Absolute frequency measurement of a diode laser locked on the 5S1/2-5D5/2 two-photon transitions of rubidium (lambda equals 778.1 nm, nu equals 385.3 THz)

A frequency chain, derived from the one used to measure the absolute frequency ((nu) $= 473 THz) of the He-Ne/I 2 optical standard, is currently being implemented in order to measure the frequency of a diode laser stabilized on the two-photon transition of rubidium vapor. The measurement scheme is based on the comparison of the frequency of this near-IR potential secondary standard to the 13th harmonic frequency of the R(12)-CO 2 /OsO 4 LPTF secondary standard at (nu) equals 29.096 THz. Recent results on the frequency synthesis are reported, enabling the testing of long-term stability of this Rb-locked system with respect to the IR reference standard.