Converting optical frequencies with 10^(-21) uncertainty
Frequency synthesizers from audio frequency to the microwave region have been widely used in daily life, high technology and scientific research. Those frequency synthesizers can output a signal with frequency related to the input light frequency (fin) as fin/R. Meanwhile, the phase coherence, frequency stability and accuracy of the output signal inherit from the input signal. While in the optical region, there was no such a device. Since the invention of lasers, scientists are able to realize optical frequency conversion with nonlinear optical process. For example, second harmonic generation can convert optical frequencies as fout = fin/0.5, where fout is the output light frequency. However, optical frequency conversion with arbitrary ratios has not been realized for a long time.