refracted near field method

cesky

Measurement of refractive index profile of optical fibres

The properties of both single and multimode fibres are determined by the refractive index profile of the fibre. Profiling enables the spatial variations of the refractive index to be measured and hence provides information such as core and cladding diameter, concentricity and numerical aperture (NA). The optical characteristics of the fibre i.e. mode field diameter (MFD), cut-off wavelength and chromatic dispersion can also be derived from the refractive index profile. To determine a fibre's refractive index profile we use the refracted near-field technique or, more compactly, a refracted ray technique [Stewart77, White79]. First measurements, including profiling of polarization maintaining fibres prepared in IREE, were done in our institute in the mid-eighties [Stadnik87]. At present, we use a commercial fibre profiler York S14. The refractive index profile is obtained by scanning a very small spot of light across the end-face of the fibre under test. The refracted light (rays which are not guided and are refracted out through the side of the fibre) is a direct measure of the refractive index of the fibre. The fact that the amount of light refracted out from the fibre depends on the refractive index of the fibre is demonstrated in the figure 1.

Figure 1. The refracted light is a direct measure of the refractive index of the fibre:
the lower the index of the fibre, the greater the vertex angle of the emergent cone.

It can be derived that for a structure with refractive index profile independent on longitudinal coordinate z (the fibre axis), the angle of refraction (a, b in the figure) depends in fact only on the refractive index n(r) in place of the laser spot on the fibre end face and on the refractive index of the immersion fluid that surrounds the fibre. The refractive index is given by the following equation

where P(lambda) is the measured power, P_0 is the calibration optical power measured with the cell filled only with the immersion fluid, with no fiber; angle theta_i je is the vertex angle of the incident cone and angle theta_s is the refracted angle given by the shield.

Technical specifications:
Refractive index resolution: 0.0001
spatial resolution: 500 nm

References
[White79] K. I. White, "Practical application of the refracted near-field technique for the measurement of optical fibre refractive index profiles'', Optical and Quantum Electronics, 11:185, 1979
[Stewart77] W. J. Stewart, "A new technique for measuring the refractive index profiles of graded optical fibres'', Technical Digest: Int. Conf. Integrated Opt. Opt. Fibre Communications, paper C2-2, IOOC, Tokyo, 1977
[Stadnik87] B. Stadnik, F. Osvald, J. Doupovec: "Refracted near filed method for the measurement of optical and geometrical parameters of optical communications fibres", Fine mechanics and optics, 32(12): 365-71, 1987.

Figure 3. Measured refractive index profiles of several telecommunication optical fibres.

Figure 4. Experimental optical fibres for fibre sensors and lasers.