= Octave-spanning comb phase-control experiment
The 2000 experiment prepared <optical pulses> with a <mode-locked laser> and broadened them in an air-silica microstructure <optical fiber>. A wavelength-selective <beam splitter> separated red and blue light. A nonlinear crystal doubled the red branch; the blue branch received a known acousto-optic shift. A filter selected recombined light for an avalanche <photodiode>, supplying an electronic <beat>. Mirror translation controlled repetition <frequency>; mirror tilt controlled offset with coupled effects. Separate delayed-pulse <interference> tested programmed <phase> changes. Alternative envelope fits and unmatched optical reflections were examined as checks and limitations.<Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis by David J. Jones and others>[Science 288, pp.636–638, Figs.2,4–5: apparatus and temporal check; Tokushima mirror]{ref}
These readouts answer different questions. The electronic <beat> closes a feedback loop; changing its setpoint and checking the delayed-pulse <phase> tests whether that control reaches the optical waveform. Neither readout alone establishes the accuracy of an unknown optical <frequency>. That additionally requires a calibrated time reference, the correct tooth number and <beat> sign, and checks for missed cycles and drift. The ideal unshifted diagram above omits the experiment's known acousto-optic shift; its actual raw detector <frequency> must be corrected for that shift before identifying $f_0$.
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