NIST's Femtosecond Laser
A mode-locked, pulsed laser emits a pulse train.
The carrier wave has a phase velocity different from the envelope's
group velocity, resulting in a frequency offset.
The fourier transform of this into frequency space is a
comb of precisely spaced frequencies characterized only by
the offset and repetition frequencies.
The comb teeth typically have 1 GHz spacing.
The D1 transitions will be compared to a tooth in the comb
to arrive at a precise frequency measurement.
Shown below is how the offset frequency is determined.
Figures from Nature, Optical Frequency Metrology,
Hänsch, et al, March 14, 2002, 416, pg. 233.
prev
toc
next