Modal effect characterization and compensation for high-capacity, space-division-multiplexed optical networks

Abstract

Due to the exponential growth of global data traffic, a capacity shortage in the optical fiber network is expected in the near future. Space-division multiplexing (SDM), combined with multicore fibers (MCFs) and multimode fibers (MMFs), offers a promising solution to address this challenge. For this technology to become commercially viable, issues such as MCF and MMF characterization and the impact of transmission impairments on SDM optical links must be addressed. In addition, since multiple modes or cores can be jointly utilized to transmit information, energy-efficient multi-dimensional modulation formats and the corresponding transceivers, along with their digital signal processing (DSP) algorithms, should also be considered. First, we evaluate two methods, namely the mode-dependent signal delay method (MDSDM) and the mode-dependent average power method (MD-APM), for the characterization of MCFs and MMFs. In these methods, we select launch states that optimize the signalto- noise ratio (SNR) for measuring the modal dispersion (MD) and mode-dependent loss (MDL) vectors. The MD of MMFs and MCFs is estimated using time-of-flight measurements of optical pulses and multiple sets of group delay measurements. We then investigate the accuracy of MD-SDM and MD-APM in characterizing MMFs by simulating various sources of error. Additionally, we estimate the MD and MDL of MMFs and MCFs under different coupling regimes. Thermal noise at the direct-detection receiver and modal crosstalk at the fiber input are considered as two limiting factors in the estimation of the MD and MDL vectors. During the second phase of this dissertation, we study various multi-dimensional modulation formats for MMFs and MCFs. Our focus is on mode vector modulation (MVM), which was recently proposed by our group. We model and simulate transmission impairments such as rotation of the state of polarization (RSOP), MD, chromatic dispersion (CD), and modal crosstalk in SDM wavelength-division multiplexing (WDM) optical networks. Next, we investigate different DSP equalization algorithms, including least mean square (LMS), independent component analysis (ICA), Kalman filter, and extended Kalman filter (EKF). Finally, we propose an optimal adaptive equalizer for the MVM direct-detection transceiver.