Frequency stabilization of a 650 nm laser to an I-2 spectrum for trapped Ba-138(+) ions

Optical manipulation of Ba+ ions is performed mainly by a 493 nm laser for the S-1/2 - P-1/2 transition and a 650 nm laser for the P-1/2 - D-3/2 transition. Because the branching ratio between the 493 and 650 nm transitions of a single Ba+ ion is comparable, stabilization systems of both lasers are equally important for Doppler cooling, sub-Doppler cooling, optical pumping, and state detection. The stabilization system of a 493 nm laser to an absolute Te-2 reference has been well established. However, the stabilization of a 650 nm laser to the absolute reference of an atomic or molecular spectrum has not been presented before, to our knowledge. Here, we experimentally obtain the I-2 spectrum near 650 nm and provide the experimental connection to the spectrum of Ba+. We present the 20 spectral lines of I-2 in the range of 0.9 GHz above the resonance of the P-1/2 - D-3/2 transition. We stabilize the 650 nm laser through the optical cavity to the lowest one among these lines, which is about 350 MHz apart, as the absolute frequency reference. Finally, we measure the frequency differences between these iodine lines and the Ba+ resonance through the fluorescence excitation spectrum with well-resolved dark states, which is in agreement with the theoretical expectation. The presented stabilization scheme enables us to perform precise experiments with Ba+ ions. (C) 2019 Optical Society of America