Untersuchungen zur Erzeugung und Charakterisierung von höheren transversalen Moden in Multi-Mode-Glasfasern

Investigations of generation and characterisation of higher-order modes in multi-mode optical fibers.

  • Für den zukünftigen Einsatz von optischen Glasfasern mit großen Laserkernquerschnitten zur Leistungsskalierung von Faserlasern und -verstärkern ist ein erweitertes Verständins über die Entstehung und Unterdrückung höherer transversaler Moden (HTM) notwendig. Aus diesem Grund ist in der vorliegenden Diplomarbeit ein Verfahren entwickelt worden, mit dem erstmalig die Polarisationszustände und gleichzeitig die Leistungsanteile jeder einzelnen, im Ausgangsstrahl vorkommenden HTM bestimmt werden können. Der Messaufbau besteht aus einem Viertelwellenplatte-Analysator-Polarimeter und einem bezüglich HTM selektiven 3-Spiegelresonator. Die Leistungsanteile der HTM konnten bis auf genau bestimmt werden. Indem für viele verschiedene Drehwinkel der Wellenplatte jeweils ein
  • In many applications either single-mode fibers are used if a single transverse mode is needed, or strongly multi-mode fibers are used, where higher-order modes exist in such a large number that the power disturtribution to the individual modes is of low interest. However, in the case of weakly multi-mode or few mode fibers higher-order modes are important and their characterisation can help either to suppress or to use them. One field of research, where the propagation of light in few-mode fibers is of interest is the field of high-power fiber amplifiers and lasers. For this purpose (fiber amplifiers and lasers) optical fibers with a large mode area and a small numerical aperture are necessary to avoide nonlinear effects like stimulated Brillouin Scattering and to obtain very high output power and good beam quality (M2<1.2). Such LMA fibers often support higher-order transverse mode guidance. One known technique to suppress high order modes is for example bend-induced mode distortion. The efficiency of all current used techniques depends on the core diameter and the modal power distribution. Therefore a better understanding of higher-order modes can help to suppress higher order modes which allows to increase the core diameter a little further. Consequently an accurate measurement of the mode content as well as of the polarisation state is important for better characterisation and understanding of few-mode fiber systems. To achieve this goal we built up a system that allows polarisation-sensitive measurement of higher-order modes. At first to generate higher-order modes the radiation of a 1064nm single frequency non-planar-ring-oscillator (NPRO) is carefully coupled into a few mode fiber with core diameter of 25 µm and a numerical aperture of 0.07. Through carefully squeezing of the fiber it was possible to excite higher order modes in a controlled way. To analyse the higher order modes we used a measurement system that consists of a general compensator-analyser-polarimeter and a scanning-three-mirror ring resonator. The ring resonator was used for the mode analysis and the polarimeter to investigate the state of polarisation. The experimental result shows that each excited higher order mode had different polarisation. The distinct modes were nearly fully polarized. The total beam, however, showed a worse degree of polarisation, as it consisted of differently polarized modes. We explain the different polarisation state of each mode as an effect of transverse variation of birefringence, which results from imperfections in the manufacturing process as well as by the squeezing and bending the fiber.

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Author:Thomas Theeg
Advisor:Peter HartmannGND, Carsten FallnichGND
Document Type:Diploma Thesis
Language:German
Name:Institut für Angewandte Physik der Universität Münster, Arbeitsgruppe Optische Technologien
Corrensstr. 2-4, 48149 Münster
Date of Publication (online):2008/05/27
Year of first Publication:2008
Date of final exam:2008/02/22
Tag:Faserverstärker; Glasfasern; Polarisation; höhere transversale Moden
fiber amplifier; higher order modes; optical fibers; polarisation
Page Number:79 Seiten, 46 Abb., 6 Tab., 26 Lit.
Faculty:Westsächsische Hochschule Zwickau / Physikalische Technik, Informatik
Release Date:2008/05/27