Achieving strong doubling power by optical phase-locked Ti: sapphire laser and MOPA system

We show two external cavity-enhanced second-harmonic generations of 922 nm with periodically poled potassium titanyl phosphate crystal, whose doubling cavities are locked separately with Hansch-Couillaud and intra-modulation methods. The outputs of second-harmonic generation reach 310 mW, 54.8% of the conversion efficiency from the Ti: sapphire laser with the crystal length of 10 mm, and 208 mW, 59% of the conversion efficiency from the MOPA system with the crystal length of 30 mm. It consists of heterodyning the Ti: sapphire laser and the MOPA system, and compares the phase of the beat frequency signal with the phase of a reference RF local oscillator. The resulting phase error is used as a feedback signal and fed back to the reference cavity of the Ti: sapphire laser to lock the two lasers in phase. A stable blue power of 520 mW is obtained, which supplies enough power for the cooling and trapping step of the strontium (Sr) optical lattice clock. Four stable isotopes of Sr, ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr, are detected by probing the laser during a strong 460.7-nm cycling transition (5 _s ²¹S ₀- 5 _s5 _p ¹P ₁).