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Japanese Journal of Applied Physics

Stabilization of a 729 nm Laser Diode for High-Resolution Spectroscopy of Calcium Ions

Kenji Toyoda, Hiroshi Kawasaki, Tomohiko Ikeda, Masayoshi Watanabe, Shinji Urabe

Published Mar 15, 2003• 8 citations
Publisher

Abstract

An absolutely stabilized laser diode system for high-resolution spectroscopy of the 4 2 S 1/2 –3 2 D 5/2 electric-quadrupole-allowed transition in calcium ions at 729 nm is developed. Both its short-term and long-term stabilities are examined. A system for laser cooling of single calcium ions using an ultraviolet laser diode for excitation of the 4 2 S 1/2 –4 2 P 1/2 transition is also constructed, and by using it we have observed quantum jumps of single calcium ions with a high stability and a high signal-to-background ratio.

DOI
10.1143/jjap.42.1268

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An absolutely stabilized laser diode system for high-resolution spectroscopy of the 4 2 S 1/2 –3 2 D 5/2 electric-quadrupole-allowed transition in calcium ions at 729 nm is developed. Both its short-term and long-term stabilities are examined. A system for laser cooling of single calcium ions using an ultraviolet laser diode for excitation of the 4 2 S 1/2 –4 2 P 1/2 transition is also constructed, and by using it we have observed quantum jumps of single calcium ions with a high stability and a high signal-to-background ratio.
Calcium ions are laser cooled in a Paul trap by use of a grating-stabilized UV laser diode. Unusual quantum jumps or fluctuations in the fluorescence signal are observed without application of any shelving lasers. It has become clear that such phenomena can be ascribed to shelving transitions induced by background 393-nm spontaneous photons emitted from the laser medium. Shelving transitions are suppressed by use of an interference filter, and the ions are successfully cooled to crystallization temperature with sufficient reproducibility.
The 4 2S(1/2)-3 2D(5/2) electric quadrupole transition in calcium ions, which is expected to be used in calcium-ion optical frequency standards, is spectroscopically investigated with a laser system that uses only fundamental waves of diode lasers as light sources. Four Zeeman components allowed by the selection rules for the electric quadrupole transition are identified. As for one Zeeman component, one large carrier and accompanying smaller first sidebands are observed, which implies that Lamb-Dicke confinement of the ion is achieved. The compact and reliable system for spectroscopy of single calcium ions described is advantageous for realization of practical optical frequency standards.
The National Institute of Information and Communications Technology in Japan is proceeding with development of an optical frequency standard using an electric quadrupole transition of Ca+ ions in a small radio-frequency trap. The single, trapped 40Ca+ ions are laser cooled to a temperature of <10 mK using extended-cavity diode lasers at 397 nm and 866 nm. In order to measure the electric quadrupole transition, the linewidth of the clock laser diode at 729 nm is reduced to ~66 Hz using an ultra-low-expansion (ULE) cavity. We observed the electric quadrupole transition with a linewidth of ~600 kHz, together with the ionic motional sidebands. We are also developing optical frequency combs without photonic fibers broadening the spectrum. The experimental is now improved in order to observe the narrower linewidth of the quadrupole transition.
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.
We have fabricated an rf ion trap and laser-cooling light sources to observe shelving transition in a single calcium ion. All the radiation required for laser cooling and observation of the transition can be prepared with diode lasers in this system. Continuous-wave radiation at 397 nm for laser cooling was generated by frequency doubling of a GaAlAs laser in an external enhancement cavity. As a preliminary experiment for observing the shelving transition, laser cooling of one hundred calcium ions was demonstrated using this radiation and another diode laser.