Diferencia con el Principio de Precaución 68

at realizable levels. The work developed along this thesis addressed the aforemen- tioned challenges.

1.2

Objectives and Organization of the Thesis

The main objectives of the present thesis have been the research and development of MDM-FMF systems. The main impairments of MDM-FMF systems are identified and modeled, enabling the development of several techniques to reduce their impact, namely: DMD, modal XT, MDL/mode dependent gain (MDG) and IM-NLs. A nonlinear semi-analytical model (NSAM) for transmission over FMFs is developed. The design of MMUXs/MDMUXs based on mode selective phase masks is investigated regarding the mask resolution, mask phase noise and mask alignments. The design of FMFs with low DMD suitable for long-haul transmission is addressed, considering up to 12 non-degenerate LP modes. Moreover, three methods to extend the transmission reach of MDM-FMFs by limiting the accumulation of DMD and MDL/MDG along transmission are proposed. Finally, an extensive study of the inter-modal nonlinear effects in MDM-WDM-FMF systems is presented, where the dominant effects as well as possible techniques to reduce their induced penalty are identified.

This thesis is structured in 7 chapters and 3 appendices that support the main document. In chapter 2, a brief summary on the theory of modal propagation in FMFs is presented, in order to gain a clear insight on the major properties of FMFs. In this chapter, the main concepts and notions used throughout the thesis are intro- duced. Moreover, an extensive review of the literature about FMFs is presented with emphasis on fiber properties such as DMD and modal XT. The basic system concept common to the majority of the MDM-FMF systems proposed in the literature is ex- plained, and different implementations of system components are reviewed, namely: MMUXs/MDMUXs, MMAs and MIMO-DSP.

In chapter 3, a method for the semi-analytical solution of the coupled linear differential equations that describe the linear XT in FMFs due to waveguide imper- fections is proposed. A set of multimode coupled nonlinear Schr¨odinger equations (MM-CNLSE) for nonlinear simulation of FMF transmission including the linear XT is derived, considering a pulse envelope per mode per polarization. The MM-CNLSE obtained can be solved modifying the split-step Fourier method (SSFM) to include the respective linear effects and to allow the simulation of FMFs with M modes. Moreover, MMUXs/MDMUXs based on phase masks (PMs) are optimized in order

to enable the study of MDM transmission in the following chapters. Afterwards, the general simulation setup of a MDM-FMF system used along this thesis is presented. Finally, simulation results for the transmission over a SI-TMF are obtained, allowing a qualitative validation of experimental results provided in the literature.

In chapter 4, the design of FMFs with up to 12 non-degenerate LP modes using a GCCT or a multiple-step index (MSI) profile is proposed. The profiles parameters are optimized in order to achieve the lowest DMD and macro-bend losses (MBL) lower than the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) recommendation. Firstly it is shown that, for 2 non-degenerate LP modes, by optimizing the core grading exponent and the dimensioning of the trench of a GCCT profile, a given DMD target (between -200 ps/km and 200 ps/km) can be attained with a deviation lower than 1.8 ps/km over the C-band. A simple design rule considering just those two parameters is derived, allowing the design of ILD-FMFs and DC-FMFs. Afterwards, the design of FMFs with up to 12 non-degenerate LP modes is studied considering a GCCT profile and a MSI profile. The optimization results show that the MSI profiles present lower DMD compared to the minimum obtained for a GCCT profile. Moreover, it is shown that the optimum DMD and the MBL increase with the number of modes for both profiles. Finally, the impact of the fabrication margins on the optimum DMD is analyzed. The probability of having a manufactured FMF with 12 non-degenerate LP modes and a DMD lower than 12 ps/km is 4 times higher for a GCCT profile than for a MSI profile.

In chapter 5, three techniques for the improvement of the distance reach of MDM- FMF transmission systems are proposed. The first technique proposed is the in- troduction of discrete modal XT through fiber splices, in order to reduce the CIR spread. The effectiveness of the method is assessed through simulation considering 3 × 136 Gbit/s MDM-PDM-QPSK ultra-long haul transmission systems employing ILD-FMFs or DC-FMFs. The maximum distance reach is optimized by varying the optimum number of splices per span and the splice XT level. Furthermore, alternative methods for distance reach improvement mitigating the MMAs MDG are presented, using strong coupling modes and maximum-likelihood (ML) detection.

In chapter 6, the impact of IM-NLs on MDM-FMF systems carrying WDM channels is investigated. A set of multimode multi-wavelength coupled nonlinear Schr¨odinger equations (MM-MW-CNLSE) including linear XT is derived, considering one pulse envelope per wavelength in each polarization and mode. The derived MM- MW-CNLSE allows studying each IM-NL process separately, namely: inter-modal