Modelling of multimode Erbium-doped fibre amplifiers for mode-division multiplexed transmission systems

Abstract

This PhD thesis, undertaken within the framework of MODEGAP, covers the design and optimization of high-performance in-line Multimode Erbium-Doped Fiber Amplifiers (MM-EDFAs) or Few-mode (FM) EDFA for next-generation SDM transmission systems based on Mode-Division Multiplexing (MDM). In the MM-EDFAs, minimizing the differential modal gain (DMG) is of paramount importance to prevent system outage.By using an experimentally validated commercial amplifier simulator, I proposed a 2-mode-group EDF design incorporating ring doping that allows accurate modal gain control amongst the two-mode groups using a simple and much more practical LP01 pump mode. Subsequently a 2-mode-group ring-doped EDF according to my design was fabricated in-house and a portable 2-mode-group EDFA with low DMG built and tested, confirming my predictions. My 2-mode-group EDFA design lay at the heart of several successful 2-mode-group fiber based transmission experiments, as listed in this thesis.To investigate the vector modes effects in FM-EDFAs, we developed our own MM-EDFA simulator capable of modelling both the Linear Polarized (LP) modes and the full vector solutions. We have concluded that, in practice, the LP amplifier model is valid and sufficient enough to predict the FM-EDFA performance. I proposed a 4-mode-group EDFA design that offered DMG < 1dB across four-mode groups using a customized pump profile. As the number of guided modes increases, the required pump power also increases which means expensive single-mode pump diodes are needed in the core-pumping approach. Cladding pumping is an alternative way to provide pump radiation with the advantages of reducing the costs. Consequently, I upgraded our in-houseamplifier simulator to a cladding-pump-able MM-EDFA design tool incorporating an optimization algorithm (i.e. Genetic Algorithm) that accepts customized criteria and allows a large number of free parameters to be optimized simultaneously. Using this tool, I proposed the designs and optimizations of cladding-pumped 4 and 6-mode-group EDFAs.Apart from the standard step-index MM-EDFAs, I also investigated novel fiber amplifiers with ring-index profiles for SDM applications. The first type of ring-index fiber discussed in this thesis is of solid core and weakly guiding. The solid-core ring core fiber has an advantage of reducing digital signal processing complexity in MDM transmission. I proposed a 6-mode-group ring core multimode erbium doped fiber amplifier (RC-MM-EDFA) capable of providing almost identical gain among the six mode groups within the C band using either core- or cladding-pumped implementations. The second type of ring-index fiber is an air-core fiber that enables the stable transmission of Orbital Angular Momentum (OAM) modes, which can be used as another degree of freedom for information multiplexing. I have created a new variant of my amplifier model targeting OAM modes and have achieved DMG lower than 0.5 dB for 12 OAM modes in an air-core EDF