CSEM Scientific and Technical Report 2008

TIME AND FREQUENCY
Alain Maurissen
Since the invention of the pendulum clock (C. Huygens 1656),
time and frequency have been the physical quantities that can
be measured with the greatest precision. It has become good
strategy to translate other physical quantities into time or
frequency references (f. i. the meter is now defined as the
length of the path travelled by light in vacuum during a time
interval of 1/299 792 458 of a second).
Nowadays, measuring time merely translates into counting
cycles of a known frequency (such as the one provided by an
atomic clock, and therefore called “frequency standard”).
Counting cycles offer the major advantages inherent to the
digital domain (in particular resistance to noise disturbances).
For decades, the original frequencies used for this purpose lie
in the microwave domain (1 to 100 GHz) and a great amount
of efforts has been spent in the associated technologies and
upward higher frequencies knowing that they provide better
time resolution and open the way to increased precision.
Given that the physical size of a traditional microwave
component (source, resonator …) is somehow related to the
associated wavelength (speed of light/frequency) of the
associated electromagnetic wave, for the above-mentioned
frequencies, miniaturizing below the centimeter size becomes
more and more difficult.
The invention of the stabilized frequency comb resulting in the
Nobel price award to T. W. Hänsch and John Hall in 2005,
somehow electrified the time and frequency community by
providing a replacement for the old complex and cumbersome
frequency chains used in some metrology laboratories hence
allowing convenient optical to microwave frequency division of
optical frequency standards.
The state of the art technology is not yet at the technological
level required for impressive integration but it is already clear
that the way is traced and that such “all optical clocks” will
emerge in a not so far distant future.
The research activity in the new division takes full advantage
of ongoing research and development in optical components.
This means that the available “cycles counters” will soon
operate in the hundreds of terahertz range rather than the
tens of gigahertz range. The direct advantages in terms of
precision and resolution are evident and the jump from
gigahertz to terahertz clearly outdates all efforts performed in
the microwave domain where little is still to be gained.
This technological jump will allow the approach to new fields
of application, where bulky technologies of today are
inadequate (portable devices, GSMs, watches …).
Research has fully taken benefit from the available technology
and research platforms of the peripheral divisions of CSEM
(Photonics, Microelectronics, micro and nanotechnology,
packaging…) all available under the “one roof” concept.
In 2008, the major efforts of the division were mainly devoted
to:
• Strengthening its competencies in the stabilized
femtosecond lasers paving the way for a new optical
frequency comb research activity.
• Perform advanced research in the field of low-power and
ultra compact cell atomic clocks for the inter-division
development of a demonstrator to be build in 2009-2010.
• Enhancing time of flight and lidar technology by
complementing CSEM current PRN technology with the
3D camera developed by the Photonic division and for
new lidar-camera for space exploration missions.
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