1. Introduction

Cladding-pumped Tm-doped fibre lasers operating in eye-safe 2-µm spectral region have attracted growing interest in recent years owing to their numerous applications in areas such as LIDAR and medicine [1, 2]. In contrast to conventional ‘bulk’ solid-state lasers, fibre-based sources benefit from a geometry that allows relatively simple thermal management and hence offer the prospect of higher output power and improved beam quality. A particular attraction of Tm-doped fibre lasers is the very broad transition linewidth offering the prospect of wide tunability over the ~1700–2100 nm regime. Direct pumping of double-clad Tm-doped silica fibre lasers with diode lasers at ~790 nm [3] and Tm-doped silica fibre lasers sensitized by co-doping with Yb at 975 nm [4] has been demonstrated with output powers up to 85 W and 75 W respectively. In the latter, the slope efficiency was rather low (~32%), due, in part, to the relatively high quantum defect heating (~51%), hence suggesting that power scaling may prove rather difficult via this scheme due to the relatively high fractional heat loading. For a tunable cavity configuration, a wavelength tuning range of 230nm from 1860–2090nm has been demonstrated at multi-watt power levels [5], with the tuning range at the short wavelength end limited by the relatively long device length that is typical for cladding-pumped configuration.. An alternative, and arguably more promising, approach for power scaling of Tm fibre lasers is to pump directly into the upper level manifold (³F₄) with laser sources at 1.55–1.62 µm. This pumping scheme has been demonstrated at low output power (<20mW) with slope efficiency as high as 71 % for Tm-doped silica fibres (pumped with an Er fiber laser [6]) and 84% for Tm-doped fluoride fibres (pumped with a colour centre laser [7]). This approach has the attraction of a very high Stokes efficiency (~0.75–0.85) and hence low quantum defect heating, opening up the prospect of very high lasing efficiencies and should facilitate further power scaling. Moreover, the good beam quality available from high-power Er-doped fibre lasers allows direct pumping into the of the Tm-doped fibre’s core (or into the inner-cladding of a Tm-doped double-clad fibre with a much smaller inner-cladding diameter than is typically required for direct diode pumping), leading to the possibility of much shorter device length. This is critically important for extending the tuning range of Tm fibre lasers to shorter wavelengths region. In this paper we report efficient operation of Tm-doped silica fibre lasers, pumped by a high-power cladding-pumped Er,Yb fibre laser at 1565nm, with much higher output power than previously reported using this pumping scheme. In addition, we have demonstrated the capability of widely tunable operation. In a cladding-pumped Tm fibre laser we obtained up to 19.2 W of output at 1991 nm with a slope efficiency of 69% with respect to absorbed pump power, and in a core-pumped laser we obtained a maximum output power of 12.1W at 1860 nm limited by available pump power. Using tunable cavity configurations, the core-pumped and cladding-pumped Tm fibre lasers could be tuned from 1723 to 1973 nm and from 1859 to 2061 nm at multi-watt power levels respectively, giving a combined wavelength tuning range of 338 nm. To the best of our knowledge this represents the broadest tuning range and shortest operating wavelength from a high-power Tm-doped fibre laser reported to date.

2. Experiments and results

The Er,Yb-doped fibre laser used in our experiments was constructed in-house and comprised ~3 m of double-clad fibre with a 30 µm diameter (0.22 NA) Er,Yb-doped phospho-silicate core surrounded by a 400 µm diameter D-shaped pure silica inner-cladding with a calculated NA of 0.49. Pump power was provided by two spatially-combined diode-stacks at 975 nm. A simple resonator configuration was employed with feedback for lasing provided by an external cavity, comprising a collimating lens and a high reflectivity plane mirror, at one end of the fibre and, at the opposite (pump launching) end by a simple perpendicularly-cleaved fibre facet acting as the output coupler. Laser output at 1.565 µm was extracted from the pump launching end with the aid of a dichroic mirror. With this arrangement, that laser generated up to 58 W of output at 1565 nm with a linewidth of ~2.6 nm (FWHM) and with a beam quality factor of M²=1.9. Further details of the Er,Yb-doped fiber laser pump source can be found in ref. 8. The Tm fibre used in these experiments had a Tm-doped alumino-silicate core of 20 µm diameter and 0.12 NA, surrounded by a pure silica D-shaped inner-cladding of 200 µm diameter and 0.49 NA (calculated), with a low refractive index polymer outercladding. The Tm-doping level was ~1.4wt.% and the effective absorption coefficient of the Tm-doped fibre for cladding-pumping at 1.565 µm was measured to be ~1.5 dB/m. A schematic diagram of cladding-pumped Tm fibre lasers for free-running and tunable operation is shown in Fig. 1. The 1565 nm output from the Er,Yb fibre laser was launched into the Tm:fibre with the aid of a 50 mm focal length lens with anti-reflection coatings at ~1.5 µm. The launch efficiency was estimated to be ~86%. Both end sections of the fibre were mounted in water-cooled V-groove heat sinks to prevent possible thermal damage to the fibre coating due to a small fraction of unlaunched pump power and by heat generated in the core due to quantum defect heating. The laser cavity for free-running operation [shown in Fig. 1(a)], was formed between the perpendicularly-cleaved fibre end-facet at the pump launch end of the fibre (which also served as the output coupler), and a simple external cavity comprising a plane dichroic mirror with high reflectivity at 1860–2180nm (>99.8%) and high transmission at 1530–1600 nm (>92%), and 50 mm focal length antireflection coated collimating and focussing lenses at the other end of the fibre. The unabsorbed pump power was monitored just behind the dichroic mirror. Another dichroic mirror with the same coating was used to extract the 2 µm output from the pump launch end of the fibre. The Tm fibre laser output power as a function of launched pump power for 2.6 m and 5 m fibres are shown in Fig. 2. The laser reached threshold at a launched pump power of 3.1 W for the 5 m fibre and produced an output power of 19.2 W at 1991 nm for a launched pump power of 38.2 W. The unabsorbed 1565 nm pump light was measured to ~6.7 W at the highest pump power of 38.2 W and the slope efficiency with respect to the absorbed pump power was ~69%. A lower output power of 15 W was generated at 1977 nm from the 2.6 m fibre due to lower pump absorption. The unabsorbed pump power was measured to reach 14 W for 38 W launched pump power and the slope efficiency with respect to the absorbed pump power was ~72%, which compares favourably with the upper theoretical limit of ~79% determined by the Stoke efficiency. It can be seen from Fig. 2, for both the 2.6 m and 5 m fibres, that the output power increases linearly with pump power suggesting there is scope for further power scaling by simply increasing the input pump power. Wavelength tuning was achieved by modifying the external cavity design to include an antireflection-coated (T>98% at 2 µm) Infrasil planoconvex collimating lens of 25 mm focal length, and a simple diffraction grating with 600 lines/mm in the Littrow configuration to provide wavelength selective feedback, as shown in Fig 1(b). The grating was blazed at wavelength of 1.9 µm and had measured reflectivities of 90% (polarised perpendicular to the grooves) and 70% (polarised parallel to the grooves) at 2 µm. In these experiments, a shorter length of fibre (~2.5 m) was employed and the unabsorbed pump light after a single-pass was retro-reflected, by an additional plane mirror with high reflectivity at 1500–1650 nm to improve the pump absorption efficiency. It is estimated that, with this set-up, ~65% of the unabsorbed pump power after the first pass was re-launched back into the fibre yielding an overall absorption efficiency of ~74%. The fibre-end nearest the grating was angle-polished at ~8° to suppress broadband feedback from the uncoated face that might otherwise compete with the wavelength-dependent feedback provided by the grating and thus restrict the tuning range. The external cavity in Fig. 1(b) was first optimized to maximise the output power for the free-running configuration by placing a plane mirror with high reflectivity at 2 µm mirror perpendicular to the collimated beam from the fibre end and reflecting the 2 µm light directly back into the fibre. This yielded a maximum output power of ~19 W for 38.2 W of launched pump power, which was almost the same as the power generated from the 5 m fibre in the configuration of Fig. 1(a). The corresponding slope efficiency with respect to launched pump power was ~55%. With the plane mirror tilted by a small angle (~10°) and the diffraction grating aligned in the Littrow configuration [as shown in Fig. 1(b)], we obtained a maximum output power of 17.4 W at 1941 nm for ~38 W of launched pump power (see inset of Fig. 3) with a launched threshold pump power of ~3 W and slope efficiency with respect to launched pump power of 50%. The lasing wavelength could be tuned over 202 nm from 1859 to 2061 nm and the output power exceeded 15W over a tuning range of 150 nm from ~1875 to 2025 nm (see Fig. 3) with an output linewidth (FWHM) of <0.5 nm. The power stability of the laser output was monitored with a high speed InGaAs detector (band width of 50 MHz) and a 100 MHz digital oscilloscope. No self-pulsing was observed and the short-term stability was measured to be <0.9% (RMS) on a time scale of 10 ms for operating wavelength <2010 nm. The laser became noisier at longer wavelength as described in Ref. [5]. The short-term stability was ~2% at 2025 nm and the output signal showed regular pulsing at ~2051 nm with a repetition rate of ~3.6 kHz and with amplitude fluctuations of ~45%.

 

Fig. 1. Schematic diagram of Tm-doped fibre laser cladding-pumped by a 1565 nm Er,Yb fibre laser, (a) free-running operation; (b) external cavity for tunable operation.

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Fig. 2. Output power of cladding-pumped Tm fibre laser versus launched pump power at 1565 nm.

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Fig. 3. Tunable Tm fibre laser output power versus operating wavelength for cladding pumped 2.6 m fibre. Inset: Output power at 1941 nm versus launched pump power.

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In the core pumped laser configurations [Figs. 4(a) and 4(b)], a much shorter length of fibre (~24 cm) was selected due to the high core absorption efficiency (~150dB/m). Pump light was launched into the core using an anti-reflection coated 25 mm focal length lens with high transmission at 2 µm (>98%) and a transmission of 91% at the pump wavelength of 1.565 µm. Feedback for laser oscillation was provided by the 3.6% Fresnel reflection from the perpendicularly-cleaved fibre end facet at the pump launch end of the fibre (which served as the output coupler), and, at the opposite end, by a dichroic mirror with high reflectivity at 1660–1980 nm (>99.8%) and high transmission at 1530–1570 nm (>80%) butted directly to the fibre end. A dichroic mirror with the same coating was used to extract the output from the pump beam at the pump launch end. The higher pump absorption efficiency for core pumping compared to cladding-pumping leads to a much higher thermal loading density, and hence more aggressive cooling of the fibre is needed. For this reason, in the core-pumped laser configuration, the entire length of the fibre was sandwiched between two water-cooled V-groove heat-sinks to facilitate heat removal and minimise the risk of damage. Only a small section of uncoated fibre (~4 mm long) was left protruding from the heat-sink at the pump incoupling end of the fibre to minimise the risk of damage to the outer-coating from stray pump light. The unabsorbed pump power was monitored at all times. Most of the unabsorbed pump results from light that is launched into the inner-cladding and not the core. For the non-tunable cavity configuration [shown in Fig. 4(a)], a maximum output power of 12.1 W at 1861 nm was generated for 23.1 W absorbed pump power (see Fig. 5) with a threshold pump power (absorbed) of ~2.5 W and a slope efficiency of 59% with respect to the absorbed pump power. It can be seen from this figure that the laser output power is essentially linear with respect to pump power up to the highest power even though the fibre had a very strong absorption of ~150dB/m. This means the maximum output power appears to be limited mainly by the available pump power and thermal effects do not appear to be an issue at this power level. We attribute this to both the low quantum defect heating of this pump scheme and the use of an active cooling arrangement for the fibre. It is worth noting that, with the pump launching scheme in this experiment, only ~55% of pump power that was incident on the fibre end facet was coupled into the core. Thus, a further increase in output power could be achieved by either improving the launching efficiency. Using larger core fibre or small cladding-to-core area ratio fibre design are the simplist options given the slightly multimode nature of the Er,Yb fibre pump laser.

 

Fig. 4. Schematic diagram of the Tm-doped fibre laser core-pumped by a 1565nm Er,Yb fiber laser, (a) free-running operation; (b) external cavity for tunable operation.

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Fig. 5. Output power of 24-cm core-pumped Tm fibre laser versus absorbed pump power at 1565 nm.

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For tunable operation, an external cavity configuration was employed, as shown in Fig. 4(b), comprising the same diffraction grating as was employed in the cladding-pumped laser and a 50 mm focal length collimating lens with anti-reflection coatings at 2 µm. A dichroic mirror with high reflectivity at 1.66–1.98 µm (>99.8%) and high transmission (>80%) at 1.53–1.57 µm was used to separate the 2 µm signal and the unabsorbed pump light in the external cavity. With this arrangement, we obtained up to 8.4 W of output at 1827 nm with a threshold pump power of 2.7 W (absorbed) and with a slope efficiency of 46% with respect to absorbed pump power (see inset of Fig. 6). The operating wavelength was tunable over 250 nm from 1723–1973 nm (see Fig. 6). The output signal was monitored with a high-speed photo-detector during the whole experiment and no self-pulsing was observed over the whole 250 nm tuning range at all power levels. The short-term stability was verified to be <1% (RMS) for all operating wavelengths. Combined with the cladding-pumping Tm fibre laser results, an overall tuning range of 338 nm from 1723 to 2061 nm has been achieved. To the best of our knowledge this represents the broadest tuning range and shortest operating wavelength from a high-power Tm-doped silica fibre laser reported to date.

 

Fig. 6. Tunable laser output power versus operating wavelength from a 24-cm core-pumped Tm fibre. Inset: Output power at 1827 nm versus absorbed 1565nm pump power.

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3. Conclusion

In conclusion, we have reported ultra-efficient and widely tunable Tm-doped silica fibre lasers in cladding-pumped and core-pumped configurations, pumped in-band by a 1565 nm Er,Yb fibre laser. Output powers up to 19.2 W at 1991 nm were generated from the cladding-pumped cavity configuration for ~38.2 W of launched pump power corresponding to a slope efficiency of 69% with respect to absorbed pump power. For the core-pumped configuration, a maximum output of 12.1 W was generated at 1851 nm for 23.1 W absorbed pump power using only ~ 24 cm of Tm-doped fibre. Wavelength tuning was realized by use of an external cavity containing a diffraction grating. A maximum output power of 17.4 W at 1941 nm was generated from cladding-pumped laser for 38.2W of launched pump power and the lasing wavelength could be tuned over 202 nm from 1859 to 2061 nm. The operating wavelength of the core-pumped Tm fibre laser could be tuned over 250 nm from 1723–1973 nm at multi-watt power levels. Further power-scaling should be possible by employing a higher power Er,Yb fiber laser at 1.57 µm as the pump source, or by using multiple Er,Yb fibre pump lasers in combination with a modified double-clad Tm fibre design with a small inner-cladding-to-core area ratio to accommodate more pump power.