Generation of quasi-continuous wave 389-nm coherent light by frequency doubling of a Ti:sapphire laser for nuclear spin polarization of3He atoms

Abstract

Magnetic Resonance Imaging based on the hyperpolarized helium-3 (³He) gas has been attracted as a non-destructive testing technique for the porous media and the medical imaging. In order to produce nuclear spin polarization of ³He, optical pumping is the efficient way using a resonant line. However, there is no resonant light source to the line from the ground state of ³He. Then, we have been focusing on the nuclear spin polarization in a discharge cell using the metastability exchange optical pumping (MEOP) technique. We aim at the optical transition 2³S1→2³P0 at λ=389nm that has never been investigated for the polarization. Therefore, at first, we developed a single-frequency 389-nm coherent light source based on the second harmonic generation of a single-frequency 778-nm continuous-wave Ti:sapphire laser light with a BiB₃O₆ (BiBO) nonlinear crystal in an external cavity for the enhancement. As a result, we obtained the 389- nm output radiation with the high conversion efficiency of 56%. Additionally, we also demonstrated the frequency doubling of a quasi-continuous wave Ti:sapphire laser for the optical pumping of multiple optical transitions.