CSEM Scientific and Technical Report 2008

Compact Cell Atomic Clocks and Semiconductor Lasers
S. Lecomte, J. Haesler, R. Matthey
Compact cell atomic clocks for low-power high-end applications will have a major impact in future applications requiring a better mid- and long-term
frequency reference than currently available quartz crystals. Such atomic clocks require a semiconductor laser, the properties of which can
massively impact the clock performances. The developments towards a miniature atomic cell clock as well as the new fully operational laser test
bench are presented here.
In the last generation of atomic clocks, diode lasers are
extensively used for atomic pumping and detection purposes.
For example, the current smallest form-factor atomic clocks
require a VCSEL (Vertical-Cavity Surface-Emitting Laser) and
optically-pumped caesium beam clocks make use of DFB
(Distributed FeedBack) laser diodes (two clock types that are
currently under development at CSEM).
Since the laser properties can strongly impact the clock
performance, a dedicated optical bench for in-depth laser
characterization has been developed (see Figure 1). This
bench allows systematic, fast and reliable laser spectral
property measurement of sources emitting in the near-infrared
spectral range (650 – 900 nm). It has already been
successfully used to measure different types of laser diodes
implemented in caesium beam and rubidium cell atomic
clocks. The gained information has allowed selecting the best
devices.
Figure 1: Picture of the laser test bench
A main research direction of the Advanced Optical Systems
section is the development of miniaturized atomic clocks. The
availability of very small atomic clocks with package volume
on the order of some cubic centimeters and ultra-low power
consumption allowing battery operation will open up many
new applications where the superior mid- and long-term
frequency stabilities compared to usual unreferenced
oscillators (like quartz oscillators) will be a dramatic
improvement. Fields like telecommunication and global
positioning receivers are both good future application
examples.
In order to prepare a two year long CSEM-wide effort starting
in 2009 and aiming at the development of a miniaturized
atomic clock (foreseen clock dimensions around 1 cm3 ), a
laboratory clock representative breadboard has been set up.
Adaptable laboratory elements like laser head, atomic cell
support, and photodetector head have been developed and
form a complete and tunable clock physics package. This
setup efficiently allows testing of the different clock
subsystems (like the laser, the atomic cell, the photodetector,
96
etc) and their respective impact on the clock signal-to-noise
ratio. Detailed understanding and measurement of the many
experimental parameters are able to define the future
miniature atomic clock design and specifications.
Characteristic clock signals have already been measured
using the CPT (Coherent Population Trapping) scheme (see
Figure 2).
Figure 2: Typical clock CPT signal obtained with a current-modulated
VCSEL.
Clock signal noise measurements have also been conducted
and the measured clock signal-to-noise ratio predicts the
realistic clock frequency stability. With very compact glass
cells (some millimetre linear dimensions), relative frequency
stability values around 5⋅10 -11 at 1-second integration time
have been calculated.
To conclude, CSEM has set up and validated a laser test
bench suitable for the complete characterization of laser
diodes commonly used in atomic clocks. Moreover and in the
perspective of developing a miniature cell atomic clock in the
frame of a CSEM-wide effort, a versatile clock laboratory
setup is now fully operational. CPT clock signals have been
measured and the future miniature clock functional
parameters are now identified. It is expected that CSEM, by
the combination of its broad and well-suited competencies, will
strongly impact the field of miniature atomic clocks.