Forthcoming capacity scaling requirements of optical networks and advances in optical fiber communications beyond the omnipresent single-mode fiber operating over the conventional band introduces new opportunities and challenges for exploiting the expanded spectral and spatial resources available for fiber-optic network designs. The spectral and spatial degrees of freedom introduce utilization tradeoffs that can be leveraged under different applications. After briefly reviewing the supporting network building elements (emerging fiber types, multiplexers, optical switches, and amplifiers), we consider networking scenarios such as intra/inter datacenter, terrestrial, undersea, and wireless 5G/6G hauling networks and identify the best utilization plan and key attributes that can be harnessed by the offered spectral and spatial degrees of freedom for each scenario and how optical switching can be employed therein for traffic management. Networks supporting these scenarios are experiencing tremendous traffic growth pushing their existing implementations to their physical capacity-distance limits. New network builds must offer significant capacity multipliers in an efficient and cost-effective manner, requiring new switching solutions for supporting the unabated traffic growths foreseen in the years to come. We discuss how space-division multiplexed networks can help address this challenge in key communication scenarios.

A novel optical high order quadrature amplitude modulation (QAM) transmission system for high-speed short links is described. Dual-polarization (DP) QAM and twin local lights are generated from one light source in the system, and these lightwaves are simultaneously transmitted via standard single mode fiber. The receiver can be constructed simply because it does not require a coherent light source under wavelength control. The system enables a 3.1 Gbaud DP-16-QAM signal to be successfully demodulated after 80-km transmission without using an optical dispersion compensator. It also achieves high tolerance against phase noise in the signal light source.

A channel modeling method is developed and proposed to practically model the various effects of mode couplings for optical communication systems with multi-core fiber (MCF) and/or few-mode fiber (FMF). The method is similar to existing channel modeling methods in which strong coupling is exclusively considered but extends them to cover both strong and weak couplings. With the method, the entire channel is divided into K statistically independent sections where each section may represent an optical device or the span of a few-mode fiber transmission line that maintains coherence. Each section is modeled by a channel matrix model comprising full random unitary matrices and a differential mode delay (DMD) matrix. To further enhance the modeling of weak coupling, the matrix is modified by segregating it into partial block matrices and inserting correlation terms among blocks. The method is evaluated with data obtained in two representative cases: midrange (40 km) and long-haul (527 km) transmissions. Its effectiveness and practicality are validated from agreement obtained between simulation and measured results for the two cases.

The design and performance of repeater circuits based on Si and GaAs MESFET process technologies are described. Repeater circuits were designed and fabricated for around 10 Gb/s repeater systems using Si and GaAs IC processes. The Si ICs operated up to 9 Gb/s, and the GaAs ICs exceeded 10 Gb/s. It was verified that regenerative repeater systems using these ICs and optical amplifiers exhibit a stable operation at 10 Gb/s. The performance of the 10 Gb/s repeater using these monolithic ICs and photonic circuits is discussed.>