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.