Electro-optical frequency division and stable microwave synthesis

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Science  18 Jul 2014:
Vol. 345, Issue 6194, pp. 309-313
DOI: 10.1126/science.1252909

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A more simple route to metrology standards

The time standards set by atomic clocks are usually the domain of national metrology laboratories. Li et al. modulated the phase of two closely spaced laser lines to generate a comb of stable and equally spaced frequencies. They then used that comb to generate stable microwaves. Being somewhat simpler than existing optical comb generation techniques and also tunable, the approach may offer a new era of metrology and precision timekeeping.

Science, this issue p. 309


Optical frequency division by using frequency combs has revolutionized time keeping and the generation of stable microwave signals. We demonstrate optical frequency division and microwave generation by using a tunable electrical oscillator to create dual combs through phase modulation of two optical signals that have a stable difference frequency. Phase-locked control of the electrical oscillator by means of optical frequency division produces stable microwaves. Our approach transposes the oscillator and frequency reference of a conventional microwave frequency synthesizer. In this way, the oscillator experiences large phase noise reduction relative to the frequency reference. The electro-optical approach additionally relaxes the need for highly linear photodetection of the comb mode spacing. As well as simplicity, the technique is also tunable and scalable to higher division ratios.

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