Fig. 1. Schematic of a multiwavelength all-FMF ring laser. LD, laser diode; MSC, mode-selective coupler; FMF-WDM, few-mode fiber wavelength division multiplexer; FM-EDF, few mode erbium-doped fiber; PD-ISO, polarization dependent isolator; PC, polarization controller; OSA, optical spectrum analyzer; CCD, charge-coupled device, infrared camera.

Fig. 2. Spectral response of an all-FMF Sagnac interferometer. Transmission spectra with different launch modes: (a) LP01 mode and (b) LP11 mode. (c) and (d) Transmission spectra from 1540 to 1550 nm.

Fig. 3. Spectra of multiwavelength operations when adjusting the PCs. Output spectra of successively tunable (a) single-, (b) dual-, (c) triple-, and (d) quadruple-wavelength lasing operations.

Fig. 4. Stability of triple-wavelength output spectra. (a) Repeated scans of the triple-wavelength output spectrum every two minutes. (b) Power fluctuation (black) and central wavelength (blue) of each channel.

Fig. 5. Near-field intensity distributions of the FMF laser. (a) Single-wavelength LP11 mode and (b) corresponding tunable OVBs when pressing the end of the FMF. (e) Triple-wavelength LP11 mode and (f) corresponding tunable OVBs. (c1)–(c4) and (g1)–(g4) Near-field intensity patterns with rotation of a polarizer. (d1)–(d4) and (h1)–(h4) Corresponding interference patterns. The white arrows indicate the polarization orientation.

Fig. 6. Schematic of Q-switched all-FMF ring laser. CNT-SA, carbon nanotube saturable absorber.

Fig. 7. (a) Output optical spectrum of a Q-switched fiber laser. (b) Typical Q-switched pulse train. (c) Single pulse envelope. (d) RF spectrum.

Fig. 8. (a) Average output power and pulse energy versus pump power. (b) Pulse duration and repetition rate versus pump power.

Fig. 9. Near-field intensity distribution of TM01 (top) and TE01 (bottom) modes with rotation of a polarizer.