Ytterbium-doped large-core fibre laser with
`1 kW of continuous-wave output power
`
`Y. Jeong, J.K. Sahu, D.N. Payne and J. Nilsson
`
`A ytterbium-doped fibre laser with up to 1 kW of continuous-wave
`output power at 1.09 mm with a slope efficiency of 80% and a good
`beam quality (M2¼ 3.4) is reported. The fibre was pumped by two
`diode-stack sources launched through opposite ends. No undesirable
`roll-over was observed in output power with increasing pump power.
`
`Introduction: In recent years the output powers of ytterbium (Yb3þ
`)
`doped fibre lasers have grown dramatically and they are now compet-
`ing with conventional bulk solid-state lasers (e.g. Nd:YAG lasers) in
`high-power application areas such as material processing, medicine,
`and range finding. The technique of cladding-pumping combines the
`attractions of fibre lasers with high-power, low-cost, multimode-diode
`pumping and leads to fibre lasers with high-brightness, even diffrac-
`tion-limited, output, even when low-brightness diode lasers are used
`as pump sources. An excellent conversion efficiency (>80%) and a
`broad tunability of Yb-doped fibre lasers make them exceptional high-
`power sources in the 1–1.1 mm wavelength range, e.g. 10 kW of
`output power has been reached from highly multimoded devices
`that combined the output power from several fibre lasers [1]. The
`power achieved in high-brightness output beams from single-fibre
`configurations is lower, but has nevertheless reached 500 W in
`30–40 m-long fibre devices [1, 2].
`In this Letter, we describe further power-scaling of the single-fibre
`laser configuration to an output power of 1 kW. For this to be possible,
`we used a higher pump power, a larger inner cladding, and a larger core
`than before.
`A large inner cladding is necessary to accommodate the large pump
`beams required for a kW fibre laser in our end-pumped configuration. A
`large core is also required for many reasons: with a thick inner cladding,
`a conventional core diameter of e.g. 10 mm or less would lead to
`excessively long fibres in which background loss would degrade the
`output power. Therefore, a large core is needed to reach sufficient pump
`absorption with an acceptable Yb-concentration. Another reason is the
`suppression of nonlinear-scattering [2–4], achieved with the lower
`power density and shorter fibre possible with a larger core. There is
`also the possibility of fibre facet damage: in the case of pure silica the
`damage threshold intensity is 10 GW=cm2,
`i.e. 100 W=mm2 in
`pulsed operation [5]; however,
`this value is likely to be lower in
`continuous-wave (CW) operation and even for a rare-earth-doped
`core, because the impurities and inhomogeneities caused by dopant
`molecules are likely to increase the susceptibility to damage. As a
`conservative estimate, we may want to keep power density at the facet
`below 1 W=mm2 for reliable operation. With this value, the modefield
`area needs to be as large as 103 mm2 if we aim for a 1 kW output
`power from a single-fibre laser, though further power scaling may well
`be possible with this core size in view of our conservative estimate of
`the damage threshold.
`
`Experiments and results: A double-clad Yb-doped large-core fibre
`was designed to meet the requirements for power-scaling to the kW
`level. It was pulled from a preform that was fabricated in-house by the
`modified chemical-vapour deposition (MCVD) and solution doping
`technique. The fibre had a 43 mm-diameter Yb-doped core with a
`numerical aperture (NA) of 0.09, centred in the preform. With this
`design,
`the modefield area for
`the fundamental mode becomes
`1100 mm2, which should allow for reliable operation with kW
`level output power. The D-shaped inner cladding had a 650=600 mm
`diameter for the longer=shorter axis after being milled. This diameter
`was chosen to enable efficient coupling of the high power pump
`sources. The fibre was coated with a low-refractive-index polymer
`outer cladding which provided a nominal inner-cladding NA of 0.48.
`The small-signal absorption rates in the inner cladding were 1.5 and
`3 dB=m at 972 and 975 nm, respectively. This corresponds to a
`-concentration of 4500 ppm by weight. The fibre length used
`Yb3þ
`in the laser experiments was 8 m.
`The experimental setup is shown in Fig. 1. We used a double-sided
`end-pumping scheme, with two pump sources launched into opposite
`ends of the fibre. Two diode-laser-stack-based pump sources were
`used, emitting at 972=975 nm, respectively. The pump beams were
`
`coupled to the active fibre via collimating and focusing lenses [3]. We
`could launch a combined maximum pump power of 1.3 kW, corre-
`sponding to 85% of the power incident on the fibre. In one end of the
`laser cavity, high-reflectivity feedback was provided by a pair of
`dichroic mirrors, with high transmission at the pump wavelength and
`high reflection at the signal wavelength. The mirrors were external to
`the fibre and coupled to it via a lens. The laser output coupler was
`formed by a 4% reflecting flat perpendicular cleave in the other end of
`the fibre. The signal was separated from the pump beam with another
`dichroic mirror. Both ends of the fibre were held in temperature-
`controlled metallic V-grooves designed to prevent thermal damage to
`the fibre coating by any non-guided pump or signal power, or by the
`heat generated in the laser cycle itself.
`
`Fig. 1 Yb-doped fibre laser arrangement used with two diode-stack pump
`sources
`
`HR: high reflectivity; HT: high transmission
`
`Fig. 2 Fibre laser output power against launched pump power, and laser
`output spectrum at full power
`
`a Output power against launched pump power
`b Output spectrum
`
`The laser output power characteristics are shown in Fig. 2, together
`with the output spectrum at full output power. The maximum laser
`output power was 1.01 kW and the slope efficiency with respect to the
`launched pump power was 80%. The pump leakage was estimated to be
`below 1.7%. The standard deviation of the temporal power was <1.2%,
`
`ELECTRONICS LETTERS 15th April 2004 Vol. 40 No. 8
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`ASML 1119
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`measured with a photodetector of 3.5 ns rise=fall time and a 400 MHz
`oscilloscope. The output power increased linearly with launched pump
`power. There was no evidence of any power limitation due to nonlinear
`scattering, nor was any stimulated Raman scattering observed.
`Compared to previous results generating up to 500 W from 30–
`40 m-long fibres with 12 and 24.5 mm core diameters [1, 2], our fibre
`was significantly shorter (8 m) and had a bigger core (43 mm diameter).
`This suggests a very high threshold for undesirable nonlinear scattering
`for our Yb-doped fibre laser, and thus, nonlinear scattering was
`completely suppressed. We measured the beam quality factor (M2) of
`3.4. This must be considered to be a good result, bearing in mind the
`relatively high V-parameter of 11.2 of the core at 1.09 mm, and given
`that no special measures were taken to suppress operation on higher-
`order modes. Given that we may be relatively far from the damage
`threshold, one could consider making the core smaller (provided
`acceptable pump absorption can be maintained). This would allow for
`an improved beam quality. The core-NA could also be reduced to
`improve the beam quality. There are also mode-selecting techniques
`such as a fibre taper and bend-loss filtering that can be used to improve
`the beam quality further in the multimode core [6, 7].
`
`Conclusion: We have demonstrated a highly efficient, high Yb
`concentration, double-clad Yb-doped large-core fibre laser with a
`CW output power of 1.01 kW at 1.09 mm based on an 8 m single fibre.
`No evidence of roll-over in laser output power at the highest launched
`pump powers (1.3 kW) was observed, suggesting that our laser
`could be scaled to even higher powers using a more powerful pump
`source or, for example, with additional multiplexed-pump sources.
`
`Acknowledgment: This work was supported in part by DARPA under
`contract MDA972-02-C-0049.
`
`# IEE 2004
`Electronics Letters online no: 20040298
`doi: 10.1049/el:20040298
`
`28 January 2004
`
`Y. Jeong, J.K. Sahu, D.N. Payne and J. Nilsson (Optoelectronics
`Research Centre, University of Southampton, SO17 1BJ, United
`Kingdom)
`
`E-mail: yoj@orc.soton.ac.uk
`
`References
`
`3
`
`1 Gapontsev, V.P., Platonov, N.S., Shkurihin, O., and Zaitsev, I.: ‘400 W
`low-noise single-mode cw ytterbium fiber laser with an integrated fiber
`delivery’. Proc. Conf. on Lasers and Electro-Optics, Baltimore, MD,
`USA, June 2003 (postdeadline paper CThPDB9)
`2 Limpert, J., Liem, A., Zellmer, H., and Tu¨nnermann, A.: ‘500 W
`continuous-wave fibre laser with excellent beam quality’, Electron.
`Lett., 2003, 39, pp. 645–647
`Jeong, Y., Sahu, J.K., Williams, R.B., Richardson, D.J., Furusawa, K.,
`and Nilsson, J.: ‘Ytterbium-doped large-core fibre laser with 272 W
`output power’, Electron. Lett., 2003, 39, pp. 977–978
`4 Platonov, N.S., Gapontsev, D.V., Gapontsev, V.P., and Shumilin, V.:
`‘135 W CW fiber laser with perfect single mode output’ . Proc. Conf.
`on Lasers and Electro-Optics, Long Beach, CA, USA, May 2002
`(postdeadline paper CPDC3)
`5 Said, A.A., Xia, T., Dogariu, A., Hagan, D.J., Soileau, M.J.,
`Van Stryland, E.W., and Mohebi, M.: ‘Measurement of the optical
`damage threshold in fused quartz’, Appl. Opt., 1995, 34, pp. 3374–3376
`6 Alvarez-Chavez, J.A., Grudinin, A.B., Nilsson, J., Turner, P.W., and
`Clarkson, W.A.: ‘Mode selection in high power cladding pumped fibre
`lasers with tapered section’. Proc. Conf. on Lasers and Electro-Optics,
`OSA Tech. Dig. Washington, DC, USA, 1999, pp. 247–248
`7 Koplow, J.P., Kliner, D.A.V., and Goldberg, L.: ‘Single-mode operation of
`a coiled multimode fiber amplifier’, Opt. Lett., 2000, 25, pp. 442–444
`
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