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Optics Letters

Amplitude-squeezed, frequency-modulated, tunable, diode-laser-based source for sub-shot-noise FM spectroscopy

Steven Kasapi, Seema Lathi, Y. Yamamoto

Published Apr 1, 1997• 49 citations
Publisher

Abstract

We have developed a frequency-modulated, tunable, amplitude-squeezed, diode-laser-based source and used it to perform FM spectroscopy on rubidium. The setup consists of a free-running diode laser injection locked by a frequency-stabilized, current-modulated diode laser. The injection-locked slave laser beam adopted the frequency spectrum of the master laser beam while rejecting residual AM in the master laser beam by more than 50 dB. Injection locking also enhanced amplitude squeezing in the slave laser beam by suppressing uncorrelated longitudinal sidemodes. The noise floor of the measurement was 0.8 dB below the shot-noise level.

DOI
10.1364/ol.22.000478

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Europhysics Letters (EPL)•Oct 1, 2003•8 citations
We have developed a frequency-modulated, tunable, amplitude-squeezed, diode-laser-based source and used it to perform FM spectroscopy on rubidium. The setup consists of a free-running diode laser injection locked by a frequency-stabilized, current-modulated diode laser. The injection-locked slave laser beam adopted the frequency spectrum of the master laser beam while rejecting residual AM in the master laser beam by more than 50 dB. Injection locking also enhanced amplitude squeezing in the slave laser beam by suppressing uncorrelated longitudinal sidemodes. The noise floor of the measurement was 0.8 dB below the shot-noise level.
We have developed a sub-shot-noise diode laser system and used it to perform sub-shot-noise FM spectroscopy. By inducing FM sidebands on a direct-current modulated external-cavity diode laser and injection locking a slave laser, we have suppressed the residual AM in the master by 50 dB and achieved a noise level of 0.8 dB below the shot-noise level. We expect that such techniques will provide useful and practical tools for future precision measurements and novel atom-photon interaction experiments.
We have demonstrated sub-shot-noise FM spectroscopy on thermal cesium atoms, using amplitude-squeezed light from a semiconductor laser. The light source used in our experiment was a custom-made GaAs transverse junction stripe semiconductor laser featuring broadband amplitude squeezing and a large FM bandwidth. We have also developed a new technique by which a semiconductor laser can be frequency modulated with infinitesimally small residual AM and with complete preservation of the squeezed-amplitude fluctuations.
We demonstrate that the large FM noise of a semiconductor laser can be combined with a squeezed AM noise spectrum to make a sub-shot-noise, FM-noise-based spectroscopic measurement. In particular, we probe the spectrum of magneto-optically trapped and cooled rubidium atoms using the FM noise on squeezed AM background noise. The natural absorption line as well as a stimulated Raman transition are clearly visible in the FM-to-AM converted noise even though the signal is below the classical shot-noise level.
Frequency-modulation (FM) spectroscopy has been performed on the D(2) transitions of rubidium atoms with an AlGaAs diode laser at 780 nm. Doppler-broadened hyperfine-structure transitions of (85)Rb and (87)Rb were resolved with no residual amplitude-modulation-induced background signal by modulating the injection current of the laser diode at a low frequency (20-50 MHz) compared with the Doppler width. To obtain Doppler-free spectra, we combined FM spectroscopy with saturation spectroscopy. The results show that the FM spectroscopy technique is sensitive and should be useful for high-resolution spectroscopy, although the resolution was instrument limited and unusual double peaks were observed.
Widely frequency-tunable quantum-correlated twin beams have been experimentally demonstrated. The noise of the intensity difference between the signal and idler beams of the twin beams is reduced to more than 3 dB below the shot noise limit (SNL) throughout a wavelength-tuning range of 5.6 nm. A continuously frequency-tuning range of 2.8 GHz appears around the center wavelength of 1041.506 nm, where the maximum noise reduction of about 5.3 dB is observed. The tunable twin beams are used in frequency modulation (FM) spectroscopy of Cs 2 molecules and a spectrum with a sensitivity of about 4 dB beyond that of the SNL is obtained.