The performance of optical transmission systems is strongly dependent on the details of the fibre refractive-index profile. In previous papers we have shown that non-destructive measurements performed on both single-mode and multimode fibre preforms a spatial filtering technique can reveal the index profile with remarkable accuracy and resolution. The method involves imaging of the transversely-illuminated preform by means of a high-quality lens and modulating the transmitted light in the lens focal plane with any of a number of types of spatial filter. With the aid of a small photodetector placed in the image plane it is possible to isolate individual rays in turn and to determine their deflection as they traverse the preform. The index profile is reconstructed from this deflection data by performing an integral transform
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