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 $4s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-4\mathrm{snp}{}_{}{}^1{}_{}{}^{}P_1^0{}_{}{}^{}$ transitions for $n=10-13\mathrm{}$ are measured to an accuracy of ±2%, more than six times better than previous measurements.