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From Chains to Combs: The Measurement of Optical Frequencies

Bernard With the invention of the laser, a new tool became available for the spectroscopic study of atomic and molecular structure, for the measurement of fundamental constants of nature, and for the creation and measurement of new standards of time and frequency and other physical units.   However, new techniques were needed to measure accurately and precisely the laser frequencies, which ranged from 100 GHz to 1000 THz and were well beyond the capabilities of conventional electronics.  For over 30 years, complicated chains of lasers and nonlinear mixers were the only devices capable of measuring optical frequencies.   This all changed at the turn of the current century with the invention of octave-spanning comb which has made optical frequency measurements commonplace and opened up new fields to precision measurement.