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Physical Review A

Observation of large continuous-wave two-photon optical amplification

Hope M. Concannon, William J. Brown, Jeff R. Gardner, Daniel J. Gauthier

Published Aug 1, 1997• 25 citations
Publisher

Abstract

We observe 30% two-photon optical amplification of a probe laser-field propagating through a laser-pumped potassium vapor. This amplification is spectrally isolated and substantially larger than that of previously reported continuous-wave two-photon amplifiers. The combination of large amplification and spectral isolation of the two-photon gain feature will greatly facilitate precise studies of the photon statistics of this highly nonlinear quantum amplifier and the development and characterization of a two-photon laser based on this gain medium. We also observe spectrally-distinct three-photon amplification $(\ensuremath{\sim}5$%) in the same system under different experimental conditions. We present a simple model of the interaction that gives qualitative agreement with our observations and explains the dependence of the two-photon gain on the various system parameters. This model predicts that the size of the two-photon gain is quite sensitive to an interference between two different quantum pathways.

DOI
10.1103/physreva.56.1519

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Two-Photon Raman Gain in a Laser Driven Potassium Vapor

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Physical Review Letters•Jan 27, 1992•172 citations
We observe 30% two-photon optical amplification of a probe laser-field propagating through a laser-pumped potassium vapor. This amplification is spectrally isolated and substantially larger than that of previously reported continuous-wave two-photon amplifiers. The combination of large amplification and spectral isolation of the two-photon gain feature will greatly facilitate precise studies of the photon statistics of this highly nonlinear quantum amplifier and the development and characterization of a two-photon laser based on this gain medium. We also observe spectrally-distinct three-photon amplification (\ensuremath∼5(\ensuremath{\sim}5(\ensuremath∼5%) in the same system under different experimental conditions. We present a simple model of the interaction that gives qualitative agreement with our observations and explains the dependence of the two-photon gain on the various system parameters. This model predicts that the size of the two-photon gain is quite sensitive to an interference between two different quantum pathways.
Atomic systems that display a linear or weakly nonlinear interaction with light are well known and well understood. However, when the interaction between light and matter becomes highly nonlinear and the light and matter strongly couple, the systems become much more difficult to understand both theoretically and experimentally. One example of a strongly coupled, highly nonlinear system is the two-photon laser that is based on the two-photon stimulated emission process. This laser has intrigued theorists and experimentalists alike. Research has been hindered, however, by the difficulties in constructing such a laser. Most two-photon gain media prove unsuitable due to small gain and the occurrence of destructive competing nonlinear effects. I have developed a new two-photon gain medium that overcomes these difficulties. It consists of a laser-driven potassium vapor in which the origin of the gain is due to the two-photon Raman scattering process. The two-photon gain feature is identified by performing spectroscopy of the laser-driven potassium vapor. To complement the experimental observations, I have developed a theoretical model of the two-photon Raman gain medium using the semi-classical density-matrix formalism. The predictions of the model are in qualitative agreement with the experimentally observed frequency- and intensity-dependence of the two-photon gain. I also describe a simplified rate-equation model of two-photon lasers through which I explore their steady-state and transient behavior. The model highlights the novel threshold behavior of two-photon lasers and the need to inject an external field to initiate lasing. This work, both theoretical and experimental, provides the first step toward a robust experimental realization of a two-photon laser.
Summary form only given.A less well understood regime of quantum optical systems is the case when they are far from equilibrium and highly nonlinear. One device that inherently operates in this regime is the two-photon laser that is based on the two-photon-stimulated emission process, whereby two photons stimulate the emission of four photons. Such lasers are predicted to have highly unusual nonlinear dynamic and quantum statistical behaviors. Recently, our group demonstrated large continuous-wave two-photon amplification in a laser-driven potassium vapor based on a multiphoton Raman scattering process. Subsequent studies have revealed that destructive interference between multiple quantum pathways significantly reduces the two-photon-stimulated emission rate for this process. To overcome this problem, we have devised a new mechanism for two-photon amplification that suppresses the quantum interference using electromagnetically induced transparency in conjunction with a novel geometry.
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.
We develop a theoretical model of two-photon amplification in laser-driven potassium atoms and use it to analyze the recent experiments reported by Pfister et al. [Phys. Rev. A 60, R4249 (1999)]. The model takes into account most of the essential factors influencing the amplification process, including the atomic hyperfine structure (which makes multiphoton emission possible) and the simultaneous interaction with intense drive and probe beams with arbitrary detunings. We determine the origin and analyze the properties of different multiphoton gain resonances that appear in the light-matter interaction. In particular, the influence of the drive and probe field amplitudes and detunings on the strength and frequency of the two-photon amplification resonance is studied in detail, showing clearly the differences with respect to the behavior of single-photon or other multiphoton amplification processes. In addition, we investigate interferences between different quantum pathways originating from the hyperfine structure and determine the conditions under which they can enhance or suppress multiphoton resonances. The predictions of the model are in good agreement with the observations, indicating that it can be used to understand recent experiments on two-photon lasing reported by Pfister et al. [Phys. Rev. Lett. 86, 4512 (2001)].
We observe and analyze a two-photon continuous-wave optical gain mechanism designed for building a two-photon laser. The two-photon stimulated emission is spectrally isolated and resonantly enhanced using the multilevel structure of 39K,{}^{39}\mathrm{K},39K, in conjunction with an alternative interaction geometry involving orthogonal beams and polarizations. The observed two-photon laser beam amplification increases linearly with low input laser beam intensity, as expected, and saturates at a gain of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at high intensity. A theoretical analysis of the observations is outlined.
We report the first observation of continuous-wave two-photon lasing in the optical regime, and demonstrate that its initiation requires the injection of a trigger pulse into the laser resonator. Successful operation of the two-photon laser relies on the use of a novel gain medium consisting of laser-driven, two-level atoms and the use of a high-finesse optical cavity to isolate the two-photon gain from competing processes. Threshold conditions for laser action are in good agreement with recent theoretical predictions.