Direct measurement of the numerical aperture of the plastic optical fibers

To use plastic optical fibers it is useful to know their numerical apertures. These fibres have a large core diameter, which is very different from those of glass fibers. For their connection a strict adjustment to the properties of the optical systems is needed, injecting the light inside and collecting it outside so as not to increase the losses resulting of their strong absorption. If it is sufficient to inject the light at the input with an aperture lower than the theoretical aperture without core stopping, it is very useful to know the out numerical aperture which is varying with the injection aperture and the length of the fiber, because the different modes may be not coupled and are not similarly absorbed. The authors propose a new method of total measurement of the whole flux transmitted by the fiber in terms of its output numerical aperture and so the selected percentage may be directly obtained. Experimental results showing that the useful core diameter of the fiber is not a restraining parameter and confirming the previous results concerning the effects of the injecting aperture and the length of the fiber are presented.