A Laser with a
Linewidth of only 10mHz
By Erika Schow
By Erika Schow
June 29, 2017 -- No one had ever
come so close to the ideal laser before: theoretically, laser light has only
one single color (also frequency or wavelength). In reality, however, there is
always a certain linewidth. With a linewidth of only 10 mHz, the laser that
researchers from the Physikalisch-Technische Bundesanstalt (PTB) have now
developed together with US researchers from JILA, a joint institute of the
National Institute of Standards and Technology (NIST) and the University of
Colorado, Boulder, has established a new world record. This precision is useful
for various applications such as optical atomic clocks, precision spectroscopy,
radioastronomy and for testing the theory of relativity. The results have been
published in the current issue of "Physical Review Letters".
Lasers were once deemed a solution without problems – but
that is now history. More than 50 years have passed since the first technical
realization of the laser, and we cannot imagine how we could live without them
today. Laser light is used in numerous applications in industry, medicine and
information technologies. Lasers have brought about a real revolution in many
fields of research and in metrology – or even made some new fields possible in
the first place.
One of a laser's outstanding properties is the excellent
coherence of the emitted light. For researchers, this is a measure for the
light wave's regular frequency and linewidth. Ideally, laser light has only one
fixed wavelength (or frequency). In practice, the spectrum of most types of
lasers can, however, reach from a few kHz to a few MHz in width, which is not
good enough for numerous experiments requiring high precision.
Research has therefore focused on developing ever better
lasers with greater frequency stability and a narrower linewidth. Within the
scope of a nearly 10-year-long joint project with the US colleagues from JILA
in Boulder, Colorado, a laser has now been developed at PTB whose linewidth is
only 10 mHz (0.01 Hz), hereby establishing a new world record. "The
smaller the linewidth of the laser, the more accurate the measurement of the
atom's frequency in an optical clock. This new laser will enable us to
decisively improve the quality of our clocks", PTB physicist Thomas Legero
explains.
In addition to the new laser’s extremely small linewidth,
Legero and his colleagues found out by means of measurements that the emitted
laser light's frequency was more precise than what had ever been achieved
before. Although the light wave oscillates approx. 200 trillion times per
second, it only gets out of sync after 11 seconds. By then, the perfect wave
train emitted has already attained a length of approx. 3.3 million kilometers.
This length corresponds to nearly ten times the distance between the Earth and
the moon.
Since there was no other comparably precise laser in the
world, the scientists working on this collaboration had to set up two such
laser systems straight off. Only by comparing these two lasers was it possible
to prove the outstanding properties of the emitted light.
The core piece of each of the lasers is a 21-cm long
Fabry-Pérot silicon resonator. The resonator consists of two highly reflecting
mirrors which are located opposite each other and are kept at a fixed distance
by means of a double cone. Similar to an organ pipe, the resonator length
determines the frequency of the wave which begins to oscillate, i.e., the light
wave inside the resonator. Special stabilization electronics ensure that the
light frequency of the laser constantly follows the natural frequency of the
resonator. The laser’s frequency stability – and thus its linewidth – then
depends only on the length stability of the Fabry-Pérot resonator.
The scientists at PTB had to isolate the resonator nearly
perfectly from all environmental influences which might change its length.
Among these influences are temperature and pressure variations, but also
external mechanical perturbations due to seismic waves or sound. They have
attained such perfection in doing so that the only influence left was the
thermal motion of the atoms in the resonator. This "thermal noise"
corresponds to the Brownian motion in all materials at a finite temperature,
and it represents a fundamental limit to the length stability of a solid. Its
extent depends on the materials used to build the resonator as well as on the
resonator's temperature.
For this reason, the scientists of this collaboration
manufactured the resonator from single-crystal silicon which was cooled down to
a temperature of -150 °C. The thermal noise of the silicon body is so low that
the length fluctuations observed only originate from the thermal noise of the
dielectric SiO2/Ta2O5 mirror layers. Although
the mirror layers are only a few micrometers thick, they dominate the
resonator's length stability. In total, the resonator length, however, only
fluctuates in the range of 10 attometers. This length corresponds to no more
than a ten-millionth of the diameter of a hydrogen atom. The resulting
frequency variations of the laser therefore amount to less than 4 × 10–17
of the laser frequency.
The new lasers are now being used both at PTB and at JILA
in Boulder
"In the future, it is planned to disseminate this
light also within a European network. This plan would allow even more precise
comparisons between the optical clocks in Braunschweig and the clocks of our
European colleagues in Paris and London Boulder
The scientists from this collaboration see further
optimization possibilities. With novel crystalline mirror layers and lower
temperatures, the disturbing thermal noise can be further reduced. The
linewidth could then even become smaller than 1 mHz.
