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 4s²¹S₀-4snp¹P₁⁰ transitions for n=10-13 are measured to an accuracy of ±{}2%, more than six times better than previous measurements.