| Due to the rapid growth of internet traffic, wavelength-division multiplexing (WDM) has been the workhorse of data networks, accommodating exponential capacity increase over the past20years. Recently, however, progress in WDM transmission capacity has remarkably slowed down as experiments are approaching the fundamental Shannon limits of sing-mode optical fiber system. Mode-division multiplexing (MDM) over few-mode fibers (FMFs), a new and disruptive form of optical multiple-input multiple-output (MIMO) transmission, is expected to further scale optical network capacities. In MDM transmission, index perturbations can induce coupling among signals in different modes, and can cause propagating fields to evolve randomly. Besides, there are important theoretical or technological problems need to be solved, such as the realization of optical components for mode-multiplexing or-demultiplexing, multimodal amplification, FMF design for low-and high-crosstalk regime, nonlinear effects in FMFs, and etc. By focusing on the random mode coupling and transmission fiber, in this dissertsation, we propose and describe several physical model and technological exploration for large-capacity MDM transmission. These works can be listed as follows:1. We propose and describe a physical model of polarization-dependent principal modes (PDPMs) in a given setting of dual-LP11mode and dual-polarization transmission over weakly-guiding FMFs. Proof-of-concept numerical simulations illustrate that the PDPMs do not suffer from both mode dispersion and polarization mode dispersion to first order of frequency variation, even in the presence of random spatial-and polarization-mode coupling. For pulse propagation within different input PDPMs, the arrival time of the pulses within the corresonding output PDPMs is generally different. This guarantees that each input PMPD corresponds to a launching condition for minimum pulse broadening. The proposed PDPM model can be a basic formalism for analyzing and controlling of mode coupling/dispersion-induced distortion, in the given optical MIMO scheme of dual-LP11mode and dual-polarization transmission over FMFs.2. By considering very strong intra-group mixing while neglecting inter-group mixing in mode group division multiplexing (MGDM) transmission, we theoretically propose and describe the physical model of intra-group principal modes (IGPMs) in graded-index (GI) multimode fibers (MMFs), from the view of fiber transmission matrices. Proof-of-concept calculations for an exemplary mode group-channel with the two lowest-order degenerate mode groups (i.e. the three lowest-order fiber eigenmodes) show that IGPMs exhibit potential possibilities of the MGDM channel with minimal mode mixing/dispersion-induced signal distortion over a GI MMF. For the mode group-channels in such intensity modulation, direct detection GI MMF links, the IGPM concept provides the potential advantages in terms of frequency-independent group delays and extended reach, especially if a mode group-channel supporting a larger number of adjacent higher-order modes is used for the transmission.3. Based on the analogy between multipath fading in wireless channels and multimode nature in MMFs, the potential of capacity enhancement and the application of space-time block coding (STBC) in coherent optical MIMO MMF links are theoretically investigated with considering negligible inter-modal coupling. Numerical simulations show that STBC technique can be applied in an effort to improve the error performance. Furthermore, a comparative study is performed by considering several schemes that employ multiple transmitters/receivers. Simulation results of these schemes, in terms of bit error rate as a function of optical signal to noise ratio, are provided.4. A2x6-Gb/s MDM transmission over70-km dual-mode microstructured fiber (MF) is numerically demonstrated by using a long-period fiber grating as the mode converter between orthogonal LPo1and LP11modes. The mode multiplexing or demultiplexing is achieved by adjusting the interaction parameters of fiber couplers to separately forward signals to straight-and cross-path. By doing this, the simulation results for the proposed MDM transmission show negligible mode crosstalk and low power penalty of less than1.86dB. Also, Approximate empirical analysis of mode power distribution carried by the fundamental mode is newly investigated based on a broadband dispersion compensating MF. The fraction of modal power in the core region is defined with the help of extending the applicability of well-established classical optical fiber theories to MFs. The influences of structural parameters and wavelength on mode power distribution characteristics of the fundamental mode are systematically analyzed based on simple physically consistent concepts of conventional fibers. |
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