Accurate measurement of relative oscillator strengths by phase-matched nonlinear optics

A new method for accurately measuring relative oscillator strengths is presented. The method is based on measurements of the frequencies at which a nonlinear-optical sum-mixing process is phase matched. By applying this technique to Ca I, the relative strengths of the $4{s}^{2}^{1}S_{0}\ensuremath{-}4\mathrm{snp}^{1}P_{1}^{0}$ transitions for $n=10\ensuremath{-}13$ are measured to an accuracy of \ifmmode\pm\else\textpm\fi{}2%, more than six times better than previous measurements.