Measurement and control of modal propagation in mode-division multiplexed optical fiber transmission systems [electronic resource]
Keywords: Electrical Engineering.

Over the last two decades traffic carried on the backbone optical fiber networks has been growing exponentially thereby creating a demand for increasing communication bandwidth. This demand has been met by introducing different kinds of multiplexing, i.e. combining of parallel data streams, including wavelength-division multiplexing (WDM), multiplexing in-phase and quadrature signals, and polarization multiplexing in single-mode fibers (SMF). However, long-haul SMF systems are rapidly approaching a fundamental information-theoretic capacity limit, also known as nonlinear Shannon capacity limit. Recently, there has been growing interest in the use of spatial mode multiplexing in multimode fibers (MMF) for long-distance communication as a possible route of overcoming this limit for further bandwidth growth. Both polarization multiplexing and spatial multiplexing are forms of mode-division multiplexing (MDM) and introduce challenges such as polarization-dependent and mode-dependent loss (PDL, MDL) and polarization-mode dispersion (PMD) and modal dispersion (MD). In this dissertation, we study a differential technique for combined measurement of PMD and PDL effects in SMF systems, and design of transmission fibers, amplifiers and mode scramblers for minimizing MD and MDL in MDM systems. In the case of SMF systems, the most common methods of PMD measurement assume no PDL and would not be applicable for an end-to-end system measurement if one of the components has a significant PDL. While PMD is caused by polarization-dependent phase effects and PDL is caused by polarization-dependent amplitude effects, in the case of both significant PMD and PDL a combined treatment of these effects is necessary for correct estimation of overall system performance. Hence, we present an optical dispersion analysis and measurement technique based on frequency derivatives of the Jones matrix. This technique allows simultaneous measurement of all scalar and polarization-dependent phase and amplitude effects over a range of wavelengths in a single wavelength sweep. Owing to its differential nature, it can be more accurate than techniques that calculate dispersion by comparing phase and amplitude measurements from adjacent wavelengths in a sweep. The method involves measuring eight elementary parameters related to the frequency derivative of the Jones matrix. An experimental setup and data analysis methods for measuring the elementary parameters are presented. Three optical devices exhibiting various dispersive effects are tested, and the ability to measure all the elementary parameters is demonstrated. In the case of spatially multiplexed long-haul systems, the properties of transmission fibers and fiber amplifiers are crucial to the ultimate feasibility of MDM systems. In transmission fibers, low group delay (GD) spread minimizes MD and receiver signal processing complexity, while large modal effective areas minimize nonlinear effects. In fiber amplifiers, low mode-dependent gain (MDG) leads to low overall system MDL and minimizes the loss of capacity and the potential for outage. We study multimode transmission fibers and multimode erbium-doped fiber amplifiers (MM-EDFAs) for 12 signal modes (including spatial and polarization degrees of freedom). We numerically compute modal fields and determine their effective areas, GDs and chromatic dispersion (CD) coefficients. We solve multimode rate equations numerically to calculate MDGs and pump power requirements in MM EDFAs. Results are compared for various numerical apertures (NAs), index profiles and doping profiles. Graded-index depressed-cladding (GIDC) fibers offer an attractive combination of low GD spread in transmission fibers with root mean-squared (RMS) value of 583 ps/km at 0.15 NA, and low MDG in MM-EDFAs (0.14 dB RMS at 25-dB mode-averaged gain with 0.15 NA). Optimized erbium doping profiles comprise a uniform cylindrical region plus an extra annulus. Even the best transmission fiber designs show that further minimization of MD is needed for practicality of spatially multiplexed systems. The GD spread arising from MD can be significantly reduced by introducing strong mode coupling via mode scramblers. We study the design of such mode scramblers implemented as long-period multimode fiber gratings (LPMFGs) for systems using D=12 modes (six spatial modes). By optimizing the grating chirp function, we minimize the MDL of the grating while ensuring full intergroup mode coupling. We find a design yielding MDL and mode-averaged loss in the C band not exceeding 0.36 dB and 0.45 dB, respectively. We also verify the effect of such mode scramblers on the GD scaling of a long-haul system, demonstrating that the scramblers reduce the scaling of GD spread with length from a linear to a square-root dependence, as expected in the strong coupling regime.