Study on the dopant concentration ratio in thulium-holmium doped silica fibers for lasing at 2.1µm

Abstract

We present the fabrication and laser performance of thulium-holmium co-doped silica fibers when cladding pumped at ∼790 nm. By using the hybrid gas phase-solution doping process in conjunction with the MCVD preform fabrication technique, the doping concentration and the Tm:Ho ratio were varied to study the energy transfer efficiency from Tm 3+ to Ho 3+. Our study indicates that for a thulium concentration that has resulted in an efficient two-for-one crossrelaxation process with 790 nm pumping, and while maintaining a Tm:Ho concentration ratio in the range ∼ 10 to 20, the energy transfer efficiency has reached above 75%. In a free-running laser cavity, the pump power limited laser output of 38W with a slope efficiency of 56% at 2.1 microns is demonstrated.

Tm3+/Ho3+ profiled co-doped core area optical fiber for emission in the range of 1.6–2.1 µm

“…Therefore, the short-length rare-earths doped fibers used for lasers and amplifiers working with single-mode regime constructions often require additional cladding modes strippers (e.g. high refractive index coatings or fiber coiling) to obtain the best possible LP 01 beam quality (M 2 ≈ 1) 42 . The significant change in the fraction of power in the fiber core propagated can be noticed vs. the propagation wavelength and only 20% of optical power (weak guidance) will be propagated in the fiber core for 2100 nm (Fig.…”

“…However, both the results of numerical calculations and measurements confirm the possibility of shaping the luminescence profile by selecting the length of the optical fiber. The Tm 3+ /Ho 3+ co-doped fibers are often investigated as fiber laser constructions in the range of 1.7–1.8 µm 41 and 2.0–2.1 µm obtaining tenths of watts output power 42 . The wideband emission-designed fibers were also investigated and demonstrated for ASE sources for flat spectral power density in the range from 1527 to 2171 nm (10 dB bandwidth) 43 .…”

Emission Wavelength Limits of a Continuous-Wave Thulium-Doped Fiber Laser Source Operating at 1.94 µm, 2.09 µm or 2.12 µm

“…The use of commercial 793 nm diodes to pump holmium-doped fibers is possible, although by co-doping holmium fibers with thulium ions. A 1:10 concentration ratio between holmium and thulium ions has been identified as favoring the thulium-to-holmium energy transfer in a fiber laser source that can then be directly pumped by 793 nm pump diodes, analogously to only thulium-doped fiber laser sources [35]. Thulium-holmium co-doped fiber lasers are currently in full expansion, essentially in single-oscillator architectures (green dots in Figure 2) [35][36][37][38][39][40].…”

“…A 1:10 concentration ratio between holmium and thulium ions has been identified as favoring the thulium-to-holmium energy transfer in a fiber laser source that can then be directly pumped by 793 nm pump diodes, analogously to only thulium-doped fiber laser sources [35]. Thulium-holmium co-doped fiber lasers are currently in full expansion, essentially in single-oscillator architectures (green dots in Figure 2) [35][36][37][38][39][40]. The two highest output powers reported in the literature were 262 W from a free-space pumping system [40] and 195 W from the only monolithic all-fiber system ever developed [39].…”

“…The two highest output powers reported in the literature were 262 W from a free-space pumping system [40] and 195 W from the only monolithic all-fiber system ever developed [39]. 62 % [28] 57 % [28] 51 % [26] 60 % [34] 55 % [32] 49 % [40] 57 % [27] Tm 49 % [10] 52 % [22] 54 % [21] 49 % [13] 50 % [24] 60 % [8] 54 % [23] 57 % [20] 50 % [18] 50 % [12] 50 % [16] 72 % [14] 62 % [11] 53 % [17] 50 % [9] 62 % [10] 58 % [25] 34 % [38] 47 % [19] 35 % [36] 42 % [35] 45 % [39] 41 % [30] 38 % [31] 49 % [33] 56 % [37] 29 % [40] Figure 2. Comparison of the maximum output power of 2 µm fiber lasers in continuous wave regime demonstrated in the literature, as a function of their emission wavelength.…”