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.

6 × 10 Gb/s polarization- and mode-multiplexed transmission, without MIMO equalization, is experimentally demonstrated over a 200-m elliptical core few-mode fiber with two PANDA stress rods. This fiber simultaneously breaks the degeneracies in space and polarization in a mode group. A polarization extinction ratio of <;-16 dB and loss <;0.63 dB/km were achieved.

Polarization-maintaining few-mode fibers (PM-FMFs) have found applications in sensors with attractive features that traditional single-mode fibers do not possess. We propose a measurement method for multi-parameter sensing based on PM-FMFs. This method is mainly based on the polarized interference effect of PM-FMF. We experimentally demonstrate the principle of the simultaneous sensing of temperature, strain and transverse force. The sensor has been verified by three groups of multi-parameter measurement experiments showing that the changes of multi-parameter derived from the wavelength shift of the polarized interference spectrum are consistent with the multi-parameter changes set in the experiments. The proposed PM-FMF based sensor has a temperature sensitivity of 0.3 nm/°C, a strain sensitivity of 0.01 nm/µε and a transverse force sensitivity of 0.0065 nm/(N/m).

We report the design of 12-LP-mode (21-spatial-mode) fiber with low differential mode group delay (DMGD), large effective area and low bending loss that is adapted to mode-division-multiplexing (MDM) system over the C+L band. Based on the designed fiber parameters, we characterize the few-mode fiber (FMF) with the DMGD, effective area and 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 μm2. 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.

We present a novel design of few-mode fiber (FMF) with a uniform differential mode group delay (U-DMGD) among propagating modes and apply the FMF to a single-fiber delay line module for implementing microwave photonic finite impulse response (FIR) filters. By optimizing key parameters such as the core grading exponent and the dimension of the trench of FMF, a U-DMGD between adjacent modes among four modes (LP01, LP11, LP02 and LP31) over the entire C band is achieved. Wavelength dependence is entirely removed. An FIR microwave photonic filter (MPF) implemented using the designed 1-km FMF is investigated through numerical simulations. The free spectral range (FSR) of the MPF is 5.7 GHz, the 3-dB bandwidth is 1.26 GHz, and the main lobe-to-side lobe ratio (MSR) is 10.42 dB. Discussions on fabrication aspects have also been presented. The proposed single-fiber delay line structure based on FMF can significantly reduce the system complexity of microwave photonic signal processing.

We experimentally demonstrate adaptive variable-forgetting-factor (VFF) recursive-least-square frequency-domain equalization (RLS-FDE) for mode-division multiplexing. The VFF-RLS-FDE algorithm improves convergence speed as transmission distance increases. For MDM transmission over a 1,000-km few-mode fiber, the convergence speed was increased by 18.7 times in comparison with LMS-FDE. In the meantime, the convergence error performance of VFF-RLS is also much superior to LMS while fixed-forgetting-factor RLS has a 0.2 dB penalty compared with LMS. The proposed VFF-RLS algorithm achieves better performances than conventional RLS in terms of convergence speed and convergence error.