Cylindrically Polarized Light in Nanobore PCF

During this work, some properties of a special kind of a solid core PCF was  studied. In this kind, there is a coaxial nanobore at the centre of core which  causes an increase in the birefringence between cylindrical modes. Due to this special design, it is called nanobore PCF. Doing some experiments on  a sample of this kind of fiber, the results are depicted through the chapter  3.

This involves excitation of cylindrical modes, attenuation coefficient measurement, and coupling between cylindrical modes. In exciting using SLM,  the cylindrical modes’ profile, azimuthal or radial, were generated beforehand  and coupled to the fiber, causing to excite the same mode inside the fiber. The quality of the beam in this method of excitation were acceptable, but  not perfect. Using a SLM device sometimes is handy, because makes it easy  to excite higher order modes, even the results are not pure.

This devices can be engineered to excite more higher order modes, which may be of interest for some people. However, it is comparably an expensive method.  Following, the loss of fiber for different modes measured.  This is an  important property, which determines the length that light could transfer in  the fiber, while some effects like absorption and scattering try to destroy it.  Roughly speaking, a mode with a higher refractive index has lower attenuation. This is because it has less interaction at the boundaries.

The measurements done in this work comply with this principle. Fundamental mode has the lowest attenuation coefficient, whereas azimuthal mode has a larger coefficient, and radial mode has a coefficient which is largest amongst.  This is a sign of correct measurement, and a new measurement in its kind  which was not done before.  Generally, when a mode is propagating inside the fiber, it keeps all its energy for itself and doesn’t share it with other modes. This is the principle of orthogonally of modes.

However, when there is some sources of stress like bending or imperfections in the fiber structure, the mode starts to share the energy with other modes. In a simple language, the orthogonally rule is broken due to stress. In some fibers, cylindrical modes, and also HE21, are very degenerate, causing coupling of those modes to each other very easily, which is not desired in some applications.

As discussed in introduction of the thesis, enclosing a nanobore in the solid core PCF will increase the birefringence between cylindrical modes. This practically increase the beat length of  fiber, and consequently those modes can propagate a longer distance without  being coupled one to another. 

Now there is a look from the other side; how the coupling between modes could be controlled? How to engineer sources of stress to govern the coupling between modes? This question was realized in this work using a mechanical grating, which performed very well. Starting from azimuthal mode, and pressing the grating on a stripped fiber, there was a smooth transition to radial mode, and to prove its correctness, some discussion were made. 

 

In the last part of experiments, Sec. 3.4, azimuthally polarized light excited using gold filled nanobore PCF. That is a novel way to produce azimuthal mode, while other modes attenuated, and a clean azimuthal mode  gained, which was not mixed up. The images taken in different angles of the polarizer, shows the symmetry, purity and quality of the mode. This is the one of the best generation methods if someone is looking for a real azimuthal  mode. Compared to the method of exciting by SLM, it also has a simpler  setup. But the only a weakness of this method is the lower output power,  while most of the energy is attenuated in the gold filled part.