Fig. 1. Simulation and experimental results of the AIFGs based on the FMF and RCF. (a) The phase-matching curves of CV modes (TE01, HE21, and TM01) for the FMF and group refractive index difference between LP01 and LP11 modes. The inset figure shows the RI profile of the FMF. (b) The transmission spectrum of a BAIFG when f=2.291  MHz. The illustration represents the pattern of the transmitted mode. (c) The PMCs for the RCF. The inset is the RI profile. (d) The transmission spectrum of the AIFG made of an RCF when f=303  kHz. The illustrations show the patterns of CV modes.

Fig. 2. (a) The PMCs for the FMF with a diameter of 34 μm. (b) The beat length difference of CV modes and the central DTP wavelength with different cladding diameters. (c) The transmission spectra of the BAIFG using different RF frequencies. (d) The relationship between dual-resonant dip wavelengths and frequencies with different diameters.

Fig. 3. Experimental setup for the CVB generation based on the BAIFG. (a) Polarized projection of the CVBs. (b) OAM detection using self-interference. TSL, tunable semiconductor laser; PC, polarization controller; SMF, single-mode fiber; FMF, few-mode fiber; RF, radio frequency; L, focus lens; P, polarizer; QWP, quarter-wave plate; M1, M2, mirrors; BS, beam splitter; CCD, charge-coupled device.

Fig. 4. Mode patterns and polarization distributions of the HE21mixed mode (a) at 1490–1590 nm with f=730  kHz, (b) at 1550 nm with different frequencies. (c) Mode patterns and polarization distributions of a single HE21even or HE21odd mode. (d) Intensity distributions and self-interference patterns of vortex beams by selecting the OAM±1 components of the HE21mixed mode.

Fig. 5. (a) The measured light in the HE21mixed mode with polarized intensity distributions and their reconstructed patterns by using the DL-based SPGD algorithm. Modal weights of the HE21mixed mode generated at (b) different wavelengths and (c) RF frequencies. (d) Modal weights of HE21even and HE21odd modes at optimized input polarizations.

Fig. 6. (a) The experimental setup based on a mode-locked fiber laser for generating femtosecond pulses in HOMs and amplifying the output power. WDM, wavelength-division multiplexer; EDF, erbium-doped fiber; DCF, dispersion compensating fiber; NPS, nonreciprocal phase shifter; OC, optical coupler; ISO, isolator. (b) Comparison of output spectra of the LP01 mode and OAM mode with the transmission spectrum of a BAIFG. The insets are mode patterns of LP01 and OAM modes. The results of the ML fiber laser include (c) output power from the CW state to ML state, (d) RF spectrum (inset: pulse trains), and (e) auto-correlation trace.

Fig. 7. (a) Mode patterns and polarization distributions of three CV modes under different polarizations. (b) Predicted modal weights of the above CV modes.

Fig. 8. (a) Measured spectra of seed and amplified LP11 mode. (b) The output power of LP01, LP11, and OAM modes varies with the pump power. (c) Mode patterns of LP11 and OAM modes with different pump powers. (d) Intensity distributions and self-interference patterns of vortex beams with the pump power of 200 mW.

Fig. 9. (a) PMCs of the TMF with diameters of 125 μm and 30 μm in the 1.0 μm waveband. (b) Transmission spectra of the BAIFG with a fiber diameter of 30 μm.

Fig. 10. (a) The PMCs of the RCF (r1/r2=3.5/7.5  μm, Δ=0.0034) when the DTP is at 2025 nm. (b) The PMCs of the designed RCF (r1/r2=3.5/6  μm, Δ=0.0034) when the DTP is at 1550 nm. (c) The structure of the RCF when the DTP is at 1550 nm and the corresponding beat length difference. (d) The PMCs of the RCF with r1 of 0.5 μm. The red dashed boxes are the DTP wavelengths of CV modes, and the illustrations show the simulated electric field distributions of CV modes.