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Optics Letters

Frequency-modulation spectroscopy of rubidium atoms with an AlGaAs diode laser

Shunsuke Nakanishi, H. Ariki, Hiroshi Itoh, Kazuki KONDO

Published Nov 1, 1987• 10 citations
Publisher

Abstract

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.

DOI
10.1364/ol.12.000864

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Optics Letters•Apr 1, 1997•49 citations
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.
A frequency-doubled diode laser spectrometer in the blue-wavelength region with a continuous-frequency tuning range of up to ∼7 GHz has been developed, comprising a powerful fundamental light source based on a master/slave laser combination coupled with a KNbO3 crystal for second-harmonic generation in a power-enhancement cavity. It has been employed for the observation of saturated absorption spectra in 52S1/2–62P1/2 hyperfine transitions of atomic rubidium with the aid of a modulation transfer spectroscopic technique, and also the frequency stabilization to one of them has been demonstrated, suggesting the possibility of the establishment of a precision optical frequency marker in the blue range. A relative stability of better than 2.8×10−10 is estimated based on the locked error signal in the preliminary stabilization. With a further study of the frequency instability and uncertainty, in addition to supplementary improvements of apparatus, one may expect a realization of a stabilized laser in the blue-spectral domain. Details of the developed blue-light source are described and the demonstrated results of the rubidium spectroscopy are also described and discussed.
We have performed high resolution spectroscopy of rubidium with a single mode continuous wave distributed feedback (DFB) laser diode. The saturation spectrum of the D 2 -line of 85 Rb and 87 Rb was recorded with a resolution close to the natural line width. The emission frequency was actively stabilized to Doppler-free transitions with a relative accuracy of better than 7 parts in 10 9 using commercially available servo devices only. An output power of 80 mW was sufficient to allow for two-photon spectroscopy of the 5S-5D-transition of 87 Rb. Further, we report on the spectral properties of the DFB diode, its tuning range and its frequency modulation properties. The line width of the diode laser, determined with high resolution Doppler free two photon spectroscopy, was 4 MHz without applying any active stabilization techniques. For time scales below 5 μs the line width drops below 2 MHz.
Distributed feedback (DFB) laser chips have recently become available at wavelengths that match the D1 and D2 resonance transitions of alkaline atoms. We investigated the spectral properties, tuning characteristics and modulation behavior of continuous wave, single-mode DFB diodes at 778-780 nm and performed high-resolution spectroscopy of Rubidium vapor. The mode-hop free tuning range of the DFB diodes was as large as 2.4 nm (1186 GHz). The line width of the laser diodes was examined both with a heterodyne beat experiment and with high-resolution Doppler-free two photon spectroscopy, yielding a half width of 2-2.5 MHz. The saturation spectra of the D2-line of 85Rb and 87Rb were recorded with a resolution close to the natural line width. The emission frequency was actively stabilized to Doppler-free transitions with a relative accuracy of better than 4 parts in 109 using commercially available servo devices only. The output power of 80 mW was sufficient to allow for two photon spectroscopy of the 5S-5D-transition of 87Rb. We conclude that the performance of the DFB laser equals that of grating-stabilized external-cavity diode lasers (ECDLs), without the mechanical complexity of the latter systems. The DFB diode is thus well-suited to high-resolution applications in alkaline spectroscopy, including laser cooling and optical manipulation of ultra-cold atoms.
We describe an experiment for investigating the 5S1∕2→5D5∕2 two-photon transition in rubidium using a single grating-feedback diode laser operating at 778.1nm (385THz). Continuous tuning of the laser frequency over 4GHz allows for the clear resolution of the Doppler-free spectral features and allows accurate measurement of the hyperfine ground-state splitting. A direct comparison between Doppler-broadened and Doppler-free spectral features is possible because both are distinctly evident in the two-photon spectra. By independently modifying the polarization state of the two laser fields, the impact of electric dipole selection rules on the two-photon transition spectra is investigated. This experiment is a valuable addition to the advanced undergraduate laboratory because it uses much of the same equipment as the single-photon saturated absorption spectroscopy experiment performed on the 5S1∕2→5P3∕2 transition in rubidium (λ=780.24nm) and provides students with an opportunity to investigate characteristics of atomic spectra not evident in the single-photon experiment.
We present an experimental scheme that produces Doppler-free spectra of the $5s \to 6p $ second resonance transition in atomic rubidium. The experiment uses the saturation of the cascade fluorescence that occurs when thermal rubidium atoms interact with two counterpropagating $420 $ nm laser beams of comparable intensity. Part of the excited atomic population goes through the 5p3/25p_{3/2}5p3/2​ level which then decays by emission of 780780780 nm photons. Narrow dips appear in this otherwise broad 780780780 nm fluorescence, which allows resolution of the 6p3/26p_{3/2}6p3/2​ hyperfine structure. A rate equation model is used to interpret the spectra. It is also shown that these narrow peaks can be used to lock the frequency of the $420 $ nm laser. Using a second beam modulated in frequency produces three sets of spectra with known frequency spacings that can be used to perform an all-optical measurement of the hyperfine splittings of the $6p_{3/2} $ manifold in rubidium.
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.