Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) have attracted great attention for capacity enhancement of short-reach applications such as optical interconnections, for which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components. In this paper, we firstly propose the design method for degenerate–mode-selective couplers (DMSC) with side-polishing processing. Three kinds of DMSCs for different degenerate linearly-polarized (LP) modes (LP ₁₁ , LP ₂₁ and LP ₃₁ ) in a weakly-coupled few-mode fiber (FMF) are designed. Then a pair of MMUX and MDEMUX consisting of cascaded mode-selective couplers (MSC) and DMSCs for all the 5 LP modes of the FMF are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low back-to-back IL lower than 5.3 dB for all the 5 modes. Finally, stable digital-signal-processing (DSP)-free 5-mode IM/DD MDM transmission utilizing conventional optical modules over 10-km FMF are successfully demonstrated. The proposed low-IL MMUX/MDEMUX is scalable to support more modes and can promote the practical application of MDM techniques.

Independent light propagation through one or multiple modes is commonly considered as a basic demand for mode manipulation in few-mode fiber (FMF)- or multimode fiber (MMF)-based optical systems such as transmission links, optical fiber lasers, or distributed optical fiber sensors. However, the insertion of doped-fiber amplifiers always kills the entire effort by inducing significant modal crosstalk. In this paper, we propose the design of doped-fiber amplifiers in FMF-based systems adopting identical multiple-ring-core (MRC) index profiles for both passive and doped fibers to achieve low modal crosstalk. We develop the direct-glass-transition (DGT) modified chemical vapor deposition (MCVD) processing for precise fabrication of few-mode erbium-doped fibers (FM-EDFs) with MRC profiles of both refractive index and erbium-ion doping distribution. Then, a few-mode erbium-doped-fiber amplifier (FM-EDFA) with a maximum gain of 26.08 dB and differential modal gain (DMG) of 2.3 dB is realized based on fabricated FM-EDF matched with a transmission FMF supporting four linearly polarized (LP) modes. With the insertion of the FM-EDFA, 60 + 60 km simultaneous LP₀₁/LP₁₁/LP₂₁/LP₀₂ transmission without inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP) is successfully demonstrated. The proposed design of low-modal-crosstalk doped-fiber amplifiers provides, to our knowledge, new insights into mode manipulation methods in various applications.

Spatial division multiplexing transmission over few-mode multicore fiber (FM-MCF) recently attracts great interests by simultaneously exploiting two more dimensions than conventional single mode fibers. In this paper, we propose an all-fiber spatial multiplexer (MUX) by cascading mode-selective fiber couplers (MSCs) with a fiber-bundle-type fan-in device, and spatial demultiplexer (DEMUX) by cascading a fiber-bundle-type fan-out device with degenerate-mode-selective fiber couplers and MSCs. Thanks to the low crosstalk of the FM-MCF, spatial MUX/DEMUX and their coupling, weakly-coupled 7-core-2-LP-mode real-time transmission over 1-km of FM-MCF is successfully demonstrated using 10-Gbps commercial enhanced small form-factor pluggable (SFP + ) transceivers.

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission is a promising candidate to enhance the capacity of short-reach applications such as optical interconnections, in which low-modal-crosstalk mode multiplexers/demultiplexers (MMUX/MDEMUX) are highly desired. In this paper, we firstly propose an all-fiber low-modal-crosstalk orthogonal combine reception scheme for degenerate linearly-polarized (LP) modes, in which signals in both degenerate modes are firstly demultiplexed into the LP₀₁ mode of single-mode fibers, and then are multiplexed into mutually orthogonal LP₀₁ and LP₁₁ modes of a two-mode fiber for simultaneous detection. Then a pair of 4-LP-mode MMUX/MDEMUX consisting of cascaded mode-selective couplers and orthogonal combiners are fabricated with side-polishing processing, which achieve low back-to-back modal crosstalk of lower than -18.51 dB and insertion loss of lower than 3.81 dB for all the 4 modes. Finally, a stable real-time 4 modes × 4λ × 10 Gb/s MDM-wavelength division multiplexing (WDM) transmission over 20-km few-mode fiber is experimentally demonstrated. The proposed scheme is scalable to support more modes and can pave the way to practical implementation of IM/DD MDM transmission applications.

Utilizing non-degenerate LP₀₁/LP₀₂/LP₀₃ modes in a weakly-coupled 10-LP-mode multiple-ring-core FMF and corresponding mode multiplexers/demultiplexers consisting of cascaded mode-selective fiber couplers, we experimentally demonstrate a real-time 300-km MDM transmission with commercial single-mode 200G OTN transceivers.

Weakly-coupled mode division multiplexing (MDM) techniques by suppressing modal crosstalk in the fiber link as much as possible to avoid intermodal multiple-input-multiple-output digital signal processing (MIMO-DSP) have attracted great interest for capacity enhancement of short-reach or even long-haul optical fiber transmission. However, the effective few-mode amplifier scheme has seldom been demonstrated for weakly-coupled MDM systems. In this paper, we firstly propose a few-mode distributed Raman amplifier (FM-DRA) scheme, in which broadband mode multiplexer/demultiplexer (MMUX/MDEMUX) consisting of cascaded all-fiber mode-selective couplers (MSCs) are adopted for simultaneous conversion of both signal and pump light so that weakly-coupled transmission condition will not be broken. Then, the proposed FM-DRA for the LP ₀₁ /LP ₂₁ /LP ₀₂ modes of 60-km few-mode fiber (FMF) supporting 4 LP modes is designed and fabricated with bi-directional Raman pumps. The achieved maximum on-off gain, mode dependent gain (MDG), and wavelength dependent gain (WDG) at the C-band for 60-km FMF transmission is 5.3, 0.6 and 0.7 dB, respectively. A recirculating-loop experimental system is established and weakly-coupled MDM transmission with dual-polarization differential quadrature phase-shift keying (DP-DQPSK) modulation and hybrid FM-DRAs/single-mode Erbium-doped fiber amplifiers (SM-EDFAs) is experimentally investigated for the first time. Simultaneous LP ₀₁ /LP ₂₁ /LP ₀₂ transmission over 360-km FMF is successfully demonstrated only adopting 2 × 2 or 4 × 4 MIMO-DSP. The proposed FM-DRA scheme is beneficial for practical applications of MDM techniques.