Absolute Optical Frequency Measurement of the Cesium D₁ Line with a Mode-Locked Laser

We have measured the absolute optical frequency of the cesium D₁ line at 335 THz (895 nm). This frequency provides an important link for a new determination of the fine structure constant α. The D₁ line has been compared with the fourth harmonic of a methane stabilized He-Ne laser at 88.4 THz ( 3.39 μm). To measure the frequency mismatch of 18.39 THz between 4×88.4 THz  =  354 THz and the D₁ line a frequency comb spanning around 244 000 modes of a Kerr-lens mode-locked laser was used. We find 1 167 688 (81) kHz for the hyperfine splitting of the 6P_1/2 state and 335 116 048 807 (41) kHz for the hyperfine centroid from which we derive α^-1  =  137.035 992 4(41).