Fig. 1. Experimental setup of the digital fiber amplifier. AMP, amplifier; PM, polarization maintaining; SMF, single-mode fiber; FMF, few-mode fiber; PBS, polarization beam splitter; DM, dichroic mirror; BS, beam splitter; PC, polarization controller; M, mirror; λ/2, half-wave plate; ISO, isolator.

Fig. 2. Scalar LP modes. (a) Examples of input and output beam profiles without input wavefront shaping. (b), (c) Modal decomposition of (b) the unshaped LP01 mode and (c) the optimized LP01 mode. (d) Measured intensity profiles of the different orders of output LP mode after input wavefront shaping. (e) Measured output intensity profiles of the rotated LP11 mode.

Fig. 3. Cylindrical vector modes. (a) Theoretical (first column) and experimentally measured (second column) far-field intensity distributions of doughnut-shaped output vector modes and the intensity profiles of the corresponding mode passing through a linear polarizer at different orientations (white arrows) as a verification of the vector polarization state. (b) Average output power of the TM01 and EH11e modes versus the launched pump power. (c) The measured temporal pulse shape having a pulse duration (FWHM) of ∼150  ps. (d), (e) Examples of vector modal decomposition analysis for (d) the TM01 mode and (e) the EH11e mode, showing that >91% of power is contained within the desired vector.

Fig. 4. OAM modes. (a)–(d) The measured intensity profiles of the linearly polarized OAM output beams with topological charge of (a) l=−1, (b) l=1, (c) l=2, and (d) l=−2, respectively. The insets show the orthogonal components of the horizontal and vertical polarization states, indicating that the linear polarization dominates the output beams. The different spiral interference patterns indicate the output beams have the desired topological charges of the OAM state. (e), (f) Examples of modal decompositions results show that >90% of the power is contained in the desired OAM mode of (e) |V,1⟩ and (f) |H,2⟩.

Fig. 5. Modal stability measurement. (a) Measured modal purity of the EH11e mode at a fixed output power of ∼11  W as a function of time, indicating that a stable vector mode was generated and preserved over 60 min. (b)–(d) Mode evolution versus output power: the measured intensity profile of the optimized EH11e mode at (b) 11 W; the intensity profile of the output beam when the output power was decreased to (c) 7 W and (d) 2 W without changing the input wavefront. (e) The intensity profile of the output beam at 2 W was restored to a doughnut shape after the input wavefront was re-shaped.