Abstract
In contrast to the mechanical scanning procedure described
in the standard ISO/DIS 11146, the use of electronically tunable focal length
lenses has proved its capability for the measurement of the laser beam propagation
factor (${\rm M} ^{2}$) without moving components. Here, we demonstrate a novel experimental
implementation where we use a low-cost programmable liquid crystal spatial
light modulator (SLM) for sequentially codifying a set of lenses with different
focal lengths. The use of this kind of modulators introduces some benefits
such as the possibility for high numerical aperture or local beam control
of the phase of the lenses which allows for minimizing systematic errors originated
by lens aberrations. The beamwidth, according to the second-order moment of
the irradiance, is determined for each focal length by using a digital sensor
at a fixed position with respect to the spatial light modulator. After fitting
the measured data to the theoretical focusing behavior of a real laser beam,
the beam propagation factor is obtained. We successfully validated the results
in the laboratory where a full digital control of the measurement procedure
without mechanical scanning was demonstrated.
© 2012 IEEE
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