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LETTERS
PUBLISHED ONLINE: 26 JUNE 2011 | DOI: 10.1038/NPHOTON.2011.121
Generation of ultrastable microwaves via optical frequency division
T. M. Fortier*, M. S. Kirchner, F. Quinlan, J. Taylor, J. C. Bergquist, T. Rosenband, N. Lemke,A. Ludlow, Y. Jiang, C. W. Oates and S. A. Diddams*
There has been increased interest in the use and manipulation of optical elds to address the challenging problems that have traditionally been approached with microwave electronics. Some examples that benet from the low transmission loss, agile modulation and large bandwidths accessible with coherent optical systems include signal distribution, arbitrary waveform generation and novel imaging1. We extend these advantages to demonstrate a microwave generator based on a high-quality-factor (Q) optical resonator and a frequency comb functioning as an optical-to-microwave divider. This provides a 10 GHz electrical signal with fractional frequency instability of 8 3 10216 at 1 s, a value comparable to that pro
duced by the best microwave oscillators, but without the need for cryogenic temperatures. Such a low-noise source can benet radar systems2 and improve the bandwidth and resolution of communications and digital sampling systems3, and
can also be valuable for large baseline interferometry4, precision spectroscopy and the realization of atomic time57.
Several photonic systems, including optical delay-line oscillators8, whispering-gallery-mode parametric oscillators9 and dual-mode lasers10 have been investigated for the generation of low-noise microwave signals. An alternative approach, based on a high-Q optical resonator and all-optical frequency division, shows promise for the generation of microwaves with excellent frequency stability6,7,1113. This is because low absorption and scatter
ing in the optical domain can yield quality factors approaching 1 1011 in a room-temperature FabryProt (FP) resonant cavity.
For a well-isolated cavity, average uctuations in the cavity length amount to 100 am on a 1 s timescale. A continuous wave (c.w.)
laser stabilized to such a cavity can achieve a fractional frequency instability as low as 2 10216 for averaging times of 110 s
(refs 1418). Transfer of this stability to a microwave signal is the topic of this paper, and we demonstrate a 10 GHz electronic signal with exceptional frequency stability and spectral purity.
Figure 1 outlines the principle of the photonic oscillator we have developed. Phase-coherent division of the stable optical signal to the microwave domain preserves the fractional frequency instability,...