A new technique for the measurement of refractive index profiles in an optical fibre preform

Abstract

This thesis is concerned with the theoretical and experimental development of a new technique for the measurement of the refractive-index profile in an optical fibre preform.

The refractive-index profile is one of the most important parameters in designing optical fibres for communication systems. It is of critical importance to signal bandwidth of the fibre. Therefore, a close control of the index profile and its accurate measurement are necessary. In particular, quality control of preforms from which the fibres are drawn demands a nondestructive method which is able to reveal the accurate and detailed index profile of both single-mode and multimode fibre preforms.

In our method, a light beam illuminates a preform transversely and the preform image is produced on a screen. We have found that various forms of the spatial filtering technique may be used to acquire the information required for the index-profile determination.

Equivalently, we measure the light deflection suffered by geometric rays traversing the preform, i.e. the deflection function. The index profile is calculated from the deflection function through a mathematical transform.

An investigation of the optical phenomena involved and a formulation of the index-profile transform is presented as a background study for the project.

Two of the spatial filtering techniques are found to be of particular importance for the development of automatic index-profiling apparatus, namely (a) the angled knife-edge method and (b) the chopper method. The former produces a visual image of the deflection function and the latter provides the deflection function in the form of a time-varying signal. The index profiling apparatus has been designed and constructed for use in routine preform inspection.

An analysis of the method is made and shows that the method requires no critical alignment, is accurate and has high resolution. However, the method requires the preform to be circularly symmetric for accurate results.

In order to obtain an accurate index profile of a noncircular preform, a new method is presented, in which the light refraction at a preform endface is used. The method reveals the three-dimensional image of the index profile directly without the need for any calculation or further processing.

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