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MEASURING GEOMETRY OF OPTICAL FIBRE COATINGS WITH TRANSVERSE INCIDENT BEAMS - US5335057

An apparatus for measuring the geometry of an optical fibre coating using dark-field illumination. The invention consists of a prism having a wedged shape at one end such that incident illumination is split into two separate beams which each reflect inside the prism and then cross each other inside a part of the prism which contains an oil-filled cell. It is a this position that the fibre to be measured is located. In the absence of a fibre the angles of the illuminating beams are such that they are greater than the collection numerical aperture of a nearby objective lens. When a fibre is present in the cell the incident light beams are refracted at the boundaries between the glass fibre and the coating layers such that their angles are deviated and they become within the collection aperture of the lens. The lens then directs the light towards, for example, a CCD camera. The image so formed consists of a dark background with bright parallel lines corresponding to the boundaries of the coating layers. By measuring the positions of these boundaries for several rotational positions of the fibre the geometrical parameters, such as diameter and concentricity error, may be determined.

In a typical RNF profiler light is focussed by a high power microscope objective onto the end of the fibre under test, which is mounted inside an oil-filled cell. The central portion of the focussed cone of light is captured by the fibre and becomes guided modes of the fibre. The outer part of the cone is refracted out through the side of the fibre at an angle that is determined by the refractive index of the fibre at the position of the focussed laser spot. To measure this angle an opaque disc is positioned on the optical axis such that only light travelling at angles greater than that subtended by the disc is detected. The angle that the opaque disc subtends at the fibre is chosen so that it is sufficiently large to block so-called 'leaky rays'. These are rays that travel in the fibre at angles that exceed the theoretical numerical aperture of the fibre but are only weakly radiative and their presence, or rather, lack of presence, in the refracted cone of rays can lead to severe distortion of the measured index profile. To suppress leaky modes and also to achieve high spatial resolution it is advantageous to use a very high numerical aperture objective. The half-angle of a typical objective is around 60 degrees and as such poses a challenge to the collection optics.

The present invention provides a means of collecting and focussing the refracted cone of light onto a photo-detector using a hemi-spherical oil-filled cell 7 and a toroidal lens 10. The lens-like property of the hemi-spherical cell serves to bend light towards the optical axis and direct it towards the lower face of the toroidal lens. The toroidal lens further bends the light towards the optical axis by the process of total internal reflection and then the inward-curving upper surface of the toroidal lens focuses it onto the photo-detector 12.