The equivalent step index (ESI) parameters associated with single-mode fibres can be evaluated by observing the changes in transmitted power on the far field when a Ronchi ruling is inserted between the end face of the fibre and a detector collecting the output intensity. Measurements as a function of wavelength allow one to determine the cutoff wavelength, ESI parameters for short lengths of fibre and effective cutoff wavelength for long lengths.
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Pocholle et al. (1983) studied this question.
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