Frequency Comb

A frequency comb is a light source whose spectrum consists of a series of discrete, equally spaced elements. Frequency combs can be generated by a number of mechanisms, including amplitude modulation of a continuous wave laser or stabilization of the pulse train generated by a mode locked laser. Much work has been devoted to the latter mechanism, which was developed around the turn of the twenty first century and ultimately lead to one half of the Nobel Prize in Physics being shared by John L. Hall and Theodor W. Hänsch in 2005.

The frequency domain representation of a perfect frequency comb is a series of delta functions spaced according to

$f(n) = f_0 + n\,f_r$

where is an integer, is the comb tooth spacing (equal to the mode locked laser's repetition rate or, alternatively, the AM frequency) is the carrier offset frequency, which is less than .

Combs spanning an octave in frequency (i.e., a factor of two) can be used to directly measure (and correct for drifts in) f0. Thus, octave spanning combs can be used to steer a piezoelectric mirror within a carrier envelope phase correcting feedback loop. Any mechanism by which the combs' two degrees of freedom (fr and f0) are stabilized generate a comb that is useful for mapping optical frequencies into the radio frequency for the direct measurement of optical frequency.

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