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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

An all-diode-laser optical frequency reference using laser-trapped calcium

Chris W. Oates, Michelle S. Stephens, Leo W. Hollberg

Published Nov 23, 2002• 7 citations
Publisher

Abstract

The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10/sup 12/) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO/sub 3/) inside a resonant build-up cavity. We use this light to trap 2/spl times/10/sup 7/ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of <3/spl times/10/sup -15/.

DOI
10.1109/freq.1997.638547

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The 657 nm calcium intercombination line is an attractive optical frequency/length reference due to its high line Q (10/sup 12/) and insensitivity to external perturbations. In this paper we describe progress towards an all-diode-laser-based reference using laser-trapped Ca. To generate 35 mW of 423 nm trap light, we double the frequency of a 150 mW, 846 nm diode laser with a nonlinear crystal (KNbO/sub 3/) inside a resonant build-up cavity. We use this light to trap 2/spl times/10/sup 7/ atoms in a compact, magneto-optic trap. Spectroscopy of the narrow 657 nm transition is performed with a frequency-stabilized diode laser system. With the trapped atoms we have measured optical Ramsey fringes with a linewidth of 6 kHz. We discuss future improvements as well as potential performance which could achieve a fractional frequency instability of <3/spl times/10/sup -15/.
Calcium atoms could be trapped and locked directly in a small cell (about 20 cm long). A high power injection lock diode laser being frequency doubled to 422.5 nm by through a nonlinear crystal KNbO/sub 3/, is used to trap calcium atoms in the "heart" of the cell. The trapped atoms will be probed by a narrow linewidth extended-cavity diode laser. The diode laser then will be locked to the calcium /sup 3/P/sub 1/-/sup 1/S/sub 0/ intercombination line at 657 nm. The direct cell trapping and locking method makes it become possible to establish a compact high precision optical frequency standard.
We will describe a next-generation active atomic frequency reference based on super-radiant pulses of laser light from the ultra-narrow, 1 mHz linewidth, optical clock transition in an ensemble of cold 87Sr atoms. Light is stimulated from the millihertz linewidth transition by confining an ensemble of laser cooled atoms inside of a high finesse optical cavity. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. We achieve a remarkable short term fractional frequency stability, 6.7 × 1016 at 1 s of averaging, absolute accuracy, 2 Hz (4 × 1015 fractional frequency), and high insensitivity to changes in the cavity length that limits the performance of todays more stable lasers. We will also discuss current work on cavity-enhanced dispersive measurements to perform high resolution spectroscopy and atom counting.
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${}^{\ensuremath{-}1}$.
We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey-Bordé interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01↔P13 intercombination line in neutral Ca40 is used as a frequency discriminator, with an extended cavity diode laser at 423 nm probing the ground state population after a Ramsey-Bordé sequence of 657 nm light-field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×10−14 at 1 s and 2.0×10−14 at 64 s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×10−14/√τ, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10−14 range frequency instability.
We present a scheme for stabilizing multiple lasers at wavelengths between 795 and 866 nm to the same atomic reference line. A reference laser at 852 nm is stabilized to the Cs D 2 line using a Doppler-free frequency modulation technique. Through transfer cavities, four lasers are stabilized to the relevant atomic transitions in 40 Ca + . The rms linewidth of a transfer-locked laser is measured to be 123 kHz over 200 ms with respect to an independent atomic reference, the Rb D 1 line. This stability is confirmed by the comparison of an excitation spectrum of a single 40 Ca + ion to an eight-level Bloch equation model. The long-term stability with respect to the same reference is measured to be 130 kHz over a period of 2 h. The high degree of stability is demonstrated by the measured Allan deviation around 10 −11 between 1 and 100 s.