Optical frequency stabilization and optical phase locked loops: Golden threads of precision measurement

Stabilization of lasers through locking to optical cavities, and the transitions of atoms and molecules has revolutionized the field of precision optical measurement since shortly after the invention of the laser. Phase locking of one laser to another, formed the basis for linking stable lasers across the optical spectrum, the resulting frequency chains exhibiting progressively finer precision through the years. Phase locking between the modes within a femtosecond pulsed laser has yielded the optical frequency comb, one of the most elegant and useful instruments of our time. This tutorial paper gives an overview of these topics, from early work through to the latest 1E-17 fractional uncertainty attained for an optical clock, and the ongoing quest for ever finer precision and accuracy.