Fig. 1. Partial energy-level diagram and transitions of Tm³⁺ ions in silica (MPR, multi-phonon relaxation; CR, cross-relaxation; ETU, energy transfer upconversion).

Fig. 2. Experimental setup of the TDFL. TDF, thulium-doped fiber; FBG, fiber Bragg grating; PM, power meter; PD, photodetector; OSA, optical spectrum analyzer; L1, L2, antireflection-coated collimating lens; M1, short-pass filter, with cut-off wavelength 1600 nm; M2, long-pass filter, with cut-on wavelength 800 nm. A, B, C, D, E, F, G and H, reference positions for measurements. Inset: microscope image of the TDF (130 μm diameter corner-to-corner).

Fig. 3. (a) Measured white light (‘cold’) cladding transmission spectrum in 15 m of TDF (RBW: 0.1 nm). (b) Combined QCW spectrum of all five 0.79 μm diode lasers at full pump power.

Fig. 4. QCW instantaneous output power combined from both ends of the TDFL versus absorbed instantaneous pump power. Blue dots, power at 1.9 nm; red triangles, power at 0.82 μm; black squares, total output power. Inset: enlarged view of the behavior near the threshold.

Fig. 5. QCW measurements. (a) Optical spectrum at different 0.82 μm output power (RBW: 0.2 nm). (b) 20 dB spectral width versus 0.82 μm output power. (c) Time traces of the laser output at 0.82 and 1.9 μm of the 0.79 μm pump at full power. (d) Enlarged view showing the relaxation oscillation process.

Fig. 6. Measured 0.82 μm beam radius in the orthogonal directions at different longitudinal positions at approximately 250 W of instantaneous power. Inset: synthetic two-dimensional (2D) 0.82 μm beam profile as reconstructed by the scanning-slit beam profiler.

Fig. 7. Measured output spectra of the wavelength-tunable QCW TDFL at over 100 W of instantaneous output power in the 0.8 μm band (RBW: 0.5 nm).

Fig. 8. Schematic of a CW TDFL with two perpendicularly cleaved ends.

Fig. 9. Left axis: individual diode laser spectra. Right axis: cladding absorption spectrum of the TDF (RBW: 0.1 nm).

Fig. 10. (a) CW output power combined from both ends of the TDFL versus absorbed pump power. Blue dots, power at 1.9 nm; red triangles, power at 0.82 μm; black squares, total output power. (b) Conversion efficiency of 0.82 μm with respect to absorbed pump power versus 0.82 μm laser output power.

Fig. 11. CW output spectrum at full power (RBW: 2 nm).

Fig. 12. CW pump leakage versus launched pump power.