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PHYSICISTS at the National Institute of Standards and Technology (NIST) have used laser technologies to generate pure and stable microwave signals.
These signals are said to be more stable and pure than those generated by conventional electronic sources. The laser-based apparatus could improve signal stability and resolution in radar, communications and navigation systems, and certain types of atomic clocks.
The low-noise apparatus is an application of optical frequency combs, which are tools based on ultrafast lasers for precisely measuring optical frequencies, or colours, of light.
Frequency combs are used in next-generation atomic clocks to convert optical signals to lower microwave frequencies, which may then be counted electronically.
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 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.
The base microwave signal is 1GHz, which is the repetition rate of the ultrafast laser pulses that generate the frequency comb. The signal can also be a harmonic, or multiple, of that frequency.
The laser illuminates a photodiode that produces a signal at 1GHz or any multiple up to about 15GHz.