Abstract

We propose a novel technique for dividing an optical frequency down to a microwave frequency through a series of nearly identical elements which halve successively smaller optical frequency differences. Each element receives two stable lasers f₁ and f₂ as inputs and contains its own tunable laser f₃. The sum frequency f₁ + f₂ is compared with the second harmonic 2f₃, generating an rf beat frequency which is used to phase lock laser f₃ so that it lies midway between f₁ and f₂. The next stage then uses f₃ and either f₁ or f₂ as inputs and an additional laser f₄ to halve the input frequency difference again and so on down the line. If the first element uses a stable laser f₀ and its second harmonic as the two input frequencies, the difference frequency after n elements will be f₀/2ⁿ. A connection between 500 nm and the 9-GHz cesium standard would require sixteen stages. This scheme has the advantage that many of the stages can use identical components since the laser frequencies rapidly converge toward a limiting value which can be chosen for convenience or so as to synthesize a desired optical frequency. The use of diode lasers would allow for a compact and inexpensive system.

© 1988 Optical Society of America