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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Study on an all-optical microwave frequency standard

Normand Cyr, M. Têtu, Marc Breton

Published Apr 30, 1993• 3 citations
Publisher

Abstract

We describe a simple method to produce a clock transition with purely optical means through modulated pumping. We observe the field-independent ground state resonance of 87Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation). We detect the 6.835 GHz resonance with a modulation frequency of 1.139 GHz. A high contrast resonance peak is observed and a condition for zero light shift is found. With a beam radius of approximately 3 mm and a buffer gas (N2) pressure of 680 Pa (5.1 Torr), a minimal linewidth of 290 Hz is observed. A theoretical study of the excitation of hyperfine coherences by a modulated laser beam is given.

DOI
10.1117/12.143684

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2007•0 citations
We describe a simple method to produce a clock transition with purely optical means through modulated pumping. We observe the field-independent ground state resonance of 87Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation). We detect the 6.835 GHz resonance with a modulation frequency of 1.139 GHz. A high contrast resonance peak is observed and a condition for zero light shift is found. With a beam radius of approximately 3 mm and a buffer gas (N2) pressure of 680 Pa (5.1 Torr), a minimal linewidth of 290 Hz is observed. A theoretical study of the excitation of hyperfine coherences by a modulated laser beam is given.
A simple method to produce a clock transition with purely optical means by modulated pumping is described. The field-independent ground state resonance of /sup 87/Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation) is observed. The 6.835 GHz resonance with a subharmonic modulation frequency of 1.139 GHz is detected. A high-contrast resonance peak is observed and a condition for zero light shift is found. The linewidth is 3 kHz (at 6.835 GHz) in this preliminary experiment, due to the small size of the light beam ( approximately 2 mm diameter) and the low buffer gas pressure (680 Pa) that was used. A theoretical model that explains the main features of the experiment is described.>
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s<τ<2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.
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.