Methodology for the conception of speckle reduction elements in the case of short pulse illumination
2015
One of the most efficient ways to decrease the speckle contrast in the field of laser illumination is to increase the
spatial diversity of coherent laser sources. For very short laser pulses such as those required for flash laser
imaging, the spatial diversity should take place instantaneously and no time averaging effect can be used. The
spatial diversity is realized by sampling the laser beam into m beamlets with increased optical path length. This
path length has to be greater than or equal to the coherence length of the laser beam. In this case, the beamlets
are no longer able to create interferences which each other. According to the Goodman’s theory of speckle
reduction, the speckle contrast is then reduced by a factor of 1/√ m . Unfortunately, in the case of multimode
lasers, the number of uncorrelated beamlets is not infinite but is limited by a periodicity function resulting from
the laser resonator length itself. The speckle reduction possibility is therefore limited and is directly linked to
each laser source where the coherence length and cavity length are defined.
In this work we present a methodology to determine experimentally the optical path length difference as well as
the number of beamlets for de-speckling a laser source. An experimental realization is presented where both,
coherence length and periodicity function are measured with a Michelson interferometer where only the speckle
contrast of the two beams from each arm is analyzed. For the validation of the method, the chosen laser source is
a single emitter 660 nm laser diode. Two cylindrical steppers made with diamond turned PMMA have been
realized. Both elements yield interesting results with close values and in accordance with the theory of spatial
diversity. The speckle contrast could be reduced from about 10% to a value close to 4%. These values confirm
and validate the methodology presented in this work.
Steppers can also be a promising solution for the reduction of interference fringes which appear when using a
lightpipe in a laser illuminator design.
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