We report the design of 12-LP-mode (21-spatial-mode) fiber with a low differential mode group delay (DMGD), a large effective area, and a low bending loss that is adapted to a mode-division-multiplexing system over the C+L band. Based on the designed fiber parameters, we characterize the few-mode fiber (FMF) with the DMGD, an effective area, and a bending loss. Over the C+L band, the maximum DMGD is 0.106 ps/m, and the effective area is in the range of 150∼485 μm 2. The bending loss of the designed FMF reduces to zero when the bending radius is greater than 9 mm, and the DMGD is below 0.0413 ps/m at the wavelength of 1550 nm.

This paper provides an overview of latest progress on the novel advanced digital signal processing (DSP) techniques for long-haul mode division multiplexing (MDM) systems with high capacity. Space-division multiplexing (SDM) techniques have been developed for a period to increase the capacity of optical communication system by at least one order of magnitude through MDM techniques using few-mode fibers (FMFs) or multi-core multiplexing (MCM) using multi-core fibers (MCFs). The signals in MDM links are mainly impaired by the linear and nonlinear effects in FMFs, making DSP techniques become necessary to undo these impairments. In this paper, we not only review the advanced multiple-input multiple-output (MIMO) DSP techniques for compensating linear impairments in FMFs, but also enclose the state of the art of novel DSP techniques to deal with nonlinear effects. Firstly, we introduce the MIMO schemes for equalizing modal crosstalk and modal dispersion. Then, we focus on the fast tracking of time-varying (TV) channels in FMF links through frequency-domain (FD) recursive least square (RLS) algorithm. Besides, we also cover the mainstream DSP solutions for mode-dependent loss (MDL) and several possible methods to compensate nonlinearity in FMF. Moreover, artificial intelligence (AI) technologies are also discussed for its high nonlinearity tolerance and may bring a revolution in MDM systems on the process of channel equalization, link monitoring, etc. In the end, a brief conclusion and perspective will be provided.

Using principal modes (PMs) can avoid the inter-mode crosstalk in fiber-optic communication and reduce the complexity of digital signal processing. In this paper, a new detection method for PMs based on spatially and spectrally resolved imaging (S 2) is used to recover their intensity distributions and inter-mode dispersion parameters. This method collects the optical interference information on a two-dimensional plane at different frequencies. The collected data can be used to characterize the principal modes, including their patterns and differential mode group delays. Due to the frequency invariance of PMs even in mode coupling state, which is studied carefully, this proposed method is shown to be robust. Analyses based on a four-mode fiber show that the distributed fluctuation of the coupling coefficients break the frequency invariance of PMs. We experimentally measure the mode characteristics of a four-mode fiber with S 2 method. The results show that the mode group delays of degenerate modes can be separated in mode coupling state, which is exactly appropriate for the modified measurement method.

The delay-bandwidth product in double-ring resonators (DRRs) is optimized using reinforcement learning. Then, the optimized DRRs are used to build an all-optical reservoir for optical packet header recognition, enabling a word-error rate as low as 9×10-4.