Optics: measuring and testing – For optical fiber or waveguide inspection
Patent
1996-05-03
1997-05-27
McGraw, Vincent P.
Optics: measuring and testing
For optical fiber or waveguide inspection
356128, G01N 2143
Patent
active
056337086
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus for the measurement of optical properties, for example the radial refractive index profile, of a quasi cylindrically symmetric transparent object, such for example as an optical fibre preform. In particular, the invention enables the measurement of certain types of optical fibre preforms which are not amenable to analysis by the apparatus and methods of prior art.
An apparatus which analyses the optical properties of a quasi cylindrically symmetric transparent object by the illumination of the object generally within the plane orthogonal to the axis of rotational symmetry of the object is termed a transverse-type analyser. Specific examples of transverse-type analysers used for the measurement of the refractive index of an optical fibre preform are disclosed in patent specification GB-A-2071315, and by P. L. Chu in Electronics Letters, 24th November 1977, Vol.13, No.24, pp.736 to 738, and are the ones that have been used most widely in the past ten years. Improvements to the apparatus and methods have been disclosed recently in patent specifications PCT/GB89/01352 and PCT/GB91/00716.
There are two major limitations to the current transverse-type analysers when used for measuring the refractive index of an optical fibre preform. Firstly, there are a number of different types of optical fibre preforms which are of considerable importance but which cannot be satisfactorily measured by prior art. In some cases the measurement is possible but is in error, and in other cases the apparatus fails to detect an appropriate beam of electromagnetic radiation and so the measurement itself fails. The reason for this is that a significant part of the measurement beam interacts with regions of varying refractive index at almost a right angle to a radius of the optical fibre preform within the plane orthogonal to an axis of rotational symmetry of the optical fibre preform. With certain optical fibre preform types, this results in undesirable reflection and diffraction, which then lead to measurement problems. Secondly, in almost all cases it is necessary to surround the optical fibre preform being measured by a region of refractive index liquid in order to obtain a satisfactory optical interface between the optical fibre preform and the measurement apparatus. This is because the measurement beam passes through substantially every part of the circumference at some point during the course of a measurement.
The present invention seeks to overcome both types of limitation by using a significantly different type of interaction between the measurement beam and the transparent object to that used by current transverse-type analysers. An aim of the present invention is to provide apparatus suitable for the measurement of quasi cylindrically symmetric transparent objects not measurable by currently available apparatus. Another aim of the present invention is to provide improvements to the optical interface between the quasi cylindrically symmetric transparent object and the measurement apparatus.
According to a non-limiting embodiment of the present invention, there is provided apparatus for the measurement of optical properties of a quasi cylindrically symmetric transparent object, comprising source means for providing a source of electromagnetic radiation, illumination forming means for forming an illuminating beam of electromagnetic radiation for illuminating the transparent object, region selecting means for selecting a localised radial region within the transparent object illuminated by the illuminating beam, region scanning means for scanning the localised radial region through a range of radial positions within the transparent object, reflection detecting means for detecting a reflected beam of electromagnetic radiation reflected out of the illuminating beam at the localised radial region, beam measuring means for measuring the component of the position of the point where the reflected beam leaves the transparent object along a direction parallel to an axis of rotational symmetry of the tr
REFERENCES:
patent: 3879128 (1975-04-01), Presby et al.
patent: 4553841 (1985-11-01), Coppa et al.
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