Fig. 1. Experimental setup. TL, tunable lase; DL, delay; VOA, variable optical attenuator; FPC, fiber polarization controller; ISO, isolator; MSPL, mode selective photonic lantern; FMF, few-mode fiber; HWM, homemade wavelength multiplexer; FM-EDF, few-mode erbium-doped fiber; FM, flip mirror; BPF, bandpass filter; CCD, coupled charge device; QWP, quarter-wave plate; HWP, half-wave plate; SLM, spatial light modulator; SMF, single-mode fiber; OSA, optical spectrum analyzer; f1, f2, and f3, lenses with focal lengths of 11 mm, 300 mm, and 3.1 mm, respectively; DM, dichroic mirror. The inset shows the configuration of the HWM, which is sealed in a tube with the diameter and length of 5.5 and 38 mm. The dashed lines denote the free-space transmission.

Fig. 2. (a) Cross section of the erbium ions and (b) the profile of the relative refractive index and erbium ion concentration in the FM-EDF.

Fig. 3. Modal patterns of the 980 nm pump at the (a) input and (b) output of the FM-EDF, respectively.

Fig. 4. Modal patterns of the signals in different spatial modes measured at the (a) input and (b) output ports of the FM-EDFA, respectively. The wavelength of the signal is 1550 nm.

Fig. 5. (a) Modal gains and (b) NFs for the signal in different spatial modes as a function of the pump power. The results are corrected by taking the crosstalk effect into account. In the measurement, the signal wavelength is 1550 nm.

Fig. 6. (a) Modal gains and (b) NFs for the signal in different spatial modes as a function of signal wavelength. The results are corrected by taking the crosstalk effect into account. In the experiment, the pump power is fixed at 550 mW.