Highly accurate measurements of the refractive index of polymers

Contacts:

Henrik Schiøtt Sørensen, DTU Fotonik, 4677 4598 (hsch@fotonik.dtu.dk)

Michael Frosz, DTU Fotonik, 4525 6369 (michael.frosz@fotonik.dtu.dk)

Peter Andersen, DTU Fotonik, 4677 4555 (peta@fotonik.dtu.dk)

The preform of a microstructured polymer optical fibre. The hole structure is clearly seen and is retained when the preform is drawn into a fibre less than 500 µm in diameter.

A recently developed measurement technique can measure the refractive index of a sample within an accuracy of ~10^-6. The robust setup comprise of a coherent light source capable of changing wavelength over a wide range. By analyzing the scattered light pattern from the test sample ones is able to measure the refractive index of the sample material, which is highly important when used for an optical waveguide, because the refractive index variation with wavelength determines the dispersion of the waveguide. Optical fibers are typically drawn from silica, but can also be drawn from polymers such as poly(methyl methacrylate), or PMMA. Measuring the dispersion of PMMA will allow for optimization of the fibers drawn from this material. Further, an accurate measurement of the refractive index can also be used to calculate the loss of the material, by using the so-called Kramers-Kronig relation. The project will involve both theoretical aspects of the method and extensive laboratory work on real samples. You should first learn the physical basis for the technique, and then complete building your own setup to perform tests.