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Optics Letters

High-sensitivity nonlinear spectroscopy using a frequency-offset pump

J. J. Snyder, Rama K. Raj, Daniel Bloch, M. Ducloy

Published Apr 1, 1980• 94 citations
Publisher

Abstract

We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation-spectroscopy signals. With this technique, which we believe will also be effective in two-photon and other types of nonlinear spectroscopy, we have routinely observed background-free saturation signals in I2 at 514.5 nm with signal-to-noise ratios within an order of magnitude of the fundamental quantum limit.

DOI
10.1364/ol.5.000163

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Review of Scientific Instruments•Feb 1, 1997•3 citations
We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation-spectroscopy signals. With this technique, which we believe will also be effective in two-photon and other types of nonlinear spectroscopy, we have routinely observed background-free saturation signals in I2 at 514.5 nm with signal-to-noise ratios within an order of magnitude of the fundamental quantum limit.
We report the development of a simple method for eliminating coherent background noise and fluctuations from saturation spectroscopy signals. With this technique, which utilizes a traveling-wave acousto-optic modulator to simultaneously frequency-shift and modulate the saturating beam, we have routinely observed zero background, Doppler-free saturation resonances in the I2I_{2}I2​ at 514.5 nm. With a low-noise laser or high pump modulation frequency, we have achieved detection sensitivity approaching the fundamental quantum limit.
Doppler-free saturation spectroscopy in the regime of strong pumping intensities and optically thick atomic samples is investigated experimentally and theoretically. It is shown that a very high signal-to-background ratio can be obtained and, at the same time, subnatural linewidths can be reached. Further contrast and linewidth improvements can be attained by supplementing the primary depleted pumping beam with a second pumping beam. By using the signal beam from a first setup as the pumping beam for a second identical arrangement, extreme values for contrast and linewidth should be attainable.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.
A technique for reducing background and improving the signal-to-noise ratio in optical spectroscopy of weak atomic transitions is described. In this technique an incident laser is square-wave amplitude modulated and the resonance fluorescence from the atom is then detected out of phase with the excitation when the laser is off. Due to the finite lifetime of the atom’s excited state, appreciable signal is observed when the laser is off and instrumental scattering background is virtually eliminated. Applying this technique to an experiment on single laser-cooled In+ ions, we have observed a factor of 500 reduction in background and a factor of 5 increase in the signal-to-noise ratio.