PhD defence by Antonio Astrorino

Title:  Higher-order modes in optical communication systems

Supervisor
Principal supervisor:  Prof. Karsten Rottwitt, DTU Fotonik, DK

Evaluation Board
Ass. Prof. Darko Zibar, DTU Fotonik, DK
Proff. Svetlana Avramov-Zamurovic, United States Naval Academy, USA
Lecturer Zhixin Liu, University College London, UK 

Master of the Ceremony
TBA

Abstract
The orthogonality of fiber modes enables spatial mode-division multiplexing (MDM) which, combined with the already well-established multiplexing techniques in time and wavelength  domains, allows to increase the channel capacity in optical communication systems. The study of MDM has led to a growing interest in higher order modes in cylindrical waveguides. If exactly calculated, those modes consist in a very elaborate electromagnetic field and are named full-vectorial or vector modes. In the weakly-guiding approximation, instead, the so-called linearly polarized (LP) and orbital angular momentum (OAM) modes are obtained. Such an approximation
successfully describes scenarios in which the mode order is low or the refractive index contrast in step-index waveguides is negligible. On the other hand, full vectorial modes must be considered to describe atypical conditions. This Ph.D. thesis is focused on full-vectorial modes. A novel and more accurate approximation to obtain OAM modes from full-vectorial ones, instead of LP modes, is proposed. The excitation of OAM modes based on the proposed approximation and using one phase-only spatial light modulator (SLM) is addressed. An algorithm to generate SLM phase masks and optimize some setup parameters is presented. An error-correction algorithm to modify the calculated phase masks, assuming a circular symmetry in the OAM excitation setup, is also  proposed and demonstrated experimentally.