Article published PULSED LASERS
A truly reliable solution for measuring temporal shape
To control the temporal shape of laser beams, the CEA has developed a passive
optical component allowing output measurement with 1% precision. With
good tolerance for pointing errors, it can be used with a wide range of energies.
The technology has been patented and is being transferred to an SME.
240 laser beams, each of which
must be controlled at several
points, with pulses lasting 100
picoseconds to several nanoseconds.
These specifications,
dictated by the constraints of the
Megajoule project, have played an
important role in the development
of the new component, which
is dedicated to measuring the
temporal shape of a pulsed laser.
From the beginning, the researchers
opted for passive solutions;
an active device produced in
large numbers would have totally
exceeded budgetary constraints.
They also aimed for a high degree
of precision, as the instabilities
of 20% or higher observed for the
most widely used systems are not
acceptable.
The result? A compact component
which can be connected with
optical fibre and which costs less
per unit than existing systems. And
it performs better than the often
«homemade» solutions of design
and R&D laboratories using pulsed
lasers.
In most cases, these laboratories use
surface diffusers whose roughness
partially disrupts laser coherence,
excites a maximum number of
propagation modes, and thereby
generates a «mean» optical output
signal (i.e. less noise), but one that
is still far from perfect.
The CEA's solution is volume
diffusion: the laser beam passes
through a material inserted into
a metal light guide. Each point of
this material excites several modes
of propagation, which effectively
multiplies the action of the surface
diffuser over a depth a few hundred
times the beam's wavelength. This
limited thickness enables preserving
the temporal shape.
The resulting temporal curve can
achieve precision on the order of 1%.
The rate of beam coupling varies
by less than 10%, even if there are
pointing errors, for example from
temperature variations. But the
most surprising result is that the
choice of diffuser material is vast
and the materials themselves quite
ordinary: «We were able to qualify
ceramics, nylons, Teflon, etc. on our
test benches», notes Pascal Leclerc,
from the CEA's Military Applications
Division.
The prototype developed for the
Megajoule Laser enabled correcting
certain aspects, such as hot points
liable to deform the materials (and
thus skew measurement) at the
point where the beam penetrates
the component. This technology
has now been patented, and a
transfer is being planned to an SME
who will make it available to those
pulsed laser users concerned about
precision in the temporal shape. An added advantage of the diffuser is
that it can be used with lasers of all
energies, either for sampling the
beam during firing, or for reception
of the entire beam if its energy is
less than one millijoule.
