4 Femtosecond comb

The ticks of an optical clock - which occur at a frequency f_opt - can be counted using a "femtosecond comb". This device consists of a series of regularly spaced modes, like the teeth of a comb. The mode spacing f_rep and the offset frequency f₀ are referenced to a microwave standard. The value of f₀ can be determined by taking a low-frequency infrared mode with a value (nf_rep + f₀), passing it through a nonlinear crystal so that its frequency doubles to (2nf_rep + 2f₀) and then mixing it with a green mode with a frequency of (2nf_rep + f₀). The two signals form a beat at a frequency f₀, which can be measured with a photodiode. The frequency f_opt of an optical standard equals pfrep + f₀ + f_beat, where f_beat is the beat frequency between the optical standard and the nearest comb mode p.

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