Fig. 1. Output power trend of TDFLs at 2 μm^[20,24-33]

Fig. 2. Concept of TDF and its main limits. (a) Amplification process of TDF as a gain medium and its thermally-induced mode instability (TMI) at high power operation; (b) non-linear effects of TDF output laser such as amplified spontaneous emission (ASE), stimulated Brillouin scattering (SBS), and parasitic lasing

Fig. 3. Spectral properties of Tm³⁺ ions. (a) Energy level bands of Tm³⁺ ions^[35]; (b) absorption cross-section of Tm³⁺-doped fused silica around 790 nm in the ³H₄ level (red), and absorption cross-section of Tm³⁺-doped fused silica for inband pumping in the upper laser level ³F₄ (blue) together with the emission cross-section (black)^[40-41]

Fig. 4. Thermodynamic simulation comparison of TDF pumped at 793 and 1692 nm^[45]. (a) Experimental and simulated (solid lines) laser conversion efficiency at both pump wavelengths; (b) heat load (solid lines) and core temperature (dashed lines) evolution along the fiber for both pump wavelengths (assuming a surface water-cooled fiber to 20 ℃); (c) measured spectral power density at the highest output power with the 793 nm (red) and the 1692 nm pump (blue) wavelengths, respectively; (d) output beam quantity at the highest output power for both pump wavelengths

Fig. 5. TDFs based on germanosilicate glass system and their laser performance. (a) Schematic of the two-stage laser optical path of a single-mode polarisation-maintain germanosilicate glass TDF and its output laser characteristics ^[50]; (b) germanosilicate glass cladding-matched TDF and its laser output performances^[62]

Fig. 6. Relationship between doping concentration of Tm³⁺ in TDF and laser conversion efficiency^[35]

Fig. 7. Nonlinear suppression based on Al/Ge co-doped TDF. (a) Schematic of components in a pedestal fiber with Tm, Ge, and Al co-doping in the core^[75]; (b)‒(c) schematic of TDF system for SBS testing and its threshold value result^[32]; (d)‒(f) end-face, component distribution and output laser conversion efficiency for TDF^[77]

Fig. 8. Output laser performances of the step-index TDF. (a) End-face of 25/400 μm TDF^[80]; (b) schematic showing experimental setup of kW wavelength-tunable TDFL^[33]; (c) final output power versus the pump power at different wavelengths, where the insets show output power at 1980 nm recorded by a power meter and the maximum output power measured at different wavelengths^[33]; (d) output spectra of the wavelength-tunable MOPA system at maximum power level for different wavelengths^[33]; (e) evolution of laser spectrum versus output power at 1943 nm^[33]

Fig. 9. Nonlinear effects at high power output of the step-index TDF. (a) Schematic of the three-stage TDF amplifier^[22]; (b) laser output stability of the TDFL^[22]; (c) schematic illustrating the layout of the cladding-pumped TDFL and the test setup characterizing thermal mode instability (TMI)^[37]; (d) calculated relative intensity noise (RIN) for the different lengths of TDF^[37]; (e) beam quality obtained by measurement^[37]

Fig. 10. Structure design of pedestal TDFs. (a) Measured refractive-index profile of large mode area active and passive fibers with initial splice^[90]; (b) cross-section of TDF with different core and pedestal sizes^[88]; (c) effective overlap as a function of pedestal diameter for different core diameters^[88]; (d) fiber splicing with different angles^[32]; (e) backwards power measured versus output power, where a sharp rise in the backwards power is indicative of parasitic lasing here^[32]; (f) optical spectra of both splice recipes measured at nearly the same output power of 680 W^[32]

Fig. 11. Structures of nested-ring or M-type TDFs. (a) Schematic of refractive-index profile of M-type fibre^[92]; (b) effective refractive index of few modes at different wavelengths with their corresponding electric field intensity profile^[92]; (c) refractive index profile of the designed and fabricated nested-ring fiber preform with 8.2 μm diameter core^[93]; (d) TDF power curve of 62 W with slope efficiency values and the cross-sectional view of nested-ring TDF^[93]; (e) beam quality^[93]

Fig. 12. Photonic crystal TDFs. (a) Cross-section of photonic crystal TDF^[96]; (b) experimental setup of the high-power TDF chirped pulse amplification system^[31]; (c) measured average output power at 1960 nm centeral wavelength before (black crosses) and after (blue crosses) using the Treacy compressor versus launched pump power at 793 nm, where the red solid line represents the result of the simulated amplification process in a steady state^[31]; (d) schematic of the TDF chirped-pulse amplification system^[34]; (e) retrieved pulse and its corresponding Fourier limit^[34]; (f) beam quality ^{measurement of the laser at the highest output power[34]}