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Conference on Precision Electromagnetic Measurements

Analysis of the laser phase noise conversion by an atomic vapor

Messaoud J. Bahoura, Philippe Laurent, N. Dimarcq, Andr  Clairon

Published Dec 24, 2002• 0 citations
Publisher

Abstract

We present theoretical and experimental studies of the influence of diode laser phase noise on the detection of the absorption signal in an atomic vapor cell, and we compare the noise of the absorption and fluorescence signals. Excess noise is observed on both sides of the atomic resonance.
DOI
10.1109/cpem.1996.547096

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In order to clarify the mechanism of increased laser relative intensity noise (RIN) in the diode laser pumped gas-cell atomic clock, we have performed experiments examining the influence of laser linewidth, /spl Delta/v/sub L/, on RIN. Our measurements are consistent with the phase noise (PM) to intensity noise (AM) conversion theory: laser phase fluctuations induce fluctuations in the absorption cross section of the atomic vapor and these yield fluctuations in the transmitted intensity of the laser. In the experiments, transmitted RIN of a beam from a 100 kHz linewidth diode laser was compared with that from a 60 MHz linewidth diode laser after each was passed through a cell containing natural rubidium vapor. We present some results of computational theory showing that for laser linewidths less than the atomic dephasing rate of the atomic vapor, RIN is proportional to /spl radic/(/spl Delta/v/sub L/); however, for laser linewidths greater than the atomic dephasing rate RIN is a decreasing function of /spl Delta/v/sub L/.
When resonant laser light passes through a vapor, the laser's intrinsic phase fluctuations induce random variations in the atomic coherence, which in turn give rise to fluctuations in the medium's absorption cross section. Hence, laser phase modulation noise (PM) is converted to transmitted laser intensity (i.e., amplitude) modulation noise (AM). Here, we consider the influence of collisional dephasing on the PM-to-AM conversion process. Specifically, we measure the relative intensity noise of a diode laser beam, resonant with the Rb D1{D}_{1}D1​ transition at 794.7 nm, after it has passed through a Rb87/N2{\mathrm{Rb}}^{87}/{\mathrm{N}}_{2}Rb87/N2​ vapor as a function of nitrogen number density. Our results demonstrate that when collisional dephasing is very rapid, the spectral density of cross-section fluctuations is reduced, so that there is a significant decrease in the efficiency of PM-to-AM conversion at low Fourier frequencies. These results imply that, in general, when laser PM-to-AM conversion is the dominant noise process, pressure broadening can actually increase spectroscopic sensitivity.
When laser light propagates through a resonant medium, the transmitted beam can exhibit excess intensity noise [amplitude modulation (AM)]. In a semiclassical description of the phenomenon, laser phase noise (PM) induces fluctuations in the medium's electric susceptibility, which in turn cause fluctuations in the transmitted intensity. The process provides an efficient means for PM-to-AM conversion, and intuition suggests that large linewidth lasers should exhibit much greater PM-to-AM conversion than narrow linewidth lasers. Here we measure the relative intensity noise (RIN) for two diode lasers whose linewidths $\ensuremath{\Delta}{\ensuremath{\nu}}_{L}$ differ by more than 102,{10}^{2},102, after the lasers have propagated through a resonant rubidium vapor. Though the RIN of the narrow linewidth laser is only reduced by a factor of about 6 compared to the broad linewidth laser, our results are nonetheless consistent with numerical simulations of the PM-to-AM conversion process. In particular, both computation and analytical theory indicate that RIN is a nonlinear function of $\ensuremath{\Delta}{\ensuremath{\nu}}_{L}.$ For single-mode laser linewidths less than the atomic dephasing rate, RIN increases like $\sqrt{\ensuremath{\Delta}{\ensuremath{\nu}}_{L}},$ while for linewidths greater than the atomic dephasing rate RIN is a decreasing function of $\ensuremath{\Delta}{\ensuremath{\nu}}_{L}.$
We report on oscillating complex noise spectra obtained when a diode-laser beam passes through a resonant dense Doppler-broadened cesium-vapor cell. Atomic coherence converts the laser phase noise into amplitude noise in the transmitted beam. We have found that the level of amplitude noise is orders of magnitude above the intrinsic laser noise. As a function of laser detuning, this noise extends over several inhomogeneous widths, depending on the spectral frequency. Numerical calculations based on a simple theory remarkably mimic the details of the experimental noise spectra.
The optically thick vapors employed in gas-cell atomic clocks provide for very efficient conversion of laser phase noise (PM) to amplitude noise (AM). Here, we discuss the role of PM to AM conversion in determining gas-cell clock performance. First, a brief overview of the PM to AM conversion process will be given, highlighting the fact that laser phase noise creates fluctuations in the atom's optical absorption cross-section which have an exponential influence on the laser's transmitted intensity. Experimental results will then be presented showing that the PM to AM conversion process can increase the relative intensity noise of transmitted laser light by orders of magnitude, and that this can, under certain conditions, cause laser-pumped clock performance to be poorer than lamp-pumped clock performance.