Test procedures 1 of Measurement methods and test procedures Numerical aperture Scope This part of document establishes uniform requirements for measuring the numerical aperture of optical fibre thereby assisting in the inspection of fibres and cables for commercial purposes The numerical aperture( NA) of category Al graded-index multimode fibre is an important parameter tha describes a fibre's light-gathering ability. It is used to predict launching efficiency, joint loss at splices and micro/macrobending performance Overview of method This test procedure describes a method for measuring the angular radiant intensity( far-field) distribution from an optical fibre. The numerical aperture of a category A1 graded-index multimode optical fibre can calculated from the results of this measurement using equation(11) for NA in the far field, NAff, as described in 8.1 The maximum theoretical NA of a graded index multimode fibre is defined as follows: NAth= sin em (1) where NAth is the maximum theoretical numerical aperture em is the largest incident meridional ray angle that will be guided by the fibre In terms of the fibre index profile here n, is the maximum refractive index of the core, and n2 is the refractive index of the cladding NA 2△ n for△<<1 here n NA can be determined from a far-field radiation pattern measurement on a short length of fibre or from a measurement of a fibre s refractive index profile. Using the far-field method, the intensity pattern, 1(0), of a fibre is acquired, and the NAff (numerical aperture in the far field)is defined as the sine of the half- angle where the intensity has decreased to 5 % of its maximum value 3 Apparatus 3.1 Input system 3.1.1 Light source Use an incoherent light source capable of producing an area of substantially constant radiance (variations of less than 10 in intensity) on the endface of the specimen. It shall be stable in intensity and position over a time interval sufficient to perform the measurement 3.1.2 Input optics Use a system of optical components to create a monochromatic(<100 nm full width half maximum substantially constant radiance spot larger in diameter than the endface of the specimen and with a numerical aperture greater than that of the specimen Unless otherwise specified, the centre wavelength shall be 850 nm t 25 nm. Provide a means of verifying the alignment of the endface. Optical filters may be used to limit the spectral width of the source 3.1.3 Fibre input end support and alignment Provide a means of supporting the input end of the specimen to allow stable and repeatable positioning vithout introducing significant fibre deformation. Provide suitable means to align the input endface wit respect to the launch radiation. 3.1.4 Cladding mode stripper Provide means to remove cladding light from the specimen. Often the fibre coating is sufficient to perform this function. Otherwise, it will be necessary to use cladding mode strippers near both ends of the test specimenTest procedures Page: 1 of 5 Subject: Originated by: Wu Jia Measurement methods and test procedures – Numerical aperture 1 Scope This part of document establishes uniform requirements for measuring the numerical aperture of optical fibre, thereby assisting in the inspection of fibres and cables for commercial purposes. The numerical aperture (NA) of category A1 graded-index multimode fibre is an important parameter that describes a fibre's light-gathering ability. It is used to predict launching efficiency, joint loss at splices, and micro/macrobending performance. 2 Overview of method This test procedure describes a method for measuring the angular radiant intensity (far-field) distribution from an optical fibre. The numerical aperture of a category A1 graded-index multimode optical fibre can be calculated from the results of this measurement using equation (11) for NA in the far field, NAff, as described in 8.1. The maximum theoretical NA of a graded index multimode fibre is defined as follows: NAth = sin θm (1) where NAth is the maximum theoretical numerical aperture; θm is the largest incident meridional ray angle that will be guided by the fibre. In terms of the fibre index profile: (2) where n1 is the maximum refractive index of the core, and n2 is the refractive index of the cladding or (3) where (4) NA can be determined from a far-field radiation pattern measurement on a short length of fibre or from a measurement of a fibre's refractive index profile. Using the far-field method, the intensity pattern, I(θ), of a fibre is acquired, and the NAff (numerical aperture in the far field) is defined as the sine of the half￾angle where the intensity has decreased to 5 % of its maximum value. 3 Apparatus 3.1 Input system 3.1.1 Light source Use an incoherent light source capable of producing an area of substantially constant radiance (variations of less than 10 % in intensity) on the endface of the specimen. It shall be stable in intensity and position over a time interval sufficient to perform the measurement. 3.1.2 Input optics Use a system of optical components to create a monochromatic (<100 nm full width half maximum), substantially constant radiance spot larger in diameter than the endface of the specimen and with a numerical aperture greater than that of the specimen. Unless otherwise specified, the centre wavelength shall be 850 nm ± 25 nm. Provide a means of verifying the alignment of the endface. Optical filters may be used to limit the spectral width of the source. 3.1.3 Fibre input end support and alignment Provide a means of supporting the input end of the specimen to allow stable and repeatable positioning without introducing significant fibre deformation. Provide suitable means to align the input endface with respect to the launch radiation. 3.1.4 Cladding mode stripper Provide means to remove cladding light from the specimen. Often the fibre coating is sufficient to perform this function. Otherwise, it will be necessary to use cladding mode strippers near both ends of the test specimen