Laser system for Doppler cooling of ytterbium ion in an optical frequency standard

A laser system for Doppler cooling of ytterbium ion on the ²S_1/2 → ²P_1/2 transition in a single-ion optical frequency standard is developed. The second harmonic of a semiconductor laser with a wavelength of 739 nm is used for cooling. The laser frequency is doubled in a nonlinear BiBO crystal embedded in a ring resonator, which also serves as a reference for laser frequency stabilisation. Second-harmonic power of ~100 μW is generated at a wavelength of 369.5 nm. Diode laser radiation is modulated by an electro-optic modulator at 14.75 GHz to generate a sideband exciting the ²S_1/2 (F = 0) → ²P_1/2 (F = 1) hyperfine component of the cooling transition that is not excited by resonant cooling light. The sideband relative intensity of a few percent proved to be sufficient to reduce the ion dwelling time in the ²S_1/2 (F = 0) state to less than 10^-4 s and increase the cooling efficiency.