Knowledge Bank

Applying heterodyne detection $schemes1$ to laser polarization $spectroscopy2$ increases the sensitivity (S/N ratio) $considerably3$. An acousto-optical modulator (AOM) shifts the pump frequency relative to the probe frequency by the amount of the driving frequency of the AOM thereby avoiding coherent noise due to interference of pump and probe/signal beams. At the same time the pump bean can be amplitude modulated at high frequencies (up to the MHz region) by means of the AOM thereby shifting the detection frequency towards more favorable regions of the laser noise spectrum. Interference of an optimized fraction of the probe bean with the spectroscopic signal generated through the non-linear mixing of the pump and probe beams by the medium finally amplifies the signal at a sensitivity close to the shot noise limit. Examples are given showing spectra of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi u\leftarrow 1\Sigma g+$ transitions of $Na2(\approx 104$ times less intense compared to the ``allowed"" $A1\Sigma u+\leftarrow X1\Sigma g+$ $transition4$) and weak two photon absorption in a Na-heat pipe. Detection of components of the Na-D lines was accomplished in the vapor over the metal at room temperature and with $103-104$ atoms in the interaction volume. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$J. J. Snyder, R. K. Raj, D. Bloch, and M. Ducloy, Opt. Lett. 5. 163 (1980). $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$C. Wieman and T. W. H""{a}nsch, Phys. Rev. Lett. 36,(1170). $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$M. D. Levenson and G. L. Eesley, Appl. Phys. 19, 1 (1979). $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$J. B. Atkinson, J. Becker, and W. Demtr""{o}der, Chem. Phys. Lett. 87, 92, 128 (1982).