Acousto-optic filter properties of elliptical core and photonic crystal two-mode fibers

Abstract

This thesis investigates acousto-optic properties of novel all-fiber wavelength filters based on intermodal coupling in an elliptical core two-mode fiber and a photonic crystal two-mode fiber. Firstly, a novel apodization technique for an elliptical core two-mode fiber acousto-optic tunable filter by twisting the fiber and adjusting the acoustic polarization is proposed and demonstrated. The magnitude of the maximal sidelobes in the filter spectrum is reduced from -9.7 to -15.5 dB at 1300 nm for an acoustic frequency of 3.4 MHz, which improves serious performance deterioration such as inter-channel crosstalk when this filter is used in wavelength division multiplexing systems. The influence of acoustic birefringence on filter output spectrum as a function of a twisting angle is successfully explained. Both theoretical and experimental results are in good agreement. Further apodization using an AOTF with a double-pass scheme is also suggested to accomplish the side mode suppression ratio of below -30 dB. Secondly, an ultra-widely tunable acousto-optic filter employing a photonic crystal two-mode fiber is proposed and demonstrated. A photonic crystal two-mode fiber in our experiment is designed to have two core modes in the optical wavelength region below 3400 nm. Experimental output spectra show that a single notch coupled to LP11 core mode is continuously tuned from 700 to 1600 nm for acoustic frequencies from 0.86 to 3.1 MHz. The tuning range of 900 nm is a few times larger than that of acousto-optic filters with a conventional step-index fiber. This demonstration will allow extremely broadband devices to be developed. Other cladding mode couplings whose notch depths are negligible (lower than 0.2 dB) are also observed. For apodization of photonic crystal two-mode fiber acousto-optic filters and stale lobe orientation of the LP11 core mode, a proper design of a photonic crystal two-mode fiber with two larger holes of both sides of the core is proposed....