 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 <unk> 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 . 
