Accuracy Comparison of Absolute Optical Frequency Measurement between Harmonic-Generation Synthesis and a Frequency-Division Femtosecond Comb

Using an iodine-stabilized He-Ne laser as a transfer oscillator, we compare absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb. The agreement between these two measurements, both linked to the Cs standard, is $220\pm 770$ Hz, yielding a measurement accuracy of $1.6\times {10}^{-12\mathrm{}}$. We report $473612353604.8\pm 1.2$ kHz as a preliminary updated value of the absolute frequency of the “ $f$” component for the He-Ne laser international standard at 633 nm.