Measuring the frequency of light with a mode-locked laser

Ever since it became possible to directly measure optical frequencies in the early 1970s, large and delicate harmonic frequency chains where necessary for this task. Because of this tremendous effort, only a few of these chains have been realized worldwide. A harmonic frequency chain, that is designed to measure only one particular optical frequency, starts from a well known reference frequency either in the radio frequency or in the optical region. With a chain of frequency multiplication steps (and possibly sum - and difference frequency generation) the unknown target frequency is phase coherently linked to the reference frequency. This approach makes it necessary to traverse the whole electromagnetic spectrum between the reference and the target frequency with different oscillators, detectors and nonlinear devices. The use of a femtosecond mode locked laser as an active frequency comb generator immediately enabled the measurement of large optical frequency differences by referencing the mode spacing and counting a large number of modes.