Why you need an optical clock
The typical oscillation frequency of an optical transition is about 100,000 times higher than a microwave transition. Similar to a length measurement with a fine scale rather than a coarse scale, time measurements counting those fast optical oscillations can be orders of magnitude more precise than using microwave transitions.
Optical frequency combs allow for transferring the fast optical oscillations into a measurable signal without limiting the clock precision during the readout process. With 10-19 frequency uncertainty throughout its entire spectral range Menlo's FC1500-250-ULN is the first clockwork that guarantees high precision standards for today’s most demanding applications including optical clocks.
YouTuber Doktor Whatson discusses in his recent video ( link here) the technology defining our universal time and how a redefinition of the SI second based on optical clocks will eventually influence our lives.