Photonic crystal fiber acousto-optic tunable filters

Abstract

This thesis is concerned with all-fiber acousto-optic tunable filters (AOTFs) using a solid-core two-mode (TM) photonic crystal fiber (PCF). Firstly, the origin of multiple resonance peaks observed in an all-fiber acousto-optic tunable filter built with a TM PCF having structural imperfections is investigated in detail. Small disorder in the six-inner air holes around the core can break the mode symmetry of the PCF. For this reason, first higher-order modes of the PCF are classified into the even and the odd LP11 modes resembling those of an elliptical-core (e-core) two-mode fiber (TMF). The PCF has also non-circularity in outer cladding shape, which causes to the acoustic birefringence of the fiber. A model for the acousto-optic mode coupling in the PCF is formulated for the systematic analysis on the origin of the distorted spectral peaks. Numerical calculations and simulations based on the coupling model were performed, and the results are in good agreement with the experimental observations. Nextly, the resonance peak splitting in the output spectrum of a TM PCF-based AOTF under the fiber twist of up to multiple turns is described. The PCF guides the even and the odd $LP_{11}$-like modes in the core as a first higher-order mode. For the twist rate of our interest, lobe orientations of the $LP_{11}$-like modes follow the fiber twist, whereas the initial acoustic vibration direction is maintained along the fiber. Accordingly, the acoustic amplitude contributing to the mode coupling of the $LP_{01}$ mode to the $LP_{11}$ modes is sinusoidally modulated along the fiber. This is substantially the same to amplitude beating between two acoustic waves with difference propagation constant. Therefore, the fiber twist leads to the two phase-matching conditions that result in two notches in the transmission spectrum. Theoretical calculations based on the coupled-mode theory provide an accurate explanation of the resonance peak splitting. The peak separation is ...