is examined theoretically to determine the extent to which this process can lead to an increase in the intensity noise of an intense laser beam as the beam propagates through an atomic vapor. Numerical results for the modification of the intensity fluctuations and the noise figure are presented for both incoherent and coherent incident light. The results show that although saturated absorption is a real process for the generation of excess noise, it is not sufficient to account for the noise observed in laboratory experiments.
There is great interest in developing low-noise optical amplifiers that can be used in optical communication systems. Optical amplifiers have been constructed from laser amplifiers, optical fibers, Raman cells, χ(2) nonlinear crystals, and atomic vapors. Optical amplifiers that make use of two- beam coupling in atomic vapors are particularly important since they are able to amplify a continuous-wave multiple-transverse-mode optical signal with a single-pass gain greater than 10 at the bandwidths necessary for optical communications.1 For these reasons, we present an experimental and theoretical study of the noise properties of a probe beam amplified through two-beam coupling in potassium vapor. We also describe the conditions under which this optical amplifier can operate as a quantum-noise-limited linear amplifier.