Optična vlakna

• Main Author: Brenčič, Maja
• Other Authors: Vaupotič, Nataša
• Format: Bachelor Thesis
• Language: Slovenian
• Published: M. Brenčič 2012
• Subjects: Mnogorodovna optična vlakna; numerična odprtost; izgube v vlaknih; Gaussovi snopi; divergenca laserskega snopa; preslikave snopov z lečami.; Multi-mode optical fibres; numerical aperture; attenuation in optical fibres; Gaussian beams; divergence of a laser beam; transformation by a lens.; info:eu-repo/classification/udc/53(043.2); Snopa; The Waist; sami
• Online Access: https://dk.um.si/IzpisGradiva.php?id=22690; https://dk.um.si/Dokument.php?id=29406&dn=; https://plus.si.cobiss.net/opac7/bib/19127816?lang=sl

V diplomskem delu obravnavamo optična vlakna, ki za vodenje svetlobe izkoriščajo pojav popolnega odboja na meji dveh sredstev. Optična vlakna delimo glede na število resonančnih valovanj ali rodov, ki jih vlakna vodijo. Enorodovna optična vlakna vodijo le eno resonančno valovanje ali rod, mnogorodovna optična vlakna pa vodijo več resonančnih valovanj ali rodov. Podrobneje obravnavamo mnogorodovna optična vlakna, ki jih lahko opišemo z žarkovnim modelom, ki temelji na geometrijski optiki. Pri žarkovnem modelu razširjanje svetlobe v dani smeri predstavimo s premico, ki jo imenujemo žarek. Najprej opišemo sestavo in delitev optičnih vlaken, nato z žarkovnim modelom predstavimo način vodenja svetlobe skozi vlakno in izpeljemo numerično odprtost vlakna, ki določa največji kot, pod katerim vlakno še lahko sprejme in prenaša svetlobo. Izpeljemo tudi resonančni pogoj za razširjanje valovanja v valovodu in opišemo vzroke za izgube v optičnih vlaknih: absorpcijo v snovi, sipanje, disperzijo in ukrivljenost vlakna. Obravnavamo najpreprostejše nevzporedne laserske snope, ki jih imenujemo Gaussovi snopi. Izpeljemo enačbo, ki nam pove, kako se jakost električnega polja E(r,z) spreminja v odvisnosti od polarnih koordinat r in z. Laserske snope lahko ožimo in širimo z lečami. Obravnavamo preslikavo laserskega snopa z zbiralno lečo in izpeljemo lego in polmer grla laserja po preslikavi z lečo, če poznamo goriščno razdaljo leče in polmer grla pred preslikavo z lečo. V eksperimentalnem delu najprej opišemo pripravo optičnega vlakna za eksperimentalne namene in pripomočke, ki jih pri tem potrebujemo. Nato pokažemo, kako laserski snop zožimo z mikroskopskim objektivom in kako je premer snopa v gorišču objektiva odvisen od povečave objektiva. Manjši kot je polmer snopa v gorišču objektiva, večja je divergenca laserskega snopa. V nadaljevanju predstavimo poskuse, s katerimi izmerimo numerično odprtost vlakna in poskus, s katerim izmerimo izgube v vlaknu v odvisnosti od valovne dolžine svetlobe. V zadnjem poglavju diplomskega dela predstavimo vključitve optičnih vlaken v osnovnošolski in srednješolski pouk. Predlagamo poskuse, ki jih učitelji demonstrirajo učencem in poskuse, ki jih učenci lahko izvedejo sami. In the graduation thesis we study optical fibres which transport light by means of total internal reflection at the boundary between two media. Optical fibres are classified according to the number of resonant waves or modes that they transmit. Single-mode optical fibres carry only one resonant wave or mode, while multi-mode optical fibres transmit several modes. Multi-mode optical fibres in which propagation of light can be treated within the geometric optics have been treated in particular detail. Within the ray model, the propagation of light in a given direction is presented by a line, which we refer to as a ray. We begin by describing the structure and the types of optical fibres, continue by outlining how light propagates through the fibre with the aid of the ray model, and follow up by deriving the numerical aperture which determines the largest angle at which the fibre is still capable of accepting or emitting light. We also derive the resonance condition for the wave propagation in a waveguide and describe the mechanisms of attenuation in optical fibres (transmission losses), which are: absorption in matter, light scattering, dispersion and curvature of optical fibres. We treat the Gaussian beams which are the simplest non-parallel laser beams. We derive the dependence of the electric field intensity E(r,z) on the polar coordinates r and z. The waste size of a laser beam can be either increased or decreased with the use of appropriate lenses. We treat the transformation of a laser beam by a converging lens and derive the beam waist size and its position after the transformation by a lens, knowing the values for the focal length of the lens and the laser beam waist size and position in front of the lens. We begin the experimental part of the thesis by outlining the procedure of preparing optical fibres for experimental purposes and by introducing the required laboratory equipment. Then we demonstrate that the beam waist is narrowed when the laser beam propagates through a microscope objective lens, and show how the waist diameter depends on the magnification of the microscope objective lens. We show, that the divergence of a laser beam increases when the beam waist decreases. Next, we present a set of experiments to determine the numerical aperture of the fibre, and take a look at an additional experiment by which we can determine the attenuation levels in optical fibres depending on the wavelength of light. Finally, we discuss the possibility of introducing optical fibres into the applicable Physics-related primary and secondary school curricula. We suggest the experiments which teachers can use to introduce the concept of optical fibres, as well as examples of experiments, which pupils can perform on their own.