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13 October 2015 Methodology for the conception of speckle reduction elements in the case of short pulse illumination
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Abstract
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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Yves Lutz and Jean-Michel Poyet "Methodology for the conception of speckle reduction elements in the case of short pulse illumination", Proc. SPIE 9649, Electro-Optical Remote Sensing, Photonic Technologies, and Applications IX, 96490L (13 October 2015); https://doi.org/10.1117/12.2193744
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