- Email: [email protected]
A 103 W erbium–ytterbium co-doped large-core fiber laser
Optics Communications 227 (2003) 159–163 www.elsevier.com/locate/optcom
A 103 W erbium–ytterbium co-doped large-core fiber laser J.K. Sahu, Y. Jeong *, D.J. Richardson, J. Nilsson Optoelectronics Research Centre, University of Southampton, Highfield, Bldg 46, Southampton SO17 1BJ, UK Received 6 May 2003; received in revised form 1 September 2003; accepted 5 September 2003
Abstract We report a highly efficient cladding-pumped erbium–ytterbium co-doped large-core fiber laser, generating up to 103 W of continuous-wave output power at 1.57 lm with a beam quality (M 2 ) of 2.0. The overall slope efficiency was 30% (40% at low powers) with respect to the launched pump power. A roll-off in output power was observed at higher powers caused by the onset of co-lasing at 1.06 lm due to the increased ytterbium-excitation at high pump powers. Our result highlights the impressive power capacity of erbium–ytterbium co-doped fibers. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.55.W; 42.55.X Keywords: Fiber lasers; Diode-pumped lasers
The output powers of cladding-pumped fiber laser systems have grown rapidly over recent years thanks mainly to improvements in fiber design and fabrication and in the performance of pump diode sources (including multi-emitter lasers diodes, diode bars and diode stacks) [1–4]. To date most work has focused on ytterbium (Yb) doped fiber lasers operating at wavelengths around 1.1 lm largely because the efficiency that can be achieved is extremely high (>80%). However there is now increasing interest in fiber lasers operating at socalled eye-safe wavelengths in the range 1.5–2.0 lm. The 1.5–1.6 lm wavelength regime is partic-
*
Corresponding author. Tel.: +44-23-8059-3141; fax: +44-238059-3142. E-mail address: [email protected] (Y. Jeong).
ularly important in telecommunications. This drove the development of the erbium–ytterbium co-doped fiber (EYDF) for efficient high-power operation at 1550 nm, in core-pumped [5] as well as cladding-pumped configurations [6]. While power requirements for fiber-optic communication systems seldom exceed 1 W, far higher output powers may be required for many other applications such as free space and satellite optical communications and LIDAR. Fiber devices are prominent contenders in this wavelength region, because of the lack of good crystalline gain media, preferred for the competing ÔbulkÕ laser technology, and also because of the wide availability of pigtailed components with outstanding performance developed for telecom applications such as DFB lasers, isolators, and modulators. Power-scaling of EYDF devices is therefore very appealing.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.09.022
160
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
Recently, operation of EYDFs at the 100 W level was reported [7], and 80 W devices are on sale commercially [8]. For such devices, optimization of fiber materials and/or geometry, pumping arrangements, and heat-sinking are critical. However, until now, a more modest 17 W laser has remained the highest-power EYDF device described in the literature [9]. In this paper, we report a highly efficient Er3þ – Yb3þ co-doped fiber laser (EYDFL), claddingpumped by a 975 nm diode stack source and generating up to 103 W of continuous-wave (cw) output power at 1.57 lm. A large core diameter and high pump absorption per unit length of our fiber allowed for a 5 m device length that provided complete suppression of intracavity nonlinear scattering, with a good spatial beam quality (M 2 ¼ 2:0). The double-clad Er–Yb co-doped large-core fiber used in our experiments was designed and drawn from a preform that was fabricated inhouse using the standard modified chemical-vapor deposition (MCVD) and solution doping technique with a phosphosilicate core material [10]. Before being drawn to fiber, the preform was milled to have a D-shape in order to improve the characteristics of the overlap of the claddingmodes with the Er–Yb co-doped core, and thus the pump absorption [11,12]. The 24-lm diameter core had a numerical aperture (NA) of 0.20. The
Signal output @ ~1550 nm
D-shaped inner cladding had a 400/360 lm diameter for the longer/shorter axis, and was coated with a low-refractive-index polymer outer cladding which provided a nominal inner-cladding NA of 0.48. The small-signal absorption at the pump wavelength was 2.5 dB/m. Based on these data, we evaluate the Yb3þ -concentration to 2% by weight. The experimental setup is shown in Fig. 1. Our pump source comprised multiple beam-shaped laser diode stacks at 975 nm, and was vastly superior to those previously used for pumping Er–Yb fibers. Even so, the focused beam from a diode stack source is necessarily rather large: the beam parameter product of the pump source was 80 mm mrad 60 mm mrad. Thus, we had to use an inner cladding as thick as 400 lm. The pump beam was coupled into a 5 m length of the double-clad Er– Yb fiber using a simple combination of lenses comprising a first collimating arrangement followed by a 6 mm focal length lens that focused the pump beam onto the end of the fiber in an endpumping scheme. Simple end-pumping seems most appropriate with a diode stack pump source, since alternative side-pumping techniques are likely to fail at high pump powers, and since they generally use too thin fibers for an efficient pump launch. We could launch as much as 340 W of pump power, with an estimated launch efficiency of >90% relative to power incident on the fiber. Both ends of the fiber were perpendicularly cleaved
HT @ 975 ~1100 nm HR @ ~1550 nm
Diode stacks @ 975 nm
HT @ 975 nm HR @ ~1060 nm Signal output @ ~1060 nm
Unabsorbed pump Signal output @ ~1060 nm
Double-clad Er-Yb co-doped fiber
HT @ 975 ~1100 nm HR @ ~1550 nm
Fig. 1. Experimental setup of Er–Yb co-doped fiber laser end-pumped by a diode-stack pump source. HR: high reflectivity, HT: high transmission.
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
relative to the fiber axis. The laser cavity was formed between the facet at the pump launch end of the fiber and a lens-coupled dichroic mirror with high reflectivity at 1550 nm positioned at the other end of the fiber. A dichroic mirror (high reflectivity at 1550 nm) separated the output beam, taken from the pump launch end, from the pump beam. Another dichroic mirror (high reflectivity at 1060 nm) was inserted between the pump laser and the launch end to separate out any lasing component at 1060 nm that may arise as the ytterbium-excitation is increased at high pump levels. In addition, both ends of the fiber were held in temperature-controlled metallic V-grooves that were designed to prevent thermal damage to the fiber coating by any non-guided pump or signal power, or by the heat generated in the laser cycle itself. Thermal damage is primarily a problem at fiber ends, since this is where signal and pump powers, and thus heat generation, reach their maximum levels, and since any unguided beams would primarily be present at the fiber ends and absorbed there. The laser output power characteristics are shown in Fig. 2, together with an output spectrum (inset). The maximum laser output power was 103
Fig. 2. Fiber laser output power with respect to the launched pump power (inset: output spectrum).
161
W at the maximum diode drive current. The output power looked stable when viewed on an oscilloscope. The laser spectrum was centered at 1565 nm. The overall (average) slope efficiency with respect to the launched pump power was 30% (34% with respect to the absorbed pump power), while the initial, low-power slope efficiency was 40%. The output power increased linearly at low powers, but there was a roll-off at higher powers, caused by lasing of the ytterbium ions at 1.06 lm as the ytterbium-excitation increased at high pump levels. Thus, the energy transfer between Er and Yb ions was Ôbottle-neckedÕ at high powers: since the erbium ions are primarily pumped indirectly via energy transfer from ytterbium, a higher pump rate implies a larger number of excited Yb ions. At some point the Yb-excitation reaches a threshold level where 1.06 lm lasing commences. The more efficient the energy transfer is the higher pump powers can be used before this occurs. With a fiber with more efficient energy transfer, we believe that the 1.06 lm lasing could be suppressed and the slope efficiency at 1.57 lm improved. We note that slightly more efficient Er–Yb fibers, with smaller cores, have been reported at high power densities, without the onset of Yb-lasing [9]. This suggests there is indeed room for improvements in the Er– Yb gain medium of our present, large-core, fiber. Alternatively, considering that the fiber had perpendicular facets providing 4% broadband feedback in both of its ends, it would also be possible to mitigate 1.06 lm lasing by suppressing the broadband feedback with an angled fiber facet in the external-mirror end of the cavity. On the other hand, the simultaneous generation of 1.06 lm and 1.57 lm radiation may even be desirable rather than undesirable for some applications. Fiber design is critical for high-power fiber lasers. This is especially true, for example, when a single-mode, single-polarization, and/or narrowlinewidth output is required. Thermal management is a particular issue with Er–Yb doped fiber lasers because of their low efficiency (compared to Yb-doped fiber lasers). However, the fiber geometry is ideal from a heat dissipation perspective, and Brown and Hoffmann have estimated fracture limits in optical fibers at over 100 W/cm of generated heat [13]. Our fiber generated, and was able
162
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
to handle, an estimated over 100 W of heat per meter at the pump launch end. Nonlinear scattering mechanisms such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) constitute another obstacle. SRS can lead to a roll-off in output power [2]. SBS is the major problem when a single-frequency output is required [14]. Short fibers with high pump absorption per unit length and large core diameters are preferred for increasing the threshold of the stimulated scattering processes and nonlinear degradation in general, as long as an adequate spatial mode quality can be preserved and thermal loading is acceptable. Additional advantages to be derived from the use of short fibers are less modecoupling (with multimode fibers), lower total propagation losses, and, obviously but importantly, less fiber material usage. In our fiber, nonlinear scattering was completely suppressed. We measured the beam quality factor (M 2 ) to 2.0, which represents a good result, considering the relatively high V-parameter of 9.6 of the core at 1565 nm, and given that no special measures were taken to suppress operation on higher-order modes. To improve the beam quality further, a fiber taper can be used [15]. A smaller core could improve the beam quality, but we believe that single-mode laser operation of a fiber with a large multimode core is preferable, because of nonlinear degradation and damage limitations. Because of the ability of EYDF-based devices to combine high gain with high power conversion efficiency, efficient EYDFLs can be realized in a wide variety of configurations. Here, we chose to use one of the simplest cavities that provided a single-ended output. The use of a perpendicular facet in the external-mirror end of the cavity leads to an often undesired 4% broadband feedback, but it also reduces the coupling losses of the reflected laser field. Coupling losses can otherwise be high with an angled facet, increasing the likelihood of damage at high power operation. We also investigated alternative configurations, e.g., with 4% reflecting facets in both ends of the fiber. We found a similar efficiency also in this case. In conclusion, we have provided the first description of a double-clad Er–Yb co-doped largecore fiber laser with a cw output power in excess of
100 W at an eye-safe wavelength of 1.57 lm. This is several times more power than previously published from fiber lasers in this wavelength regime. Advances in pump and fiber technology have enabled this radical progress. The overall slope efficiency was 30% with respect to the launched pump power (34% with respect to the absorbed pump power). Although there was a roll-off in 1.57 lm laser output power at higher pump powers, we believe that this can be resolved by suppressing the ytterbium-lasing at 1.06 lm with appropriate wavelength selective elements within the cavity, or wavelength-selective feedback, or perhaps with improvements in the gain medium. In addition, with further improvements in fiber and core design and suppression of higher-order modes we believe that it will be possible to achieve diffraction-limited output, without reducing core size and thus without compromising damage threshold or nonlinear threshold values.
Acknowledgements This work was supported in part by DARPA under Contract MDA72-02-C-0049.
References [1] V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P.S. Yeh, E. Zucker, Electron. Lett. 35 (1999) 1158. [2] N.S. Platonov D.V. Gapontsev, V.P. Gapontsev, V. Shumilin, in: Proceedings of the Conference on Lasers and Electro-Optics 2002, Long Beach, USA, May 19–24, 2002, postdeadline paper CPDC3. [3] V. Gapontsev, W. Krupke, Laser Focus World (2002) 83. [4] J. Limpert, A. Liem, H. Zellmer, A. T€ unnermann, in: Proceedings of the Advanced Solid-State Photonics 2003, San Antonio, USA, Feb. 2–5, 2003, postdeadline paper PD1. [5] J.E. Townsend, W.L. Barnes, K.P. Jedrzejewski, S.G. Grubb, Electron. Lett. 27 (1991) 958. [6] J.D. Minelly, W.L. Barnes, R.I. Laming, P.R. Morkel, J.E. Townsend, S.G. Grubb, D.N. Payne, IEEE Photon. Technol. Lett. 5 (1993) 301. [7] J. Nilsson, J.K. Sahu, Y. Jeong, W.A. Clarkson, R. Selvas, A.B. Grudinin, S.-U. Alam, in: Proceedings of the SPIE Meeting on Advances in Fiber Lasers, Photonics West, San Jose, California January 25-31, 2003.
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163 [8] Press release, IPG Photonics Corporation, March 25 2003 (www.ire-polusgroup.com/html/188_IPG_Photonics_Announces_Shipment_of_First_Commercial_80_Watt_Erbium_Fiber_Laser.cfm). [9] J. Nilsson, S.U. Alam, J.A. Alvarez-Chavez, P.W. Turner, W.A. Clarkson, A.B. Grudinin, IEEE J. Quantum Electron. 39 (2003) 987. [10] G.G. Vienne, J.E. Caplen, L. Dong, J.D. Minelly, J. Nilsson, D.N. Payne, J. Lightwave Technol. 16 (1998) 1990. [11] M. Muendel, in: Proceedings of the Conference on Lasers and Electro-Optics, OSA Technical Digest Series, vol. 9, Optical Society of America, Washington, DC, 1996, p. 209.
163
[12] H. Zellmer, A. T€ unnermann, H. Welling, V. Reichel, in: Proceedings of the Optical Amplifiers and their Applications, Technical Digest, Optical Society of America, Washington, DC, 1997, p. 251. [13] D.C. Brown, H.J. Hoffmann, IEEE J. Quantum Electron. 37 (2001) 207. [14] S. H€ ofer, A. Liem, J. Limpert, H. Zellmer, A. T€ unnermann, S. Unger, S. Jetschke, H.-R. M€ uller, I. Freitag, Opt. Lett. 26 (2001) 1326. [15] J.A. Alvarez-Chavez, A.B. Grudinin, J. Nilsson, P.W. Turner, W.A. Clarkson, in: Proceedings of the Conference on Lasers and Electro-Optics, OSA Technical Digest, Optical Society of America, Washington, DC, 1999, p. 247.
A 103 W erbium–ytterbium co-doped large-core fiber laser J.K. Sahu, Y. Jeong *, D.J. Richardson, J. Nilsson Optoelectronics Research Centre, University of Southampton, Highfield, Bldg 46, Southampton SO17 1BJ, UK Received 6 May 2003; received in revised form 1 September 2003; accepted 5 September 2003
Abstract We report a highly efficient cladding-pumped erbium–ytterbium co-doped large-core fiber laser, generating up to 103 W of continuous-wave output power at 1.57 lm with a beam quality (M 2 ) of 2.0. The overall slope efficiency was 30% (40% at low powers) with respect to the launched pump power. A roll-off in output power was observed at higher powers caused by the onset of co-lasing at 1.06 lm due to the increased ytterbium-excitation at high pump powers. Our result highlights the impressive power capacity of erbium–ytterbium co-doped fibers. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.55.W; 42.55.X Keywords: Fiber lasers; Diode-pumped lasers
The output powers of cladding-pumped fiber laser systems have grown rapidly over recent years thanks mainly to improvements in fiber design and fabrication and in the performance of pump diode sources (including multi-emitter lasers diodes, diode bars and diode stacks) [1–4]. To date most work has focused on ytterbium (Yb) doped fiber lasers operating at wavelengths around 1.1 lm largely because the efficiency that can be achieved is extremely high (>80%). However there is now increasing interest in fiber lasers operating at socalled eye-safe wavelengths in the range 1.5–2.0 lm. The 1.5–1.6 lm wavelength regime is partic-
*
Corresponding author. Tel.: +44-23-8059-3141; fax: +44-238059-3142. E-mail address: [email protected] (Y. Jeong).
ularly important in telecommunications. This drove the development of the erbium–ytterbium co-doped fiber (EYDF) for efficient high-power operation at 1550 nm, in core-pumped [5] as well as cladding-pumped configurations [6]. While power requirements for fiber-optic communication systems seldom exceed 1 W, far higher output powers may be required for many other applications such as free space and satellite optical communications and LIDAR. Fiber devices are prominent contenders in this wavelength region, because of the lack of good crystalline gain media, preferred for the competing ÔbulkÕ laser technology, and also because of the wide availability of pigtailed components with outstanding performance developed for telecom applications such as DFB lasers, isolators, and modulators. Power-scaling of EYDF devices is therefore very appealing.
0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.09.022
160
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
Recently, operation of EYDFs at the 100 W level was reported [7], and 80 W devices are on sale commercially [8]. For such devices, optimization of fiber materials and/or geometry, pumping arrangements, and heat-sinking are critical. However, until now, a more modest 17 W laser has remained the highest-power EYDF device described in the literature [9]. In this paper, we report a highly efficient Er3þ – Yb3þ co-doped fiber laser (EYDFL), claddingpumped by a 975 nm diode stack source and generating up to 103 W of continuous-wave (cw) output power at 1.57 lm. A large core diameter and high pump absorption per unit length of our fiber allowed for a 5 m device length that provided complete suppression of intracavity nonlinear scattering, with a good spatial beam quality (M 2 ¼ 2:0). The double-clad Er–Yb co-doped large-core fiber used in our experiments was designed and drawn from a preform that was fabricated inhouse using the standard modified chemical-vapor deposition (MCVD) and solution doping technique with a phosphosilicate core material [10]. Before being drawn to fiber, the preform was milled to have a D-shape in order to improve the characteristics of the overlap of the claddingmodes with the Er–Yb co-doped core, and thus the pump absorption [11,12]. The 24-lm diameter core had a numerical aperture (NA) of 0.20. The
Signal output @ ~1550 nm
D-shaped inner cladding had a 400/360 lm diameter for the longer/shorter axis, and was coated with a low-refractive-index polymer outer cladding which provided a nominal inner-cladding NA of 0.48. The small-signal absorption at the pump wavelength was 2.5 dB/m. Based on these data, we evaluate the Yb3þ -concentration to 2% by weight. The experimental setup is shown in Fig. 1. Our pump source comprised multiple beam-shaped laser diode stacks at 975 nm, and was vastly superior to those previously used for pumping Er–Yb fibers. Even so, the focused beam from a diode stack source is necessarily rather large: the beam parameter product of the pump source was 80 mm mrad 60 mm mrad. Thus, we had to use an inner cladding as thick as 400 lm. The pump beam was coupled into a 5 m length of the double-clad Er– Yb fiber using a simple combination of lenses comprising a first collimating arrangement followed by a 6 mm focal length lens that focused the pump beam onto the end of the fiber in an endpumping scheme. Simple end-pumping seems most appropriate with a diode stack pump source, since alternative side-pumping techniques are likely to fail at high pump powers, and since they generally use too thin fibers for an efficient pump launch. We could launch as much as 340 W of pump power, with an estimated launch efficiency of >90% relative to power incident on the fiber. Both ends of the fiber were perpendicularly cleaved
HT @ 975 ~1100 nm HR @ ~1550 nm
Diode stacks @ 975 nm
HT @ 975 nm HR @ ~1060 nm Signal output @ ~1060 nm
Unabsorbed pump Signal output @ ~1060 nm
Double-clad Er-Yb co-doped fiber
HT @ 975 ~1100 nm HR @ ~1550 nm
Fig. 1. Experimental setup of Er–Yb co-doped fiber laser end-pumped by a diode-stack pump source. HR: high reflectivity, HT: high transmission.
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
relative to the fiber axis. The laser cavity was formed between the facet at the pump launch end of the fiber and a lens-coupled dichroic mirror with high reflectivity at 1550 nm positioned at the other end of the fiber. A dichroic mirror (high reflectivity at 1550 nm) separated the output beam, taken from the pump launch end, from the pump beam. Another dichroic mirror (high reflectivity at 1060 nm) was inserted between the pump laser and the launch end to separate out any lasing component at 1060 nm that may arise as the ytterbium-excitation is increased at high pump levels. In addition, both ends of the fiber were held in temperature-controlled metallic V-grooves that were designed to prevent thermal damage to the fiber coating by any non-guided pump or signal power, or by the heat generated in the laser cycle itself. Thermal damage is primarily a problem at fiber ends, since this is where signal and pump powers, and thus heat generation, reach their maximum levels, and since any unguided beams would primarily be present at the fiber ends and absorbed there. The laser output power characteristics are shown in Fig. 2, together with an output spectrum (inset). The maximum laser output power was 103
Fig. 2. Fiber laser output power with respect to the launched pump power (inset: output spectrum).
161
W at the maximum diode drive current. The output power looked stable when viewed on an oscilloscope. The laser spectrum was centered at 1565 nm. The overall (average) slope efficiency with respect to the launched pump power was 30% (34% with respect to the absorbed pump power), while the initial, low-power slope efficiency was 40%. The output power increased linearly at low powers, but there was a roll-off at higher powers, caused by lasing of the ytterbium ions at 1.06 lm as the ytterbium-excitation increased at high pump levels. Thus, the energy transfer between Er and Yb ions was Ôbottle-neckedÕ at high powers: since the erbium ions are primarily pumped indirectly via energy transfer from ytterbium, a higher pump rate implies a larger number of excited Yb ions. At some point the Yb-excitation reaches a threshold level where 1.06 lm lasing commences. The more efficient the energy transfer is the higher pump powers can be used before this occurs. With a fiber with more efficient energy transfer, we believe that the 1.06 lm lasing could be suppressed and the slope efficiency at 1.57 lm improved. We note that slightly more efficient Er–Yb fibers, with smaller cores, have been reported at high power densities, without the onset of Yb-lasing [9]. This suggests there is indeed room for improvements in the Er– Yb gain medium of our present, large-core, fiber. Alternatively, considering that the fiber had perpendicular facets providing 4% broadband feedback in both of its ends, it would also be possible to mitigate 1.06 lm lasing by suppressing the broadband feedback with an angled fiber facet in the external-mirror end of the cavity. On the other hand, the simultaneous generation of 1.06 lm and 1.57 lm radiation may even be desirable rather than undesirable for some applications. Fiber design is critical for high-power fiber lasers. This is especially true, for example, when a single-mode, single-polarization, and/or narrowlinewidth output is required. Thermal management is a particular issue with Er–Yb doped fiber lasers because of their low efficiency (compared to Yb-doped fiber lasers). However, the fiber geometry is ideal from a heat dissipation perspective, and Brown and Hoffmann have estimated fracture limits in optical fibers at over 100 W/cm of generated heat [13]. Our fiber generated, and was able
162
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163
to handle, an estimated over 100 W of heat per meter at the pump launch end. Nonlinear scattering mechanisms such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) constitute another obstacle. SRS can lead to a roll-off in output power [2]. SBS is the major problem when a single-frequency output is required [14]. Short fibers with high pump absorption per unit length and large core diameters are preferred for increasing the threshold of the stimulated scattering processes and nonlinear degradation in general, as long as an adequate spatial mode quality can be preserved and thermal loading is acceptable. Additional advantages to be derived from the use of short fibers are less modecoupling (with multimode fibers), lower total propagation losses, and, obviously but importantly, less fiber material usage. In our fiber, nonlinear scattering was completely suppressed. We measured the beam quality factor (M 2 ) to 2.0, which represents a good result, considering the relatively high V-parameter of 9.6 of the core at 1565 nm, and given that no special measures were taken to suppress operation on higher-order modes. To improve the beam quality further, a fiber taper can be used [15]. A smaller core could improve the beam quality, but we believe that single-mode laser operation of a fiber with a large multimode core is preferable, because of nonlinear degradation and damage limitations. Because of the ability of EYDF-based devices to combine high gain with high power conversion efficiency, efficient EYDFLs can be realized in a wide variety of configurations. Here, we chose to use one of the simplest cavities that provided a single-ended output. The use of a perpendicular facet in the external-mirror end of the cavity leads to an often undesired 4% broadband feedback, but it also reduces the coupling losses of the reflected laser field. Coupling losses can otherwise be high with an angled facet, increasing the likelihood of damage at high power operation. We also investigated alternative configurations, e.g., with 4% reflecting facets in both ends of the fiber. We found a similar efficiency also in this case. In conclusion, we have provided the first description of a double-clad Er–Yb co-doped largecore fiber laser with a cw output power in excess of
100 W at an eye-safe wavelength of 1.57 lm. This is several times more power than previously published from fiber lasers in this wavelength regime. Advances in pump and fiber technology have enabled this radical progress. The overall slope efficiency was 30% with respect to the launched pump power (34% with respect to the absorbed pump power). Although there was a roll-off in 1.57 lm laser output power at higher pump powers, we believe that this can be resolved by suppressing the ytterbium-lasing at 1.06 lm with appropriate wavelength selective elements within the cavity, or wavelength-selective feedback, or perhaps with improvements in the gain medium. In addition, with further improvements in fiber and core design and suppression of higher-order modes we believe that it will be possible to achieve diffraction-limited output, without reducing core size and thus without compromising damage threshold or nonlinear threshold values.
Acknowledgements This work was supported in part by DARPA under Contract MDA72-02-C-0049.
References [1] V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P.S. Yeh, E. Zucker, Electron. Lett. 35 (1999) 1158. [2] N.S. Platonov D.V. Gapontsev, V.P. Gapontsev, V. Shumilin, in: Proceedings of the Conference on Lasers and Electro-Optics 2002, Long Beach, USA, May 19–24, 2002, postdeadline paper CPDC3. [3] V. Gapontsev, W. Krupke, Laser Focus World (2002) 83. [4] J. Limpert, A. Liem, H. Zellmer, A. T€ unnermann, in: Proceedings of the Advanced Solid-State Photonics 2003, San Antonio, USA, Feb. 2–5, 2003, postdeadline paper PD1. [5] J.E. Townsend, W.L. Barnes, K.P. Jedrzejewski, S.G. Grubb, Electron. Lett. 27 (1991) 958. [6] J.D. Minelly, W.L. Barnes, R.I. Laming, P.R. Morkel, J.E. Townsend, S.G. Grubb, D.N. Payne, IEEE Photon. Technol. Lett. 5 (1993) 301. [7] J. Nilsson, J.K. Sahu, Y. Jeong, W.A. Clarkson, R. Selvas, A.B. Grudinin, S.-U. Alam, in: Proceedings of the SPIE Meeting on Advances in Fiber Lasers, Photonics West, San Jose, California January 25-31, 2003.
J.K. Sahu et al. / Optics Communications 227 (2003) 159–163 [8] Press release, IPG Photonics Corporation, March 25 2003 (www.ire-polusgroup.com/html/188_IPG_Photonics_Announces_Shipment_of_First_Commercial_80_Watt_Erbium_Fiber_Laser.cfm). [9] J. Nilsson, S.U. Alam, J.A. Alvarez-Chavez, P.W. Turner, W.A. Clarkson, A.B. Grudinin, IEEE J. Quantum Electron. 39 (2003) 987. [10] G.G. Vienne, J.E. Caplen, L. Dong, J.D. Minelly, J. Nilsson, D.N. Payne, J. Lightwave Technol. 16 (1998) 1990. [11] M. Muendel, in: Proceedings of the Conference on Lasers and Electro-Optics, OSA Technical Digest Series, vol. 9, Optical Society of America, Washington, DC, 1996, p. 209.
163
[12] H. Zellmer, A. T€ unnermann, H. Welling, V. Reichel, in: Proceedings of the Optical Amplifiers and their Applications, Technical Digest, Optical Society of America, Washington, DC, 1997, p. 251. [13] D.C. Brown, H.J. Hoffmann, IEEE J. Quantum Electron. 37 (2001) 207. [14] S. H€ ofer, A. Liem, J. Limpert, H. Zellmer, A. T€ unnermann, S. Unger, S. Jetschke, H.-R. M€ uller, I. Freitag, Opt. Lett. 26 (2001) 1326. [15] J.A. Alvarez-Chavez, A.B. Grudinin, J. Nilsson, P.W. Turner, W.A. Clarkson, in: Proceedings of the Conference on Lasers and Electro-Optics, OSA Technical Digest, Optical Society of America, Washington, DC, 1999, p. 247.