They use an ultra-short laser pulse which "gates" (enables/disables) the detector for a very short window of time. They repeat the stimulus+gated detection with different offsets in order to sample different parts of the signal.

The ultra-short laser pulse can be generated through purely-optical means. You have a laser pulse traveling around a cavity. Due to a combination of non-linear optical effects, each round-trip tends to shorten and concentrate the pulse. The resulting pulses can be much shorter than what's achievable with electronic generators.