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Journal of the Optical Society of America

Magneto-Optic Modulation of a Light Beam in Sodium Vapor*

Bernhard M. Schmidt, John Michael Williams, Dudley Williams

Published Apr 1, 1964• 18 citations
Publisher

Abstract

The enhanced Faraday effect in gases for optical frequencies near absorption lines provides a basis for modulating a light beam. In the present study, incident radiation consisting of the Zeeman s components of the D lines of sodium passed through a polarizer to the Faraday cell, which employed saturated sodium vapor at approximately 240°C as an optical medium. Rotation of the plane of polarization of radiation traversing the cell was modulated at frequencies as high as 698 Mc/sec. On the basis of cw modulation studies, there appears to be no reduction in the integrated time-varying Faraday effect for frequencies as high as 265 Mc/sec; this frequency is 1100 times the Larmor frequency in the magnetic fields involved and 26 times the natural linewidth. There was no evidence of any reduction in frequency response for pulsed modulation at 698 Mc/sec.

DOI
10.1364/josa.54.000454

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The enhanced Faraday effect in gases for optical frequencies near absorption lines provides a basis for modulating a light beam. In the present study, incident radiation consisting of the Zeeman s components of the D lines of sodium passed through a polarizer to the Faraday cell, which employed saturated sodium vapor at approximately 240°C as an optical medium. Rotation of the plane of polarization of radiation traversing the cell was modulated at frequencies as high as 698 Mc/sec. On the basis of cw modulation studies, there appears to be no reduction in the integrated time-varying Faraday effect for frequencies as high as 265 Mc/sec; this frequency is 1100 times the Larmor frequency in the magnetic fields involved and 26 times the natural linewidth. There was no evidence of any reduction in frequency response for pulsed modulation at 698 Mc/sec.
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We present a combined theoretical and experimental study of magnetic resonance transitions induced by polarization-modulated light in cesium vapor exposed to a transverse magnetic field. Signals are obtained by phase-sensitive analysis of the light power traversing the vapor cell at six harmonics of the polarization modulation frequency. Resonances appear whenever the Larmor frequency matches an integer multiple of the modulation frequency. We have further investigated the modifications of the spectra when varying the modulation duty cycle. The resonance amplitudes of both in-phase and quadrature components are well described in terms of the Fourier coefficients of the modulation function. The background-free signals generated by the polarization modulation scheme have a high application potential in atomic magnetometry.
Results of a fairly straightforward experiment on resonant magneto-optical rotation by rubidium-87 atoms revealed strong time-dependence of the polarization plane of light emerging from atomic vapors following a sudden irradiation with a laser beam. The rotation of the plane appears as a not direct consequence of the influence of the magnetic field on atoms. Reported measurements conducted using a vapor cell without any buffer gas or an anti-relaxation wall coating show that transmitted light has initially the same (linear) polarization as the incident one. Rotation of the polarization plane caused by an axial magnetic field develops in time scales similar to the pace of establishing the optical pumping/relaxation equilibrium in the atomic ensemble. The traditional passive Faraday rotation picture providing working description for the resonant magneto-optical effects in steady-state conditions does not explain the observed sequence of evolution of the polarization. The picture has to be augmented with analysis of time dependence in the spatial distribution of relevant quantum states within the atomic ensemble as well as longitudinal and transversal intensity profiles of the light beam. Isolation of external effects caused by the optical pumping and relaxation processes from consequences of internal atomic coherences should uncover more details of mechanisms of the latter in the presence of magnetic field. Plentitude of data available from time-dependent polarization measurements provides strong incentives for further development and tests of comprehensive models of coherent interactions of atomic ensembles with radiation and possibly a base for additional advance in understanding and applications of these phenomena.
Results of a numerical study of the Faraday effect arising upon propagation of the light beams with the frequencies ω L1 (resonant to the nS 1/2-nP 1/2, 3/2 transitions) and ω L2 (resonant to the nP 1/2, 3/2-(n+2)S 1/2 transitions) through alkali-metal vapors are presented. Characteristics of the magneto-optical rotation spectra at each of the frequencies are strongly affected by the second intense radiation field resonant to the adjacent transition. When the atoms interact with two strong light waves, resonant to adjacent transitions, and with a magnetic field, the shape of the Faraday rotation spectra depends on the energy shifts of the atomic states that arise due to the dynamic Stark effect and the Zeeman effect (the Paschen-Back or an intermediate-type effect), as well as due to the difference of populations of these states caused by the interaction of the atoms with the fields. The results obtained show that in the frequency selection method, based on the resonance Faraday effect, the frequency of the generated narrow-band beam can be tuned by the intensity of the strong wave, resonant to the transition between the excited states.
A general equation is constructed for magneto-optical spectra. The incident laser beam has an arbitrary polarization and its direction is arbitrary relative to the magnetic field. The analyzer has an arbitrary extinction ratio function. The Faraday and Voigt formulas are deduced as special solutions. From the generalized Faraday formula, the absorption spectra of ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ light are measured without influence of the birefringence. The circular dichroism is estimated. The measured spectra clearly show the frequency shifts of ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ light. The magnitude of the shift for a magnetic field of 0.015 T is twice as large as the Doppler width for the Cs D2{\mathit{D}}_{2}D2​ line. These shifts are induced by the Larmor precession defined by the fine-structure g factor. The Larmor precession does not induce a real shift of Zeeman levels, but induces a phase shift of optical coherence through the Rabi frequency.
Larmor precession of laser-polarized atoms contained in antirelaxation-coated cells, detected via nonlinear magneto-optical rotation (NMOR), is a promising technique for a new generation of ultrasensitive atomic magnetometers. For magnetic fields directed along the light propagation direction, resonances in NMOR appear when linearly polarized light is frequency or amplitude modulated at twice the Larmor frequency. Because the frequency of these resonances depends on the magnitude but not the direction of the field, they are useful for scalar magnetometry. Additional NMOR resonances at the Larmor frequency appear when the magnetic field is tilted away from the light propagation direction in the plane defined by the light propagation and polarization vectors. These resonances, studied both experimentally and with a density matrix calculation in the present work, offer a convenient method of achieving additional information about a direction of the magnetic field.