E.M.A. for microstructured polymer fibers as a cladding modes trap.

A conventional polymer optical fibesr (POF) are manufactured mostly from a polymethyl mathacrylate (PMMA). Their clad is made of fluorinated materials. POF fibers have got the numerical aperture at level 0.5 and very high value of the core to clad area ratio. The problem of cladding modes propagation is marginal for this fibers because huge amount of the impulse energy is trapped in the large POF fiber core. Microstructured polymer optical fibers (mPOF) have a very different structure from POF fibers. Their cross-section structure cause that a refractive index characteristic of the mPOF fiber is more complicated than a classical polymer fiber. In mPOF we observe two regions with a higher refractive index. Therefore energy of input light splits and propagates both, in the core and in the solid clad area. It is very characteristic that the most of the energy propagates in the outer clad. This effect is increased by the very high fiber core to clad area ratio.
In the literature there are proposed several types of cladding modes traps for mPOF fibers. A very important method is similar to the solution used for early conventional fibers. A fiber surface is coated with a layer of the immersion liquid (with refractivity higher than a fiber material), the liquid forces absorption of photons from the clad area until all of cladding modes energy are absorbed. In the second variant the cladding modes are eliminated by disturbance of the fiber surface. A fine-grained sandpaper can be used to deform the external surface without destabilization of the inner structure of the fiber. This method base on scattering of the cladding mode energy by the heterogeneity on the fiber surface. In practice both of this methods in spite of their efficiency are not able to be used during the fiber fabrication process.
The E.M.A. (Extra Mural Absorption) term defines a specific type of the cladding modes trap. An E.M.A. device can be placed in complex optical structures and its main function is absorption of modes, which are not strictly connected to the fiber core. The absorbing layer covers the external fiber (or other optical elements) surface. As materials for E.M.A. layers are used black acrylic rubbers, substations contains a powdered carbon, black glasses and other solutions. Extra mural absorption devices are frequently used in optical elements such as fiberscopes, endoscopes or fiber optics tapers.
The technology of thin absorption coating layers for cladding modes elimination in microstructured polymer optical fibers will be presented. We use the black acrylic rubber as the material of the E.M.A. The configuration of the drawing tower allow draw and coat fiber during a one process.