1. Introduction

High-power solid-state laser sources operating in the eyesafe wavelength regime around ~1.5–1.6 µm have numerous applications in areas such as remote sensing, range finding, and free space and satellite communications. The traditional approach for producing laser output in this wavelength region is via direct diode pumping of erbium-ytterbium co-doped bulk glass or crystal lasers [1]. Power scaling of such lasers has proved rather difficult due to the high thermal loading density which results from a large quantum defect and the need for relatively high active ion concentrations. The situation is further exacerbated by energy-transfer-upconversion which leads to increased heat generation. For many of these applications, the requirement for high output power is also accompanied by the need of high efficiency and good beam quality, which are often difficult to achieve in conventional ‘bulk’ Er-doped solid-state laser owing to high thermal loading. Moreover, the combination of relatively narrow emission linewidths and rather low gains that are typical in conventional solid-state lasers restricts the range of operating wavelengths and hence further limits their applicability. Cladding-pumping of Er,Yb-doped fiber lasers (EYDFLs) offers a promising route to output in this spectral region [2] with the attraction of a geometry that has a high degree of immunity from the effects of thermal loading, and allows scaling to high power levels without decreasing efficiency or degrading beam quality [3]. Moreover, the broad emission linewidths that are typical in glass hosts allows for flexibility in the operating wavelength. To date, operation of an EYDFL with 103 W of continuous-wave output power at 1.57 µm has been reported for free-running cavity configuration (i.e. with no wavelength selection) with a slope efficiency of 30% with respect to launched pump power [4]. Tunable operation of an EYDFL has been reported with output powers up 30 W [5] and 43 W [6] cladding-pumped by 940 nm and 975 nm diode lasers respectively. In this paper we report a highly efficient Er³⁺-Yb³⁺ co-doped fibre laser, cladding pumped by a 975 nm diode-stack source, that generates up to 188 W of continuous-wave (cw) output at 1565 nm with an overall slope efficiency of 41% with respect to launched pump power. We have also demonstrated tunable operation by use of an external cavity containing a diffraction grating achieving a maximum output power of 108 W at 1538 nm for a launched pump power of ~336 W. The operating wavelength could be tuned over 36 nm from 1532 to 1568 nm at output power levels >100 W and with a linewidth of ~1 nm.

2. Experiments and results

The double-clad Er,Yb co-doped fibre (EYDF) used in our experiments was pulled from a perform fabricated in-house using the standard modified chemical vapour deposition (MCVD) and the solution doping technique [7]. The fibre had an Er,Yb-doped phospho-silicate core of 30 µm diameter and 0.22 NA, surrounded by a pure silica D-shaped inner-cladding of 400 µm diameter (~360 µm along the short axis). The latter was coated with a low refractive index (n=1.375) UV curable polymer outer-cladding to produce a high numerical aperture (~0.4 NA) waveguide for the pump and hence to facilitate efficient launch of pump light from high-power (but low-brightness) diode sources. Pump power was provided by two 975 nm diode-stacks which were spatially-combined by inter-leaving their output beams using a slotted-mirror beam-combiner. The resulting beam was then slit into two beams which were subsequently polarization-combined to produce a single beam of relatively high-brightness with beam propagation factors of ${\mathrm{M}}_{\mathrm{y}}^2$~150 (in the stacking direction) and ${\mathrm{M}}_{\mathrm{x}}^2$~200 (in the orthogonal direction), and with a maximum power of ~700 W. The resulting beam was then split into two beams of roughly equal power and with ${\mathrm{M}}_{\mathrm{y}}^2$~150 and ${\mathrm{M}}_{\mathrm{x}}^2$~100 allowing pumping of the EYDFL from both ends, as shown in Fig. 1(a). Pump light was launched into opposite ends of the fiber with the aid of anti-reflection coated lenses of 25 mm focal length and dichroic mirrors with high reflectivity (>99.5% at 45°) at the pump wavelength, and high transmission (>98%) at 1530-1570 nm to allow efficient extraction of the EYDFL output. With this pump arrangement, the launch efficiency into the fiber was estimated to be ~80%. The effective absorption coefficient for the fiber for pump light centered 975 nm was measured to be ~ 6.9dB/m, and hence a relatively short fiber length of ~3 m was selected for the free-running EYDFL. The combination of this short device length and the relatively large core diameter of 30 µm are important for preventing self-pulsing and avoiding detrimental nonlinear process that would otherwise lead to a roll-over in output power or damage at the fiber end facets. Both end sections of the fiber were carefully mounted in water-cooled V-groove heat sinks to prevent thermal damage to the fiber coating due to unlaunched pump power and by heat generated in the core due to quantum defect heating. Feedback for laser oscillation was provided by the 3.6% Fresnel reflection from a perpendicularly-cleaved fiber end facet, at one end of the fibre, and, at the opposite end, by a simple external cavity comprising a plane mirror with high reflectivity (>99.5%) at 1.5-1.65 µm and high transmission (>95%) at 940–970 nm, and anti-reflection coated 50 mm focal length collimating and focusing lenses.

 

Fig. 1. Schematic diagram of cladding pumped Er,Yb:fiber laser: (a) free-running configuration; (b) external cavity of the tunable fibre laser.

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Dichroic mirrors with high reflectivity at 1 µm and high transmission at 975 nm were inserted into the two pump arms to prevent any ~1-µm radiation, due to parasitic lasing on the Yb³⁺transition, from being fed back to the diode-stacks. The laser output was collimated with a 50 mm focal length lens with high transmission at 1.5-1.65 µm. Using this arrangement the laser reached threshold at a combined incident pump power of ~3 W and generated a maximum output power of 159 W at 1565 nm with a linewidth of ~2.6 nm (FWHM) for a total incident pump power of 582 W (466 W launched), corresponding to an average slope efficiency (with respect to launched power) of 34.1% (see Fig. 2). The laser had a higher slope efficiency of 37.3% at low pump powers (<220 W) which decreased to 31.2% at high pump power due to the onset of parasitic lasing on the Yb³⁺ transition at ~1µm. The feedback for 1 µm lasing was provided by the two perpendicularly-cleaved fiber end facets and the combined output at 1.065 µm was measured to be <28 W. The beam quality factor (M²) for the EYDFL was measured to be 1.9, which was a little better than expected given that the core has a V value of ~ 13. The power stability of the laser output was monitored with a high speed InGaAs detector (bandwidth of 50 MHz) and a 100 MHz digital oscilloscope and no self-pulsing was observed at all power levels. Removing the high reflectivity mirror from the external cavity, so that feedback for both 1 µm and 1.57 µm lasing was provided by the two perpendicularly-cleaved fiber end facets, resulted in a combined output power of 188 W at 1565 nm for a launched pump power of 466 W (see Fig. 3). The overall slope efficiency with respect to launched pump power was 41%. Similarly, the laser exhibited a high slope efficiency of 43.1% at low pump powers (< 300W), which decreased to 36.8% at high pump powers due once again to lasing on Yb³⁺ transition. In this case, lasing at 1065 nm resulted in a total output power (i.e. from both fiber ends) of <20 W. It is interesting to note that the 1µm power was significantly lower for the latter cavity configuration. This is believed to be due to lower cavity losses at 1µm for the former cavity configuration (shown in Fig.1(a)) due to residual feedback from the external feedback cavity at ~1µm. One way to avoid parasitic lasing at ~1µm is to increase the cavity loss at ~1µm by using an angle cleaved fiber end facet nearest the external cavity and by using a plane mirror with high transmission in the 1030–1100 nm regime. It is worth noting that the laser output power in Fig. 2 and Fig. 3 is still essentially linear with respect to pump power at even the high pump power if parasitic lasing at ~1µm is taken into account, with no evidence of any detrimental impact due to thermal loading. Hence, with the appropriate cavity design and with the required degree of suppression of 1µm lasing, it should be possible to obtain a further increase in output power by simply increasing the pump power.

 

Fig. 2. Output power versus launched pump power for free-running Er,Yb fiber laser with an external cavity.

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Fig. 3. Output power versus launched pump power with no external cavity.

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Tunable operation of EYDFL was demonstrated employing a simple external cavity design, as shown in Fig. 1(b), comprising an antireflection coated collimating lens of focal length 120 mm and a simple replica diffraction grating (600 lines/mm) mounted on a copper substrate to facilitate removal of waste heat. A relatively long focal length collimating lens in the external feedback cavity was selected to avoid any possible damage to the grating and to reduce the collimated beam divergence and hence increase the spectral selectivity of the grating feedback cavity. The grating was blazed for wavelength of ~1.65 µm with reflectivity of ~75% for light polarised parallel to the grooves and ~95% for light polarised in the orthogonal direction, and was aligned in the Littrow configuration to provide wavelength selective feedback and hence the means for adjusting the lasing wavelength. The fiber end facet nearest the grating was angle-polished at ~14° to suppress parasitic lasing between the two fiber end facets. A shorter fibre length of ~2m was selected for the tunable operation and the combined unabsorbed pump power (from two ends) was measured to <7 W for a total launched pump power of 336 W. Using this resonator configuration, the laser generated a maximum output power of 108 W at 1538 nm for ~336 W of launched pump power (see inset of Fig. 4). The threshold pump power (launched) was ~3.3 W and the slope efficiency with respect to launched pump power was 32%. The linear dependence of the output power on the pump power suggests that there was no severe thermal induced deformation on the bulk grating even at highest pump power, and clearly indicates that there is scope for further power scaling of this simple tunable EYDFL laser architecture by increasing the pump power. The laser output power as a function of operating wavelength is shown in Fig. 4. The lasing wavelength could be tuned from 1531 to 1571 nm, and over 36 nm from ~1532 to 1568 nm at output power levels in excess of 100 W with a linewidth (FWHM) of ~1 nm. The short-term stability was measured to be <0.9% (RMS) on a time scale of 300 µs. Moreover, the output power was very stable over longer time periods with power fluctuations of <3% over a time scale of 30 minutes, and we did not observe any degradation in performance over a period of several months of intermittent use.

 

Fig. 4. Tunable Er,Yb fiber laser output power versus operating wavelength for a 2 m fiber. Inset: Output power of the tunable Er,Yb fiber laser at 1538 nm versus launched pump power.

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3. Conclusion

In conclusion, efficient and high-power operation of an erbium-ytterbium co-doped fiber laser cladding-pumped by two 975 nm diode-stacks is reported. Up to 188W of continuous-wave output at 1.57 µm was generated with a beam-quality factor (M ²) of 1.9 and an overall slope efficiency with respect to launched pump power of 41% and 43% for low powers (>300 W). Tunable operation was demonstrated by use of an external cavity containing a diffraction grating and a maximum output power of 108 W at 1538 nm was generated for a launched pump power of ~336 W. The operating wavelength was tunable from over 36 nm from 1532 nm to 1568 nm at output power levels in excess of 100 W with a linewidth (FWHM) of ~1 nm. The short-term stability was measured to be <0.9% (RMS) and the output power was very stable over longer time periods with power fluctuations of <3% over a time scale of 30 minutes with no evidence of self-pulsing at any power level. The linearity of output power as a function of pump power for both the free-running and tunable cavity configurations suggests that there is considerable scope for further power scaling.