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We developed a robust, high-power, tunable, cw, single-mode, and deep-UV coherent light source by two-stage highly efficient frequency conversions with external cavities. In the first stage, a cw Ti:sapphire laser tuned to 746 nm is frequency-doubled. As the nonlinear crystal, we selected the 15-mm-long type I phase-matched LiB3O5 (LBO). We obtained the second harmonic power of 589 mW, corresponding to the generation of more than 653 mW in the cavity. This result shows the total conversion efficiency of more than 50% from the input power to the generated power. In the second bow-tie-type external cavity, the obtained 373 nm light and 780 nm light provided by another Ti:sapphire laser are doubly resonantly sum-frequency-mixed in the other external cavity to generate the 252 nm light. As the nonlinear crystal, we employed the 10-mm-long type I phase-matched β-BaB2O4 (BBO).