Fig. 1. Schematic of the symmetric TMF-OC.

Fig. 2. Mode effective indices of the LP01 and LP11 modes in the TMF versus different fiber diameters at the wavelength of 1550 nm; the inset represents the enlarged part with a tapered fiber diameter from 20 to 70 μm.

Fig. 3. (a) LP01 mode intensity distribution and (b) its power exchange in the coupling region; (c) LP11 mode intensity distribution and (d) its power exchange in the coupling region.

Fig. 4. (a) Relationship between the pulled length and the coupling efficiency; the inset represents the relationship between the pulled length and the diameter of the tapered fiber. (b) Output power from port 2 and port 3 of the TMF-OC. Microscopic images of the TMF-OC at the coupling region: (c) at the beginning of coupling and (d) when LP01 mode is observed from port 3 by a CCD camera.

Fig. 5. CCD images of the LP11 mode excited in the second TMF at different launching wavelengths.

Fig. 6. Schematic of the passively mode-locked all-fiber laser with a TMF-OC for CVB generation.

Fig. 7. (a) Mode-locked output spectrum and (b) corresponding pulse train at pump power of 150 mW.

Fig. 8. (a) RF spectrum of the mode-locked laser output measured with 100 Hz frequency span and 1 Hz RBW; inset is RF spectrum with span range of 1 GHz. (b) Measured autocorrelation trace and its sech2 fitting.

Fig. 9. Laser output power versus pump power (black curve from output 1 and red curve from output 2).

Fig. 10. Intensity distributions of (a) radial polarization beam and (f) azimuthal polarization beam without polarizer; (b)–(e) show the intensity distributions of the radial polarization beam after passing a linear polarizer; (g)–(j) show the intensity distributions of the azimuthal polarization beam after passing a linear polarizer. Arrows indicate the orientations of the linear polarizer.