 Optical heterodyne detection is an essential technique used in high @-@ accuracy measurements of the frequencies of optical sources , as well as in the stabilization of their frequencies . Until a relatively few years ago , lengthy frequency chains were needed to connect the microwave frequency of a cesium or other atomic time source to optical frequencies . At each step of the chain , a frequency multiplier would be used to produce a harmonic of the frequency of that step , which would be compared by heterodyne detection with the next step ( the output of a microwave source , far infrared laser , infrared laser , or visible laser ) . Each measurement of a single spectral line required several years of effort in the construction of a custom frequency chain . Currently , optical frequency combs have provided a much simpler method of measuring optical frequencies . If a mode @-@ locked laser is modulated to form a train of pulses , its spectrum is seen to consist of the carrier frequency surrounded by a closely spaced comb of optical sideband frequencies with a spacing equal to the pulse repetition frequency ( Fig . 16 ) . The pulse repetition frequency is locked to that of the frequency standard , and the frequencies of the comb elements at the red end of the spectrum are doubled and heterodyned with the frequencies of the comb elements at the blue end of the spectrum , thus allowing the comb to serve as its own reference . In this manner , locking of the frequency comb output to an atomic standard can be performed in a single step . To measure an unknown frequency , the frequency comb output is dispersed into a spectrum . The unknown frequency is overlapped with the appropriate spectral segment of the comb and the frequency of the resultant heterodyne beats is measured . 
