Experimental study on transmission characteristics of multimode optical fiber under moving conditions

Abstract

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.