Parametric four-wave mixing processes in sodium vapor.

Two types of parametric four-wave mixing are observed when a single intense laser beam propagates through Na vapor near the D(1) and D(2) resonance lines. The first is related to three-photon scattering, while the second results from the coherent parametric interaction of the laser with excited-state Raman scattering. Identification of the two processes was greatly facilitated by the use of an optical multichannel detector system to observe the total emission spectrum for each individual laser pulse. The experimental conditions that determine whether stimulated emission or one or both of the four-wave mixing processes dominates the emission spectrum are described.

Frequency-stable and ultranarrow-linewidth semiconductor laser locked directly to an atomic-cesium transition.

We describe a new method for semiconductor laser FM noise reduction. A Doppler-free Faraday resonance in Cs vapor provided optical feedback, and FM sideband saturation spectroscopy in a second Cs cell provided electronic feedback. The combined optical and electronic feedback allowed us to reduce the low-frequency FM noise power by more than 6 orders of magnitude, which resulted in a sub-100-Hz-linewidth semiconductor laser locked directly to an atomic transition frequency.

Competition between stimulated three-photon scattering and parametric four-wave mixing.

We show that destructive interference between the polarizations produced by three-photon scattering and parametric four-wave mixing suppresses the gain at the three-photon frequency under quite general conditions. The conditions for the suppression are discussed, and experimental verification in Na vapor is presented.

Amplitude noise reduction in semiconductor lasers with weak, dispersive optical feedback.

We present the theory and measurements of the amplitude noise spectrum from a semiconductor laser with weak optical feedback (P(fb)/P(out) approximately 10(-6)) from an external cavity containing an element of dispersive loss. The laser noise is found to be reduced over most of the low-frequency spectrum, although an increase in the noise is observed at frequencies corresponding to multiples of the external-cavity free spectral range. The low-frequency noise reduction closely follows theoretical predictions, and a reduction of as much as 7 dB is measured at an injection current of 1.5 times the threshold current. The potential of this method for contributing to the production of amplitude-squeezed light is discussed.

Self-quenching of the semiconductor laser linewidth below the Schawlow-Townes limit by using optical feedback.

We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow-Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 x 10(3) is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor.

Polarization dependence of resonance-enhanced three-photon scattering.

The polarization properties of stimulated three-photon scattering in a three-level atomic system are presented. In our investigation of the stimulated emission spectrum of Na, we find that interference between the amplitudes of the resonant contributions of the Na doublet states results in interesting polarization phenomena. Particularly striking is the behavior when the laser is tuned near the dispersion-free point. With the laser linearly polarized the stimulated three-photon scattering is polarized perpendicular to the laser polarization. For circularly polarized excitation, total extinction of three-photon scattering is observed.