Progress toward an optical frequency chain at NPL

^An optical frequency chain is being constructed at the National Physical Laboratory (NPL), Teddington, U.K., with the object of making absolute frequency measurements of optical frequency standards in single, cold, trapped Sr⁺ and Yb⁺ ions. The initial frequency reference is a methane-stabilized He-Ne laser system, but our ultimate goal is an all-optical chain to the caesium primary microwave frequency standard. We have constructed a CW singly resonant PPLN optical parametric oscillator, pumped by a high-power single-frequency Nd:YLF ring laser, yielding 0.45 W of light at the methane frequency. By cascaded frequency doubling of this output, a near-IR reference can be established at four times the methane frequency. We are developing optical frequency divider stages, and an optical frequency comb generator, as tools for linkages to the ion trap frequencies