IEC 60793-1-43-2015 pdf download.Optical fibres – Part 1–43: Measurement methods and test procedures– Numerical aperture measurement.
5 Apparatus
5.1 Input system
5.1 .1 Light source Use an incoherent light source capable of producing an area of substantially constant radiance (variations of less than 1 0 % 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. Class A fibres’ core geometry shall be determined by employing an illuminator at the operating wavelength of the fibre that satisfies the following spatial and angular requirements. The power per unit area in the focal plane of the fibre under test shall not vary more than ±1 0 % across the core area. The power per unit solid angle shall not vary more than ±1 0 % across the core’s acceptance cone.
5.1 .2 Input optics Use a system of optical components to create a substantially constant radiance spot larger in diameter than the endface of the specimen and with a numerical aperture greater than that of the specimen. The light source shall be incoherent but with a spectral width < 1 00 nm, full- width half-maximum. The NA ff is impacted by the measurement wavelength. For this reason, the centre wavelength is given as part of the detailed product specifications including IEC 60793-2-1 0, IEC 60793-2-20, IEC 60793-2-30 and IEC 60793-2-40. Default values for the centre wavelength are also listed in Annex B. Provide a means of verifying the alignment of the endface. Optical filters may be used to limit the spectral width of the source.
5.1 .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.
5.1 .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. Note that some detailed product specifications require longer specimen lengths to help remove cladding modes as well.
5.2 Output system and detection
5.2.1 General Four equivalent techniques may be used to detect the angular radiant intensity (far-field) distribution from the specimen. Techniques 1 and 2 are angular scans of the farfield pattern. Technique 3 is a scan of the spatial transform of the angular intensity pattern. (A small or large area scanning detector may be used.) Technique 4 uses an inverse far-field measurement.
5.2.2 Technique 1 – Angular scan (see Figure 2)
5.2.2.1 Fibre output end support and alignment
Use a means of supporting and aligning the output end of the specimen that allows alignment of the endface coincident with the axis of rotation of the optical detector and coincident with the plane of rotation of the optical detector.
For example, a vacuum chuck mounted on X-Y-Z micropositioners, with a microscope fixture for aligning the fibre end would be suitable. Examples include a goniometer or stepper-motor driven rotational stage.
5.2.2.2 Detection system mechanics Use a suitable means for rotation of the optical detector that allows the detector to scan an arc sufficient to cover essentially the full radiation cone from the specimen (for example, a calibrated goniometer). The axis of rotation of the mechanism shall intercept the endface of the specimen and shall be perpendicular to the specimen axis, and the rotation plane of this mechanism shall be coincident with specimen axis. Provide means for recording the relative angular position of the detector with respect to the specimen output axis.IEC 60793-1-43 pdf download.