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Using an optical femtosecond comb generator the absolute frequency of the electric quadrupole clock transition of the ytterbium ion can be measured. The setup is shown schematically below.
Figure: Schematic setup. PD : photodetector, PLL: phase-locked loop.
The ytterbium ion's clock transition at 436 nm is interrogated by a frequency-doubled diode laser. The fundamental frequency light at 871 nm is guided to the comb generator through an optical fiber. The clock transition frequency is given by
fYb = 2 · (fceo + m · frep + fYb-beat).
Whereas the beat note frequency of the Yb laser with a comb mode fYb-beat is simply detected with a photodiode, a precise measurement of the repetition rate frep and of the offset frequency fceo requires more effort. Instead of the repetition rate's fundamental frequency of about 100 MHz we measure its thousandth harmonic at 10 GHz in order to make additive noise and digitization error negligible. This signal is downconverted with a local oscillator to about 500 kHz and the 288th harmonic of that signal is tracked. In order to measure the offset frequency fceo , about 40000 comb modes at 1100 nm are frequency doubled and their beat note with fundamental modes at 550 nm is detected.
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