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

Calcium optical frequency standard

F. Riehle, H. Schnatz, B. Lipphardt, Uwe Sterr, T. Binnewies, G. Wilpers, Tilmann Trebst, Juergen Helmcke

Published May 3, 2001• 1 citation
Publisher

Abstract

A laser that is frequency stabilized to the intercombination transition at (lambda) equals 657 nm of laser cooled 40Ca atoms by means of time-domain atom interferences has been developed at PTB. The relative uncertainty of this optical frequency standard is 2.5 X 10-13 and the frequency of the laser has been measured with both, a conventional frequency measurement chain and a femtosecond laser. To reduce the uncertainty of this standard the dependence of its frequency on various parameters was investigated by using phase sensitive and frequency sensitive atom interferometers. Upper bounds were derived for various contributions to the uncertainty as e.g. the influence of gravitational acceleration, collisions of the ballistic atoms, or curvature of the wave fronts of the interrogating laser beams. Further reduction of the uncertainty of the standard is expected from the application of sub-Doppler cooling techniques. A method was devised based on the narrow intercombination transition that enabled us to reduce the velocity spread of the Ca atoms in one dimension close to the recoil limit. The method uses the repeated selection and accumulation of slow atoms from the pre-cooled atomic cloud and the repeated rethermalization of the remaining atoms and results in an increased visibility of the interference structure.

DOI
10.1117/12.424460

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Physical Review A•Aug 25, 2011•4 citations
A laser that is frequency stabilized to the intercombination transition at (lambda) equals 657 nm of laser cooled 40Ca atoms by means of time-domain atom interferences has been developed at PTB. The relative uncertainty of this optical frequency standard is 2.5 X 10-13 and the frequency of the laser has been measured with both, a conventional frequency measurement chain and a femtosecond laser. To reduce the uncertainty of this standard the dependence of its frequency on various parameters was investigated by using phase sensitive and frequency sensitive atom interferometers. Upper bounds were derived for various contributions to the uncertainty as e.g. the influence of gravitational acceleration, collisions of the ballistic atoms, or curvature of the wave fronts of the interrogating laser beams. Further reduction of the uncertainty of the standard is expected from the application of sub-Doppler cooling techniques. A method was devised based on the narrow intercombination transition that enabled us to reduce the velocity spread of the Ca atoms in one dimension close to the recoil limit. The method uses the repeated selection and accumulation of slow atoms from the pre-cooled atomic cloud and the repeated rethermalization of the remaining atoms and results in an increased visibility of the interference structure.
Optical frequency standards with the reference to a narrow electronic transition of a laser-cooled collection of neutral atomic particles are becoming essential tools of research in modern precision physics experiments. In the core of a building block of an optical frequency standard is the optical continuous wave laser that has a good spectral purity of the emitted light. Such a stable optical oscillator is highly desirable in high resolution spectroscopy, if it emits in a good quality beam at a short visible wavelength. This Master thesis explores efficient techniques for building such an optical frequency source intended for use in the cooling and trapping of Calcium atoms scheme. The strong dipole transition at the blue wavelength in the atomic Calcium is needed to reduce the kinetic energy of atoms by nearly six orders of magnitude. A further reduction in the thermal energy of the laser cooled atoms is required to locate with ultra-high precision the 400 Hz narrow clock transition of the stable 40Ca isotope. The experimental methods that achieve this and approach sub-microkelvin temperature of the laser cooled bosonic isotopes of alkaline earths are inspected. The blue laser with a uniform intensity distribution in the beam is useful to maintain the trapped number of cold atoms during these experiments. The spectroscopic properties of the relative transitions in Calcium atom are also reviewed following relevant publications in the area. The constructed blue laser can be used as a primary wavelength source in the lasers network for cooling and trapping of Calcium atoms. These experiments will constitute part of the project to build an optical atom clock referenced to 40Ca narrow linewidth transition. The blue laser is constructed by generating second harmonic in a Potassium Niobate crystal, which is temperature controlled to use a type-I noncritical phase-matching of the optical nonlinear process. The power of the intracavity-generated second harmonic depends on the resonance properties of the optical resonator where this nonlinear crystal is placed. The study is aimed at characterising the designed optical resonator and the experimental parameters that describe it. The formula is derived that relates the resonance power enhancement coefficient with finesse and the power coupling contrast of a passive optical cavity. The obtained relationship is verfied during the experiments. The produced efficiency of the intracavity second harmonic generation is approx. 0.0023 mWblue/(mWred)2. The research work also examines the noise characteristics of the infrared diode laser that is used as a pump source for the intracavity generated second harmonic and determines the spectroscopic precision of the produced blue light. The frequency locking experiment is analysed using the unbalanced scheme of the polarisation stabilisation technique. The designed optical buildup cavity became a part of the unbalanced frequency discriminator in such a scheme. The results demonstrate high gain of frequency noise suppression of the stabilised laser. The unbalanced arrangement of the H}ansch-Couillaud technique has been possible due to a very low amplitude noise of semiconductor lasers.
Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.
This paper presents the recent experimental results on development of an optical frequency standard based on ultra cold magnesium atoms with relative frequency uncertainty and long term stability at the level of Δ v/v <10 −16 . We stabilized the frequency of our clock laser system at 655 THz to narrow Ramsey fringes in a time separated laser fields interacting with cooled Mg atoms localized in a magneto-optical trap (MOT). The intercombination line 1 S 0 → 3 P 1 was used as the reference for frequency stabilization. The results of stabilization were studied with femtosecond comb based on Ti:Sa laser.
The thermal velocity distribution of a calcium atomic beam was compressed into a narrow velocity distribution with selectable velocities. Subsequently these slow atoms were deflected by an angle of 30° by a one-dimensional optical molasses. In the deflected beam we obtained an estimated flux of several 1010 atoms/s at mean longitudinal velocities near 35 m/s and a velocity width of approximately 13 m/s. The transverse velocities of the deflected atoms were close to the one-dimensional Doppler limit. We have observed saturated absorption signals in the deflected laser-cooled beam. The reduced transit time broadening allowed us to resolve the recoil splitting in saturated absorption.
A symmetrical atom interferometer with a thermal calcium atom beam has been developed using a narrow linewidth diode laser stabilized to the resonance of a high-finesse cavity. The linewidth of the diode laser was estimated to be less than 1 Hz relative to the cavity resonance in noise measurement over the range of 100 Hz to 1 MHz, and the phase instability of the interference fringes obtained from the Allan deviation was improved to 2 mrad at an integration time of 300 s. Using this atom interferometer, the ac Stark phase shift between the 1{}^{1}1S0{S}_{0}S0​ and 3{}^{3}3P1{P}_{1}P1​ states of a Ca atom was measured as a function of a laser power near the resonance of the ${}^{1}{S}_{0}{\ensuremath{-}}^{1}\phantom{\rule{-0.16em}{0ex}}{P}_{1}$ transition at a wavelength of 423 nm. The decay rate of the 1{}^{1}1P1{P}_{1}P1​ state was determined to be $\ensuremath{\gamma}=1.91(33)\ifmmode\times\else\texttimes\fi{}{10}^{8}$ s${}^{\ensuremath{-}1}$.