An optical frequency standard using strontium atoms

Sr atoms are cooled on the ¹S₀ → ¹P₁ transition at 461 nm, which linewidth is 32 MHz. Atoms from a thermal beam are decelerated by a Zeeman slower before capture in a magneto-optical trap (MOT). The MOT is formed by three retro-reflected laser beams of a total power 17 mW and of a waist radius 1 cm. The loading rate is 4 × 10¹⁰ atoms with up to 1.3 × 10⁹ atoms in the MOT. To generate 461 nm laser light, sum-frequency mixing of a Nd:YAG laser at 1064 nm is used and laser diodes at 813 nm in a KTP crystal. The 20 mm long crystal is placed inside a ring cavity resonant for both pump lasers with finesses of 35 at 813 nm and 75 at 1064 nm. With pump powers of 170 mW at 813 nm and 900 mW at 1064 nm, the respective coupling efficiencies are 70% and 35%. A blue light of 115 mW is obtained.