Boulder, CO--National Institute of Standards and Technology (NIST) researchers have created an ultraprecise microwave generator based on a yellow laser beam frequency-stabilized by a high-quality-factor (high Q) optical resonator. A frequency comb is produced that allows microwaves to be produced with a fractional frequency instability of less than 8 × 10â6 at 1 s at room temperature (conventional ultraprecise microwave oscillators require cryogenic temperatures). The apparatus could improve signal stability and resolution in radar, communications and navigation systems, long-baseline interferometry, and certain types of atomic clocks.
"This is the quietest, most-stable microwave generator that's ever been made at room temperature," said project leader Scott Diddams.
Phase fluctuations reduced by a factor of a thousand
The new low-noise system is so good that NIST scientists actually had to make two copies of the apparatus just to have a separate tool precise enough to measure the system's performance. Each system is based on a continuous-wave laser with its frequency locked to an optical resonator. This laser, which emitted yellow light in the demonstration but could be another color, is connected to a frequency comb that transfers the high level of stability to the microwave region. The transfer process greatly reduces--to one-thousandth of the previous level--random fluctuations in the phase of the electromagnetic waves over time scales of a second or less.
The base microwave signal is 1 GHz, which is the repetition rate of the ultrafast laser pulses that generate the frequency comb (the signal could also be a harmonic of that frequency). The laser illuminates a photodiode that produces a signal at 1 GHz or any multiple up to about 15 GHz. For example, many common radar systems use signals near 10 GHz.
REFERENCE:
1. T. M. Fortier et al., Nature Photonics (2011), doi:10.1038/nphoton.2011.121; Published online 26 June 2011.
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