Development of optical frequency standards based upon the S-2(1/2)-D-2(5/2) transition in Sr-88(+) and S-87(+)

Recent work is presented on the development of an optical frequency standar d, based on the S-2(1/2)-D-2(5/2) "clock" transition at 674 nm in two isoto pes of strontium, A approximate to 200 Hz linewidth 674 nm laser has been f requency-stabilised to the centre of the S-2(1/2)-D-2(5/2) transition in tw o Sr-88(+) ions, separately confined in two RF traps, The frequency locking algorithm for control of the laser frequency to the transition using the t wo ions is described. The mean difference of the frequency observed in the two ions is 34(33) Hz, where the number in parenthesis is the standard unce rtainty. The Allan deviation of the difference frequency stability was obse rved to range from 10 Hz to 50 Hz at 30 s. In Sr-87(+) there are "clock" tr ansitions independent of the first order Zeeman shift; laser cooled Sr-87() ions have been observed in a new trap with an oven containing isotopicall y enriched Sr-87.