Fig. 1. Design of low-modal-crosstalk few-mode-doped fiber with MRC profile in FMF-based system and the MCVD improvement for preform fabrication. (a) MRC-FMF-based system model consisting of mode control components, passive FMFs, doped FMFs, and other optical functional components. (b) Proposed DGT-MCVD processing for precise fabrication of FM-EDF with MRC profile.

Fig. 2. Realization and characterization of transmission MRC-FMF and FM-EDF. (a) Designed (blue line) and fabricated (magenta line) index profiles of transmission FMF at 1550 nm. The effective indexes (black line) of supported LP modes at 1550 nm for fabricated transmission FMF are also shown. (b) Designed (blue line) and fabricated (magenta line) index profiles of FM-EDF at 1550 nm. The effective indexes (black line) of LP01, LP11, LP21, and LP02 modes for fabricated FM-EDF are also shown. (c) Cross-section picture of the fabricated transmission FMF. (d) Cross-section picture of the fabricated FM-EDF. (e) Absorption spectrum of the fabricated FM-EDF. (f) Line scan results of the erbium-ion distributions in FM-EDF. (g) Simulative results of total modal crosstalk when light propagates from the fabricated transmission FMF to the fabricated FM-EDF. Each of the LP01, LP11a, LP11b, LP21a, LP21b, and LP02 modes is launched one by one.

Fig. 3. Realization and characterization of MMUX/MDEMUX and signal/pump WMUX. (a) Schematic structure of MMUX/MDEMUX consisting of cascaded MSCs. MSC, mode selective coupler. (b) Output mode fields of the MMUX at 1550 nm. (c) Output mode fields after 2.5-m FM-EDF at 1550 nm with pump power of 200 mW. (d) Schematic structure of the FM-WMUX for signal/pump combination.

Fig. 4. Characterization of FM-EDFA. (a) Experimental setup for characteristics measurement of the fabricated four-LP-mode FM-EDFA. MMUX, mode multiplexer; FM-WMUX, few-mode wavelength multiplexer; MDEMUX, mode demultiplexer; OSA, optical spectrum analyzer. (b) Picture of the experimental setup. (c) Measured modal crosstalk of a pair of MMUX/MDEMUX and FM-EDFA at 1550 nm. (d) Gains of the FM-EDFA with the input signal power of −15  dBm for each mode versus different pump powers. (e) Measured gain spectra at the C-band. (f) Measured NFs at the C-band.

Fig. 5. Experimental setup and results for the transmission performance of the proposed FM-EDFA. (a) Experimental setup for the transmission performance with the proposed FM-EDFA. ECL, external cavity laser; IQM, IQ modulator; AWG, arbitrary waveform generator; PM-OC, polarization-maintaining optical coupler; PBC, polarization beam combiner; OC, optical coupler; SM-EDFA, single-mode erbium-doped fiber amplifier; TDM controller, time-division-multiplexing controller; AOM, acousto-optic modulator; OBPF, optical band-pass filters; LO, local oscillator. (b) Distributed modal crosstalk coefficients h among all the LP modes of the transmission MRC-FMF. (c) Measured modal crosstalk of whole link at 1550 nm. (d) Q2-factors of simultaneous LP01/LP11/LP21/LP02 transmission with four wavelengths in each mode for both back-to-back and 60-km transmission cases. (e) Q2-factors of simultaneous LP01/LP11/LP21/LP02 transmission at C-band. (f) Q2-factors of simultaneous LP01/LP11/LP21/LP02 transmission with four wavelengths in each mode for both back-to-back and 120-km transmission cases. (g) Q2-factors of simultaneous LP01/LP11/LP21/LP02 transmission at C-band.