Higher Order Mode Fibers

This PhD thesis considers higher order modes (HOMs) in optical fibers. That includes their excitation and characteristics. Within the last decades, HOMs have been applied both for space multiplexing in optical communications, group velocity dispersion management and sensing among others. The research presented in this thesis falls in three parts. In the first part, a first time demonstration of the break of the azimuthal symmetry of the Bessel-like LP_0X modes is presented. This effect, known as the bowtie effect, causes the mode to have an azimuthal dependence as well as a quasi-radial polarization as opposed to the linear polarization of the LP_0X modes. The effect is investigated numerically in a double cladding fiber with an outer aircladding using a full vectorial modesolver. Experimentally, the bowtie modes are excited using a long period grating and their free space characteristics and polarization state are investigated. For this fiber, the onset of the bowtie effect is shown numerically to be LP₀₁₁. The characteristics usually associated with Bessel-likes modes such as long diffraction free length and selfhealing are shown to be conserved despite the lack of azimuthal symmetry. In the second part of the thesis, a new scheme for constructing chirped microbend long period gratings is presented. The method presents a versatile platform for tailoring the chirp to the phase matching profile of the targeted HOM conversion in the fiber under test. The scheme introduces the ability to implement a nonlinear chirp which is a first time demonstration. The results are modelled using coupled mode theory and it is shown that the conversion bandwidth may be increased more than four fold. In the final part of the thesis, imaging as a characterization tool for HOMs is considered. Three different characterization methods are considered. First, the divergence angle is introduced as a quality parameter to replace the conventional M² which compares the diffraction of the investigated fiber mode to that of a Gaussian and suffers from ambiguity when considering mode mixtures. Secondly, the phase retrieval method is used to retrieve the phase profile of a mode mixture in fewmoded fiber based on volume intensity measurement. A mixture of LP₀₁ andLP₁₁ is considered both using a numerical example to establish the workings of the method and experimental investigations. In the experimental investigation, both a 50/50- and 88/12-mixture is considered, and in both cases the method shows reliable results. Last, a new method for determining the group velocity dispersion of modes in the LP_0X , LP_1X , and LP_2X mode groups based on an analysis of the field profile is presented. The method is independent of the fiber length. The method reproduces the group velocity dispersion spectra obtained analyzing a test fiber with a scalar mode solver.