A Quiet Phase: Optical Tools Produce
Ultra-Low-Noise Microwave Signals
ScienceDaily (June 27, 2011) —
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.
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
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.
The base microwave signal is 1 gigahertz (GHz, or 1 billion
cycles per second), 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 1
GHz or any multiple up to about 15 GHz. For example, many common
radar systems use signals near 10 GHz.
NIST's low-noise oscillator might be useful in radar systems for
detecting faint or slow-moving objects. The system might also be
used to make atomic clocks operating at microwave frequencies,
such as the current international standard cesium atom clocks, ,
more stable. Other applications could include high-resolution
analog-to-digital conversion of very fast signals, such as for
communications or navigation, and radio astronomy that couples
signals from space with arrival times at multiple antennas.
The above story is reprinted from
materials provided byNational
Institute of Standards and Technology (NIST).
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