ESI determination from Petermann mode-field diameter measurements and refractive index profile measurements on single-mode fibres

Abstract

The two main approaches to the characterization of single-mode fibres: viz. Equivalent Step Index (ESI) and Mode Field Diameter (MFD) are still causing confusion and no real standard procedure has won the confidence of laboratory users. In a previous paper we have shown that in principle, a self consistent method can unify both the ESI and MFD approaches. For instance, it has been shown that the two parameter of the ESI approximation as derived from the moment theory of single-mode optical fibres can be determined independently from either measurement of the Petermann MFD or from measurements of the refractive index profile. This duality feature makes the moments description of single-mode fibres unique amongst a whole range of experimental and theoretical characterization methods.
In this paper we implement and critically assess our new approach using three independent measurement procedures. The ESI parameters as obtained from (i) MFD measurements, (ii) preform profile measurements and (iii) fibre profile measurements are compared for a nominally step index fibre fabricated by the MCVD process.
A statistical approach is adopted in analysing the measurement data, thereby reducing the influence of measurement error and other inherent uncertainties. Results show that while the preform and the fibre profile measurements produce essentially the same equivalent core radius and the same equivalent numerical aperture (with allowance for diffusion effects) the ESI parameters produced by the MFD are substantially different.
The discrepancy arises from the use of the "effective" cut-off wavelength, as derived from the MFD measurements, instead of the "theoretical" cut-off wavelength (corresponding to a V-value of 2.405) in the evaluation of the ESI parameters. However, it is found that if the cut-off wavelength as derived from the profile measurements, is used in the interpretation of the MFD data then, the three measurement approaches predict very similar ESI parameters. While only one fibre-preform is reported here, one other nominally step-index fibre-preform exhibited similar trends.

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