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By combining advanced laser technologies in a new way, physicists at the National Institute of Standards and Technology (NIST) have generated microwave signals that are more pure and stable than those from conventional electronic sources. The apparatus could improve signal stability and resolution in radar, communications and navigation systems, and certain types of atomic clocks.
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Matt Kirchner, a University of Colorado graduate student, fine-tunes an ultra-stable microwave generator that he helps operate at NIST.Credit: Burrus/NIST |
Described in Nature Photonics,* NIST's low-noise apparatus is a new application of optical frequency combs, tools based on ultrafast lasers for precisely measuring optical frequencies, or colors, of light. Frequency combs are best known as the "gears" for experimental next-generation atomic clocks, where they convert optical signals to lower microwave frequencies, which can be counted electronically.
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 the extremely stable length of an optical cavity with a high "quality factor," assuring a steady and persistent signal. 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 microwaves. The transfer process greatly reduces—to one-thousandth of the previous level—random fluctuations in the peaks and valleys, or phase, of the electromagnetic waves over time scales of a second or less. This results in a stronger, purer signal at the exact desired frequency.
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