Loss measurements in graded-index compound-glass fibers

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Abstract
The optical fiber scattering loss coefficient is measured directly in a scattering sphere or deduced indirectly from total loss measurements. The results show agreement for graded-index silica-based fibers, but they seem conflicting for graded-index compound-glass fibers. This is explained from the diffusion-controlled refractive index profile and the ensuing mode-dependent scattering and absorption loss due to the different optical properties of core and cladding glass. Using this model the two-lengths total loss measurement method is discussed. A detailed experiment is described that convincingly illustrates the mode of operation of the scattering sphere as used in daily practice. The wavelength-independent term in the total loss, different for fibers drawn from the same glass, is explained as being scattering partly due to 1-mode mixing of modes with the same β by imperfections that affect high-1-modes predominantly. The total loss of the glasses to be investigated can be measured using low NA excited silicone-clad fibers and safely can be decomposed into scattering and absorption contributions.