We investigate the concept of principal modes and its application for mode division multiplexing in multimode fibers. We start by generalizing the formalism of the principal modes as to include mode dependent loss and show that principal modes overcome modal dispersion induced by modal coupling in mode division multiplexing operation, even for multi-mode-fibers guiding a large number of modes, if the product of modulation bandwidth, fiber length and differential group delay is equal or less than one in each transmission channel. If this condition is not sustained, modal dispersion and crosstalk at the receiver limit the transmission performance, setting very high constraints towards modal coupling.

Single-mode VCSEL technology has advanced significantly in the past few years. The advantages of single-mode VCSELs lie primarily on the narrower linewidth, lower numerical aperture and smaller spot size compared to multimode VCSELs. They are suitable for transmitting over both multimode fibers and few-mode fibers. For multimode fiber systems, the narrow linewidth can reduce the chromatic dispersion penalty and increase the system reach. A single-mode VCSEL also allows the coupling into graded-index single-mode fiber, which is few-mode around 850 nm with high bandwidth, for few-mode transmission. We review recent progress and present new experimental results and modeling analyses of single-mode VCSEL transmission over both types of fibers. The experiments and analyses shed new light on how single-mode VCSELs can be used with multimode fibers and graded-index single-mode fibers and relative merits between 850 nm single-mode VCSELs versus 980 nm and 1060 nm single-mode VCSELs to address the needs of various applications.

A fast, accurate and simple field coupling model is presented which is capable of describing mode coupling effects due to bends and splices in multimode fibers with parabolic index profile as well as the coupling losses induced by this process. This model is validated numerically by comparing the results to the well-known coupled amplitude theory model yielding the same relative bandwidth increase behavior as long as the coupling losses are the same. It is shown, that the number of discrete segments used in this model can be reduced considerably as long as the coupling losses are kept constant. The effect of mode coupling on the differential group delay, mode dependent loss, bandwidth gain and impulse response width reduction are analyzed. It is shown that the relative bandwidth gain induced in MMF links induced by the coupling process is independent of fiber parameters or number of guided modes; it can be fully characterized by coupling induced losses. The model is compared to well-known results given by power coupling models and a good agreement is observed for high steady state loss values.

As data center interconnects surge towards a 1.6 Tbit/s data rate, achieving cost-effective and technically viable solutions present challenges. Intensity-modulation and direct-detection (IM/DD) transmission over O-Band using standard single-mode fiber has emerged as a promising low-cost option. However, understanding the limitations imposed by factors like chromatic dispersion (CD) and fiber non-linearity (FWM) is crucial, particularly in different scenarios, such as operating at 8 × 100 GBaud PAM4 in an LWDM-8 configuration. In this paper, we adopt a statistical approach to assess outage probability and consider practical fluctuations in link parameters. Numerical modeling suggests IM/DD can span distances up to 5 km with transmission power under 0 dBm using this architecture. In addition, we evaluate recently proposed architecture to achieve 800 Gbit/s and 1.6 Tbit/s using an LWDM4 configuration and assess the impact of FWM to understand the role of zero-dispersion wavelength (ZDW) of the fiber. Coherent transmission leverages more powerful signal processing capabilities which extends the transmission range. Yet, reducing coherent transmission complexity is desirable for cost-effective and power-efficient data center applications. By exploring dual wavelength transmission and DP-16 QAM transceivers, akin to IM/DD counterparts, the feasibility of streamlining this architecture is also studied. The analysis indicates that the complexity of the coherent approach can be reduced without significant penalties for distances up to 10 km.

The concept of mode-division-multiplexing (MDM) using principal modes as carriers is investigated. MDM transmission is studied for an exemplary MMF with three modes. A transmission rate of 3×40Gbit/s appears feasible using a 50km MMF with a bandwidth-length-product of 3 GHz·km.

Graded-index standard single-mode fiber offers few-mode operation around 850 nm with better modal bandwidth over step-index fiber. We investigated feasibility of VCSEL transmission through link model analysis and demonstrated 100 m VCSEL transmission at 25Gb/s.