1. Introduction

Tellurium oxide (TeO₂) based glasses possess some important advantages over other common fibre host glasses such as silicates, germanates and fluorides. Tellurite glass has a phonon energy of 600-800 cm^-1, which is significantly lower than that of both silicates (1100 cm^-1) and germanates (880 cm^-1) [1]. This extends the infra-red transparency range to ~5 µm [1] and results in low non-radiative rates, high radiative rates and long excited state lifetimes for rare earth dopants [2]. Tellurite glass also has high rare-earth ion solubility [3], enabling highly doped and hence very compact fibre lasers to be produced. The large refractive index (~2.05) of TeO₂ based glasses enhances both the absorption and emission cross sections [4,5]. Finally, tellurite glasses are more chemically and environmentally stable than fluoride glasses, and have the advantage of oxide glass fabrication techniques [1]. Thus far, lasing has been demonstrated at 1.06 µm [6], 1.6 µm [5], and 1.9 µm [7] for Nd³⁺, Er³⁺, and Tm³⁺ doped tellurite fibres, respectively.

Laser emission from Ho³⁺ at 2.1 µm has numerous biomedical [8] and sensing applications [9-11] and has been obtained from a number of different fibre laser systems. Silica has been the most commonly used host glass even though 2.1 µm lies at the edge of its transparency window; Ho³⁺ silica fibre lasers pumped either by Yb³⁺ silica fibre lasers emitting at 1.1 µm [12,13], Raman lasers operating at 1.2µm [14], or directly by diodes [15] have been demonstrated. To enhance pump absorption, Ho³⁺ silica fibres have been sensitized with either Tm³⁺ [16,17], or Yb³⁺, although the Yb³⁺ to Ho³⁺ energy transfer process proved to be inefficient [18]. Even though high powers, up to 83 W [17], have been achieved in double-clad fibres, the pump powers required to reach threshold have also been high, typically multi-Watt. This has been attributed to the high background losses in silica at 2.1 µm, which are exacerbated by the long fibre lengths necessitated by the low absorption coefficients of Ho³⁺ and Tm³⁺ in silica. Lower thresholds of 0.2-0.25 W for 2.1 µm lasing have been obtained for fluoride fibres doped with Ho³⁺ and Tm³⁺/Ho³⁺ [19,20] but, as mentioned above, fluoride glass suffers from poor chemical and environmental stability. In a single-clad Tm³⁺/Ho³⁺ doped silica fibre with a core diameter of 8.5 µm a threshold of 0.5 W was demonstrated when core pumped at 1064 nm; however, the output power was limited to just 11 mW [21].

Tellurite fibres may by sensitized to both diode-pumping at 0.98 µm and 0.94 µm, and to pumping by a Yb³⁺ silica fibre laser operating at 1.1 µm by doping with Yb³⁺. However, as in silica [18], Yb³⁺ to Ho³⁺ energy transfer efficiency is low in tellurite glass; a recent study measured it to be 17% [22]. In contrast, the energy transfer efficiency between Yb³⁺ and Tm³⁺ is nearly three times greater at 47% [4] and we have recently shown that energy transfer between Tm³⁺ and Ho³⁺ in tellurite is greater than 82% [24]. This suggests that sensitization by both Yb³⁺ and Tm³⁺ would provide a more efficient energy transfer route than sensitization by Yb³⁺ alone. Figure 1 shows the energy levels of the three ions and the relevant transitions for a 1.1 µm pumped system, including potentially deleterious processes such as excited state absorption (ESA) and up-conversion (UC). In this work, we report the fabrication and characterization of a 2.1 µm Yb³⁺/Tm³⁺/Ho³⁺ triply-doped tellurite fibre laser pumped by a Yb³⁺ silica fibre laser.

 

Fig. 1. Energy level diagram for the Tm³⁺/Yb³⁺/Ho³⁺-doped tellurite fibre. Ground state pump absorption (upward arrows marked GSA), energy transfer (curved arrows), excited state absorption (upward arrows marked ESA), non-radiative relaxation (dashed arrows), radiative transitions (down arrows) and upconversion cross relaxation (arrows marked UC) are shown.

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2. Experimental setup

The tellurite fibre was fabricated using the suction, rotation and rod-in-tube techniques [23]. The core composition was 80 TeO₂ – 10 ZnO – 10 Na₂O (mol%) doped with 1.5 wt% Yb₂O₃, 1.0 wt% Tm₂O₃, 1.0 wt% Ho₂O₃, and the cladding composition was 75 TeO₂ – 15 ZnO – 10 Na₂O (mol%). The core and cladding diameters were 7.5 ± 1 µm and 125 µm, respectively, the numerical aperture was 0.28, and the background loss of the fibre was measured using the cutback technique to be 1.2 dBm^-1 at 1476 nm, which is an improvement over the previous Yb³⁺/Tm³⁺-codoped tellurite fibre [25]. The pump wavelength of 1088 nm lies almost equally between the Tm³⁺: ³H₅ and Yb³⁺: ²F_5/2 absorption bands resulting in small absorption in both ions [25]. Figure 2 shows the absorption spectrum of a bulk sample of Yb³⁺/Tm³⁺/Ho³⁺ doped tellurite glass that has the same doping concentration as the fibre core. Despite the low absorption at the pump wavelength the long path lengths afforded by the fibre geometry allow sufficient absorption to achieve lasing.

Figure 3 shows the experimental set-up used for characterization of the tellurite fibre laser. The details of the diode pumped, Yb³⁺-doped silica fibre laser pump source can be found in Ref. 25. The multi-mode output from the Yb³⁺ fibre laser was collimated and then focused into the tellurite fibre using a 10× microscope objective. The optical cavity at the laser wavelength comprised a highly reflective mirror at the pump end and output couplers of varying reflectivity. To improve the laser performance the output couplers were also highly reflective at the pump wavelength in order to reflect the unabsorbed pump light back into the fibre. In this double pass cavity, it was calculated that >96% of the launched pump light was absorbed in the shortest fibre under test (~9 cm).

 

Fig. 2. Absorption spectrum for Yb³⁺ (1.5 wt%), Tm³⁺ (1.0 wt%), Ho³⁺ (1.0 wt%) doped tellurite glass.

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Fig. 3. Experimental set-up used to characterize the tellurite fibre laser.

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3. Experimental results

Table 1 lists the thresholds and slope efficiencies obtained with various output couplers and fibre lengths. Initial measurements with a 40% output coupler suggested an optimum fibre length of 20-25 cm, therefore experiments with other output couplers were conducted with fibres in this length range. However, it was found that for the 60% output coupler, a maximum slope efficiency was achieved with a fibre length of 17 cm, and this is shown in table 1. At 25%, the highest slope efficiency achieved is approximately half of the Stokes’ efficiency limit for this pumping scheme. Figure 4 shows the laser output power as a function of pump power for this cavity; the threshold was 15 mW and the highest output power achieved was 60 mW. The output power showed no signs of saturation and was limited by the available pump power. The highest ~2.1 µm output power achieved from a Tm³⁺/Ho³⁺ doped ZBLAN fibre laser when pumped at 803 nm was 8.8 W [20], and the improved thermal robustness of tellurite glass compared to ZBLAN glass [25] should enable output powers at the >10 W level to be realised from a tellurite fibre laser in the 2 µm spectral region.

In comparison, a Tm³⁺/Ho³⁺ doped silica fibre of length 23 cm and core diameter 8.5 µm had a threshold of 500 mW when core pumped using a 1064 nm Nd:YAG laser [21]. The cut-off wavelengths of the silica fibre and the tellurite fibre in this investigation are 2.33 µm and 2.74 µm, respectively, i.e. more than one mode is supported in both fibres at the pump and signal wavelengths. Thus both lasers were under operating very similar conditions and the factor of ~40 in minimum threshold between them demonstrates the advantages of tellurite glass at this wavelength. We attribute the reduced threshold largely to the lower absorption losses in the tellurite fibre at 2.1 µm.

Table 1. Output performance achieved for various output coupler reflectivities.

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Fig. 4. Laser output power with respect to launched pump power for a 17 cm long tellurite fibre and an output coupler reflectivity of 60%.

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Blue fluorescence was clearly observable when the fibre was pumped and was attributed to ESA populating the ¹G₄ level of Tm³⁺, as shown in figure 1. This process wastes pump excitation and is likely to be a factor in the improvement in laser performance between the Yb³⁺/Tm³⁺/Ho³⁺ triply-doped fibre described here and the Yb³⁺/Tm³⁺ doubly-doped fibre reported previously [25] because energy transfer from the ³F₄ level of Tm³⁺ to the ⁵I₇ level of Ho³⁺ reduces the number of Tm³⁺ ions available for ESA. Other possible factors responsible for the increased efficiency include the improved intrinsic fibre loss.

The output spectrum typically consisted of a small number of narrow (0.3–0.7 nm FWHM) peaks distributed over about 20 nm; a typical spectrum is shown in the inset of figure 5. This emission is attributed to the Ho³⁺: ⁵I₇→⁵I₈ transition in tellurite fibre [26]. The wavelength range detected for various output couplers is shown in figure 5 with longer wavelengths observed for higher reflectivities; increased ground-state reabsorption due to a higher cavity Q-factor favours longer wavelength oscillation [24].

 

Fig. 5. Wavelength ranges detected for different output coupler reflectivities. The fibre for the 11% output coupler reflectivity has a length of 24.7 cm and the others have a length of 42 cm. The inset shows a typical laser spectrum obtained with output coupler reflectance of 60%.

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4. Summary and conclusions

A 2.1 µm Tm³⁺/Ho³⁺/Yb³⁺ triply-doped tellurite fibre laser pumped using a 1.1µm Yb³⁺-doped silica fibre laser has been demonstrated. A maximum slope efficiency of 25% and an output power of 60 mW were obtained from a fibre just 17 cm in length. Moreover, the lowest laser threshold obtained was 12 mW, which is >40 times less than for a Tm³⁺/Ho³⁺ doped silica fibre lasers with similar pump scheme and fibre geometry. This threshold is low enough to allow pumping by single diodes and thereby enable the production of a compact and efficient source of 2.1 µm laser radiation for biomedical and sensing applications.