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Laser sources operating in the eyesafe wavelength regime around 1.5-1.6 µm have applications in a number of areas including, remote sensing, ranging and free-space communications. For many of these applications, the requirement for high output power is often supplemented by the need for high efficiency and good beam quality. This combination of operating characteristics is very difficult to achieve in conventional diode-pumped solid-state lasers based on erbium doped crystals (sensitised with Yb) owing to the relatively high fractional heat loading which results from the large quantum defect (~ 40%) and energy-transfer-upconversion. To alleviate this problem, attention has recently turned to singly-doped crystals (e.g. Er:YAG) and in-band pumping using an Er,Yb fibre laser. This approach has the advantage that most of the waste heat is generated in the fibre, which is largely immune to thermal effects, and quantum defect heating in the Er-doped crystal is very small (~7%). Using this hybrid laser scheme, we have demonstrated ~60 W of continuous-wave output from an Er:YAG laser at 1645 nm with a slope efficiency of 80 % 1 . However, for some remote sensing applications this operating wavelength is a little inconvenient, since there are atmospheric absorption lines due to methane which are in very close proximity necessitating careful selection and control of the lasing wavelength. Er:YAG also has a transition from the same upper level manifold ( 4 I 13/2 ) at 1617 nm, which lies in a region of the spectrum where there are no atmospheric absorption lies. However, this transition has a much more pronounced threelevel character and hence a much higher threshold pump power, so it has received little attention in spite of its obvious advantages for certain applications. Here, we report preliminary results for power scaling of an Er:YAG laser at 1617 nm in-band pumped by a high-power cladding-pumped Er,Yb fibre laser.The Er,Yb fibre pump laser was constructed in-house with wavelength selection provided by an external cavity containing a diffraction grating in the Littrow configuration. The Er,Yb fibre laser was pumped by two 9-bar pump modules at 975 nm and produced up to 120 W of output in a beam with M 2 ≈ 5. For efficient pumping of Er:YAG, the Er,Yb fibre laser operating wavelength was tuned to the absorption peak at 1532 nm. In this preliminary study, a simple four-mirror folded cavity was employed for the Er:YAG laser comprising a 29 mm long Er:YAG rod with 0.5 at.% Er 3+ concentration mounted in a water-cooled aluminium heat-sink, a plane pump in-coupling mirror with high reflectivity at 1617 nm and high transmission at 1532 nm, two concave mirrors of 100 mm radius of curvature and high reflectivity at 1617nm, a plane output coupler of transmission, 30% at 1617 nm, and a 100 µm thick fused silica etalon. The latter was used to select the 1617 nm line and suppress oscillation on the higher gain 1645 nm line. Figure 1 shows the Er:YAG output power at 1617 nm as a function of pump power. The Er:YAG laser yielded a ...

A simple analytical expression for threshold pump power in an end-pumped quasi-three-level solid-state laser, which takes into account the influence of energy-transfer-upconversion (ETU), is derived. This expression indicates that threshold pump power can be increased dramatically by ETU, especially in low gain lasers and lasers with pronounced three-level character due to the need for high excitation densities in the upper manifold to reach threshold. The analysis has been applied to an Er:YAG laser operating at 1645 nm in-band pumped by an Er,Yb fiber laser at 1532 nm. Predicted values for threshold pump power as a function of erbium doping concentration are in very good agreement with measured values. The results indicate that very low erbium doping levels (approximately 0.25 at.% or less) are required to avoid degradation in performance due to ETU even under continuous-wave lasing conditions in Er:YAG.

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A simple method for conditioning the pump beam in an end-pumped solid-state laser to allow direct excitation of the first order Laguerre-Gaussian doughnut (LG01) mode is reported. This approach has been applied to a hybrid (fiber-laser-pumped) Er:YAG laser yielding 13.1 W of continuous-wave output at 1645 nm in a radially-polarized LG01 doughnut beam with beam propagation factor (M(2)) < 2.4 for 34 W of incident pump power at 1532 nm. The corresponding slope efficiency with respect to incident pump power was 48%. The prospects of further power scaling and improved laser performance are discussed.