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.^[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 $f_0$.