Design and Construction of a Multi-Port Beamsplitter Based on Few-Mode-Fibers

Sammanfattning: A MBS (Multi-port beamsplitter) for higher dimensional quantum communication has been designed and constructed and the theory and method for this is presented in this thesis. It uses optical fibers in a heterogeneous structure with a single-mode fiber spliced to a multi-mode fiber and then spliced to a few-mode fiber. Three MBS:s were constructed and tested to see if superpositions between spatial modes could be generated. One with 5.65cm multi-mode fiber, one with 9cm of multi-mode fiber and one with just the single-mode fiber spliced to the few-mode fiber. The optical modes that where focused on for the superposition were the linear polarized LP01, LP11a and LP11b modes. Simulations of superpositions between these modes were performed and experiments were done to see if these simulations could be realised. The shapes of these superpositions could be seen with a camera and the stability of the different modal powers and the stability of the phases between the modes where also tested. The last experiment tested the tunability of the modes by finding their maximum and minimum output power for each individual mode. The results of these experiments show that the stability of power and relative phases are high and testing of the tunability shows that the 9cm MBS is the most tunable, the 5.65cm MBS the second best and the SMF-FMF MBS the worst. Even though the shapes of the superpositions, the stability and tunability shows very positive results, the conclusion is that more experiments are required in order to identify the superpositions and for this to be used in a quantum communication system.