Alexander L. Gaeta

Instabilities of laser beams counterpropagating through a Brillouin-active medium

Counterpropagating laser beams in a Brillouin-active medium are shown to become unstable to the growth of amplitude and phase fluctuations. Slightly above threshold, the nature of the instability is the temporal growth of sidemodes separated from the laser frequency by approximately the Brillouin frequency of the medium. This process leads to sinusoidal oscillations of the intensities of the transmitted waves. At higher input intensities the system can become chaotic; many sidemodes are excited, and the transmitted fields fluctuate wildly. The origin of the Brillouin instability is the combined action of the gain of the standard stimulated Brillouin scattering (SBS) process and of the coupling of the waves due to multiwave mixing mediated by the electrostrictive interaction. The threshold for the instability is at least several times lower than the threshold of the standard SBS process involving a single pump beam.

Exact theory of pump-wave propagation and its effect on degenerate four-wave mixing in saturable-absorbing media

An analytic solution for the intensity distribution of two counterpropagating pump waves within a saturable absorber is derived. From this distribution, the spatial variation of the nonlinear absorption and coupling constants that appear in the coupled-amplitude equations for the probe and the signal (i.e., conjugate) waves are determined. These coupled-amplitude equations are solved numerically in a noniterative manner, leading to a prediction for the phase-conjugate reflectivity. The results of the exact theory are compared with those of previously published theories. It is found that at large values of the input-pump intensities, the predicted phase-conjugate reflectivity is larger when pump-absorption effects are included in the theory.

Observation of instabilities of laser beams counterpropagating through a Brillouin medium

We show experimentally that counterpropagating laser beams in a Brillouin-active medium are unstable to the growth of intensity fluctuations. The intensities of the transmitted beams are found to oscillate at the Brillouin frequency of the medium, which is equal to 7.7 GHz for our experimental conditions of excitation of carbon disulfide at a wavelength of 532 nm. The depth of modulation is found to be as large as 25%. The threshold for this instability depends on the ratio of the intensities of the counterpropagating beams. The threshold is lowest for the case of equal input intensities and is found to be as much as 33% lower than the threshold for single-beam stimulated Brillouin scattering.

Polarization-ellipse rotation by induced gyrotropy in atomic vapors

We study theoretically and experimentally a new mechanism for the rotation of the polarization ellipse of a single laser beam propagating through an atomic vapor with a frequency tuned near an atomic resonance. The results of a theoretical treatment for the case of a J = (1/2) to J = (1/2) atomic transition show that a rotation of the polarization ellipse of the laser beam will occur as a result of ground-state optical pumping and that the angle of rotation is independent of the laser intensity over a broad range of laser intensities. The predictions of this theoretical model are tested experimentally through the use of potassium vapor and are found to agree with the experimental data.

Two-beam-excited conical emission.

We describe a conical emission process that occurs when two beams of near-resonant light intersect as they pass through sodium vapor. The light is emitted on the surface of a circular cone that is centered on the bisector of the two applied beams and has an angular extent equal to the crossing angle of the two applied beams. We ascribe the origin of this effect to a perfectly phase-matched four-wave mixing process.

NOISE PROPERTIES OF QUANTUM AMPLIFIERS WITH FREQUENCY-DEPENDENT GAIN

Abstract The power spectrum is calculated of the output of an optical amplifier with frequency-dependent gain for the case in which the input consists of one excited mode that is in an arbitrary quantum state and an infinite set of other modes that are in the vacuum state. The power spectrum is found to be independent of the particular quantum state of the excited input mode and to depend only on the shot-noise level of the amplified output and on the number of noise photons generated by the amplifier at the upper and lower frequency sidemodes of the excited input mode. For the case of a quantum-noise-limited amplifier and a coherent-state input, the results agree with the single-mode treatment of the amplifier only for the case in which the gain experienced by the sidemodes is equal to that experienced by the excited input mode.

BISTABILITY AND CHAOTIC INSTABILITIES OF LASER BEAMS COUNTERPROPAGATING THROUGH SODIUM VAPOR

Publisher Summary This chapter highlights bi-stability and chaotic instabilities of laser beams counter propagating through sodium vapor. The conceptually simple process of mutually interacting laser beams counter propagating through a nonlinear medium leads to extremely complicated dynamical behavior, including bi-stability and chaotic instabilities in the intensities and polarizations of the interacting waves. In a study described in the chapter, it was determined that the intensities fluctuate chaotically far above the threshold for instability. The origin of the unstable behavior was attributed to gain arising from a process analogous to stimulated Rayleigh-wing scattering and to distributed feedback induced by the interfering input waves. The results constitute the experimental confirmation of the prediction of polarization bi-stability due to the effects of the nonlinear coupling and the effects of propagation. For higher coupling strengths, there were no stable states of the system and the fields fluctuated in time.