Effects of Aperture Size in a Laser Doppler Velocity Meter

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  • レーザードップラー速度計における開口絞りの効果

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The paper reports the results of experimental study on the effect of an aperture stop on frequency shift and variation of frequency width of observed beat signals using a laser Doppler velocity meter, which measures linear velocity of diffusing surface of a rotating ground glass plate detecting beat frequency between diffused laser beam and reference laser beam.<br>Three kinds of the aperture stops, a circuler aperture, a horizontal slit and a vertical slit, are introduced in an optical path of the mixed laser beams of deffused and reference to restrict the effective diverging angle of the deffused beam. The experiment shows that decrease of the frequency of 20% as much as and increase of the frequency width of three times as much as the original ones of the beat signals due to increase of the size of the circuler aperture and the length of the horizontal slit from 1mm to 20mm, however increase of the length of the vertical slit has no effect on the beat signals.<br>Refering to the experiment, the dependence of the shape of the beat signal on the intensity distribution and intensity ratio of the deffused beam and the reference beam, the relation between the spot size of the laser beam, surface roughness of the ground glass plate, speckle pattern in the deffused beam, coincidence of the wave fronts of diffused and reference beams and observed interference fringes are discussed.

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