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A multimode optical fiber device for the transmission of Nd:YAG laser of 7mJ per pulse is designed. The
device is composed with an optical taper, with the front section of 5mm, and a 50m multimode fiber, with a core
diameter of 0.6mm and numerical aperture of 0.22. Applying the beam quality analyzer, energy meter, CCD
camera, rapid responding detector and other measure instruments, the transmission characteristics of the fiber
device under different conditions are experimentally investigated. In the experiment, the propagation efficiency
and it's stability, spot distribution, divergence angle, far-field beam quality, pulse broadening and other
parameters of the fiber device under different conditions are studied. It has been found that the fiber's
propagation efficiency is unequal with different incidence angle, and for normal zero angle of incidence, the
efficiency is bigger than that with oblique incidence. The divergent angle of the beam out of the fiber is varied,
and the angle is larger, when the fiber is being swung. It is definite that the output beam's divergence angle be
always less than that decided by the fiber's numerical aperture. The results of the experiment show that the beam
mode out of the multimode fiber has almost no relationship with the mode of the input laser, and mainly decided
by the fiber's modes. When the fiber of the device is being swung, the mode and intensity distribution of the
output beam of the fiber are in random variation, and the peak-to-peak value of the light spot' intensity
distribution is more than 20%. Otherwise, the fiber's propagation efficiency is invariable with the fiber in
different motion. The results of the pulse broadening caused by the fiber-optic transmission indicate that the
pulse broadening is less than 1ns for the 52m multimode fiber device, and may be ignored for a laser detecting
system with a response time of ns scale. Other output characteristics of the fiber are also studied, such as the far
beam quality in different moving conditions. The beam images out of the fiber device are illustrated in the paper.
The study may be a reference for engineering practice.
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