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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 4 — Feb. 25, 2013
  • pp: 4560–4566
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High power operation of cladding pumped holmium-doped silica fibre lasers

Alexander Hemming, Shayne Bennetts, Nikita Simakov, Alan Davidson, John Haub, and Adrian Carter  »View Author Affiliations


Optics Express, Vol. 21, Issue 4, pp. 4560-4566 (2013)
http://dx.doi.org/10.1364/OE.21.004560


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Abstract

We report the highest power operation of a resonantly cladding-pumped, holmium-doped silica fibre laser. The cladding pumped all-glass fibre utilises a fluorine doped glass layer to provide low loss cladding guidance of the 1.95 µm pump radiation. The operation of both single mode and large-mode area fibre lasers was demonstrated, with up to 140 W of output power achieved. A slope efficiency of 59% versus launched pump power was demonstrated. The free running emission was measured to be 2.12-2.15 µm demonstrating the potential of this architecture to address the long wavelength operation of silica based fibre lasers with high efficiency.

© 2013 OSA

1. Introduction

The development of high power laser sources in the 2 µm spectral region is of particular interest for a range of scientific, medical, and industrial applications. Operation in this wavelength region allows access to atmospheric transmission windows as well as water absorption features which are of interest for remote sensing and the atmospheric propagation of high power lasers [1

1. J. Wu, Z. Yao, J. Zong, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “Single frequency fiber laser at 2.05 µm based on Ho-doped germanate glass fiber,” Proc. SPIE 7195, 71951K, 71951K-7 (2009). [CrossRef]

3

3. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]

].

Over the last decade work on thulium fibre laser development at 2 µm has provided a mature platform for the development of both pulsed and CW fibre laser sources, suitable for frequency conversion [4

4. N. Simakov, A. Davidson, A. Hemming, S. Bennetts, M. Hughes, N. Carmody, P. Davies, and J. Haub, “Mid-infrared generation in ZnGeP2 pumped by a monolithic, power scalable 2-µm source,” Proc. SPIE 8237, 82373K, 82373K-6 (2012). [CrossRef]

6

6. D. Creeden, P. A. Ketteridge, P. A. Budni, S. D. Setzler, Y. E. Young, J. C. McCarthy, K. Zawilski, P. G. Schunemann, T. M. Pollak, E. P. Chicklis, and M. Jiang, “Mid-infrared ZnGeP2 parametric oscillator directly pumped by a pulsed 2µm Tm-doped fiber laser,” Opt. Lett. 33(4), 315–317 (2008). [CrossRef] [PubMed]

], and kW level power scaling [3

3. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]

]. Diode pumping of thulium fibre lasers at 0.79 µm relies on a cross-relaxation mechanism to produce efficient 2 µm operation [7

7. S. D. Jackson and T. King, “High-power diode-cladding-pumped Tm-doped silica fiber laser,” Opt. Lett. 23(18), 1462–1464 (1998). [CrossRef] [PubMed]

]. Efficient exploitation of this energy transfer mechanism requires a highly doped core with a correspondingly high refractive index and thus core numerical aperture (NA). This high core NA limits the core sizes, and hence mode areas, over which good beam quality operation can be produced. To maintain good beam quality a pedestal structure around the core can be implemented, reducing the effective NA of the core [8

8. A. Carter, J. Farroni, B. Sampson, D. Machewirth, N. Jacobson, W. Torruellas, Y. Chen, M.-Y. Cheng, A. Galvanauskas, and A. Sanchez, “Robustly single-mode polarization maintaining Er/Yb co-doped LMA fiber for high power applications,” in Conference on Lasers and Electro-Optics, CLEO, (IEEE, 2007), pp. 1–2.

]. However, this approach also increases the complexity of the fibre fabrication, and imposes practical limits on the NA, and subsequently the mode areas that can be fabricated.

In comparison to thulium fibre lasers, investigations into holmium fibre lasers have been limited. The use of holmium as the active rare earth ion presents several advantages over thulium fibre lasers for operation around 2 µm. Holmium fibre lasers allow access to an operating wavelength range extending beyond 2.1 µm, which is not efficiently addressed by thulium fibre lasers, and is of interest for mid-IR frequency conversion using ZnGeP2 optical parametric oscillators. Holmium fibre lasers also offer the potential to be tandem pumped using mature thulium fibre lasers. Tandem pumping has been widely used to power scale fibre lasers [9

9. S. U. Alam, A. T. Harker, R. J. Horley, F. Ghiringhelli, M. P. Varnham, P. W. Turner, M. N. Zervas, and S. R. Norman, “All-fibre, high power, cladding-pumped 1565 nm MOPA pumped by high brightness 1535 nm pump sources,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CWJ4.

11

11. Y. Tang, L. Xu, Y. Yang, and J. Xu, “High-power gain-switched Tm3+-doped fiber laser,” Opt. Express 18(22), 22964–22972 (2010). [CrossRef] [PubMed]

], with up to 10 kW of output power with good beam quality having been demonstrated at 1 µm [12

12. E. Stiles, “New developments in IPG fiber laser technology,” in Proceedings of the 5th International Workshop on Fiber Lasers (2009).

]. This architecture provides a robust route for the further power scaling of fibre lasers limited by the brightness of current laser diodes. Furthermore, the use of holmium as the active ion enables a simple core composition with a low dopant concentration to be used. This allows fibres with low NA cores and large mode areas to be fabricated without the need for a pedestal structure. Large mode area holmium fibres are required for producing long wavelength fibre laser sources capable of high pulse energy output, and high average power narrow linewidth operation.

For the direct pumping of singly doped holmium, the absorption bands of interest lie at ~1.15 µm, (4I7 level), and at ~1.95 µm, (5I7 level). Pumping of the 4I7 level can be addressed by 1.15 µm laser diodes [13

13. S. D. Jackson, F. Bugge, and G. Erbert, “High-power and highly efficient diode-cladding-pumped Ho3+-doped silica fiber lasers,” Opt. Lett. 32(22), 3349–3351 (2007). [CrossRef] [PubMed]

], or long wavelength operation of ytterbium fibre lasers [14

14. A. Shirakawa, H. Maruyama, K. Ueda, C. B. Olausson, J. K. Lyngsø, and J. Broeng, “High-power Yb-doped photonic bandgap fiber amplifier at 1150-1200 nm,” Opt. Express 17(2), 447–454 (2009). [CrossRef] [PubMed]

]. The large quantum defect of this pumping scheme limits its efficiency and power scaling potential. The 5I7 level can be accessed by thulium fibre lasers which can provide a mature pump source demonstrated at the kW power level. However, absorption of this pump wavelength by standard polymer coatings has precluded the demonstration of an efficient resonantly cladding pumped holmium fibre laser. This has limited the power scaling of holmium fibre lasers in comparison to other rare-earth ion doped silica fibre lasers which have benefitted from the implementation of a cladding pumped fibre.

This paper describes the efficient operation of a resonantly cladding pumped holmium fibre laser. The fibre features an all-glass fibre design utilising a fluorine-doped second cladding to provide low-loss pump guidance for resonant pumping of the holmium fibre laser at 1.95 µm. We have demonstrated both robustly single-mode and large mode area fibre lasers running at 140 W, pump power limited. The slope efficiencies of these lasers were 59% and 55% with respect to launched pump power and the lasers operated at 2.12-2.15 µm. To the best of our knowledge these are the highest power solid-state holmium lasers reported. These initial demonstrations show great promise for further power scaling of fibre lasers in the 2 µm spectral region.

2. Experiment

The cladding pumped holmium fibre laser demonstrations are based on the holmium doped silica fibre depicted in Fig. 1
Fig. 1 Fluorine-doped glass clad holmium-doped silica fibre.
. Two fibre geometries were drawn from a single preform, producing a robustly single-mode (SM) fibre, and a large mode-area (LMA) fibre. The details of the fibre geometries are listed in Table 1

Table 1. Description of the Fibres under Investigation

table-icon
View This Table
.

The lasers were operated in a free-space configuration to allow characterisation of the fibres described above and to ascertain the free-running wavelengths of these lasers. A schematic of the laser set-up is shown in Fig. 2
Fig. 2 Schematic of experimental laser layout.
.

The laser set-up allows for both single and double-ended pump architectures to be implemented. The core absorption of the fibre was ~64 dB/m at 1.95 µm. The holmium fibre length was chosen to correspond to an absorption of ~10 dB for single-ended and ~14 dB for double-ended pumping. Both the cladding-pumped holmium fibre and thulium pump laser output fibres were held in water cooled v-groove mounts. The high reflectivity (HR) end of the holmium laser cavity consisted of an Infrasil aspheric lens (f = 15 mm, AR @ 1.85-2.15 µm) and a dichroic mirror (HR @ 2.05-2.2 µm, HT @ 1.95 µm). The Fresnel reflection from the output fibre facet served as the output coupler. A pair of aspheric lenses was used to focus the output of each of the thulium pump lasers into the cladding of the holmium fibre. Residual 0.79 µm diode pump light from the 1.95 µm thulium fibre lasers was reflected by a dichroic mirror (HR @ 0.79 µm, HT @ 1.95 µm). The reflected 1.95 µm signal from these optics was monitored to determine the pump power incident on the holmium fibre. The pump coupling efficiency was determined by replacing the holmium fibre with a non-absorbing matched passive germanium doped fibre and comparing the incident and transmitted powers. The transmitted power was ~91% including Fresnel losses. A further pair of dichroic mirrors were used to separate any residual 1.95 µm pump from the holmium output before directing it onto a thermal power meter. A CaF2 wedge split off a portion of the output beam to enable monitoring of the output beam quality with a pyro-electric camera (Pyrocam III, Spiricon) and the holmium laser wavelength was monitored using an optical spectrum analyzer (AQ6375, Yokogawa).

3. Results

The output power achieved using the single-mode fibre is plotted in Fig. 3
Fig. 3 Output power from single (red) and double (blue) end pumped SM holmium fibre lasers.
. Both single-end and double-end pumped results are shown. The slope efficiency versus launched pump power of the single-end pumped laser was 59%, and the double-end pumped laser achieved 57%. The operating wavelength was measured to be a broad feature between 2.12 and 2.13 µm, as shown in Fig. 4
Fig. 4 Spectra of double-end pumped SM holmium fibre laser.
.

The output power achieved with the LMA fibre is shown in Fig. 5
Fig. 5 Output power of the double-end pumped LMA fibre laser.
. A double-end pumped laser was demonstrated. An output power of 140 W was achieved, with a slope efficiency of 55% versus launched 1.95 µm pump power. Typical spectra of the LMA laser are shown in Fig. 6(a)
Fig. 6 (a) Spectra of the double-end pumped LMA holmium fibre laser. (b) Near field beam profile at 25 W (top) and 140 W (bottom).
, with the free-running operating wavelength being in the range 2.12–2.15 µm. A typical near field profile of the laser output from the double-end pumped LMA fibre laser is shown in Fig. 6(b) at 25 W and 140 W. The beam quality was measured using a pyro-electric camera and commercial beam quality measurement software (Pyrocam III, M2-200, Spiricon-Ophir) and found to be M2 = 1.3-1.5.

4. Discussion

The sources of loss that contribute to this reduced efficiency are background IR silica losses (0.07-0.15 dBm−1) [20

20. S. Nagel, J. MacChesney, and K. Walker, “An overview of the modified chemical vapor deposition (MCVD) process and performance,” IEEE Trans. Microw. Theory Tech. 30(4), 305–322 (1982). [CrossRef]

], OH- combination mode absorption (~0.05 dBm−1ppmOH−1) [21

21. O. Humbach, H. Fabian, U. Grzesik, U. Haken, and W. Heitmann, “Analysis of OH absorption bands in synthetic silica,” J. Non-Cryst. Solids 203, 19–26 (1996). [CrossRef]

], re-absorption and non-radiative decay processes of holmium in silica. Comparison of the slope efficiency versus launched 1.95 µm pump power for the single and double-end pumped SM lasers indicate that the double-end pumped laser suffers from losses associated with the longer fibre length. The differences in the SM and LMA spectra also provide some indications as to which mechanisms dominate and further work is underway to better understand these loss processes.

The experiments presented here are pump power limited with no sign of roll-over or thermal degradation. We anticipate that further power scaling is possible by increasing the output power from each thulium pump laser, or by increasing the number of thulium pump lasers in a monolithic tandem pumping scheme.

5. Conclusion

We have presented to our knowledge the highest output power resonantly cladding pumped holmium fibre lasers. Both single mode and large mode area fibre lasers were demonstrated with output powers of 140 W achieved, limited only by pump power. The efficiency of the lasers was 55-59% with respect to launched pump power. The holmium fibre laser architecture presented in this paper will allow power scaling of both CW and pulsed holmium fibre laser sources for a range of applications. In particular the resonantly pumped holmium fibre laser architecture, utilising mature thulium fibre lasers as pump sources, shows great promise for power scaling and high power narrow line-width operation. Future work will focus on understanding and improving laser efficiency and investigating the accessible wavelength range that this laser architecture can address.

Acknowledgments

The authors would like to thank Len Corena and Dmitrii Stepanov (DSTO) for grating fabrication and Mark Hughes and Phil Davies for mechanical fabrication.

References and links

1.

J. Wu, Z. Yao, J. Zong, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “Single frequency fiber laser at 2.05 µm based on Ho-doped germanate glass fiber,” Proc. SPIE 7195, 71951K, 71951K-7 (2009). [CrossRef]

2.

J.-P. Cariou, B. Augere, and M. Valla, “Laser source requirements for coherent lidars based on fiber technology,” Compt. Rend. Phys. 7(2), 213–223 (2006). [CrossRef]

3.

P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]

4.

N. Simakov, A. Davidson, A. Hemming, S. Bennetts, M. Hughes, N. Carmody, P. Davies, and J. Haub, “Mid-infrared generation in ZnGeP2 pumped by a monolithic, power scalable 2-µm source,” Proc. SPIE 8237, 82373K, 82373K-6 (2012). [CrossRef]

5.

C. Kieleck, A. Hildenbrand, M. Eichhorn, D. Faye, E. Lallier, B. Gerard, and S. D. Jackson, “OP-GaAs OPO pumped by 2 µm Q-switched lasers: Tm:Ho:silica fiber laser and Ho:YAG laser,” Proc. SPIE 7836, 783607, 783607-8 (2010). [CrossRef]

6.

D. Creeden, P. A. Ketteridge, P. A. Budni, S. D. Setzler, Y. E. Young, J. C. McCarthy, K. Zawilski, P. G. Schunemann, T. M. Pollak, E. P. Chicklis, and M. Jiang, “Mid-infrared ZnGeP2 parametric oscillator directly pumped by a pulsed 2µm Tm-doped fiber laser,” Opt. Lett. 33(4), 315–317 (2008). [CrossRef] [PubMed]

7.

S. D. Jackson and T. King, “High-power diode-cladding-pumped Tm-doped silica fiber laser,” Opt. Lett. 23(18), 1462–1464 (1998). [CrossRef] [PubMed]

8.

A. Carter, J. Farroni, B. Sampson, D. Machewirth, N. Jacobson, W. Torruellas, Y. Chen, M.-Y. Cheng, A. Galvanauskas, and A. Sanchez, “Robustly single-mode polarization maintaining Er/Yb co-doped LMA fiber for high power applications,” in Conference on Lasers and Electro-Optics, CLEO, (IEEE, 2007), pp. 1–2.

9.

S. U. Alam, A. T. Harker, R. J. Horley, F. Ghiringhelli, M. P. Varnham, P. W. Turner, M. N. Zervas, and S. R. Norman, “All-fibre, high power, cladding-pumped 1565 nm MOPA pumped by high brightness 1535 nm pump sources,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CWJ4.

10.

M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, “415 W single-mode CW thulium fiber laser in all-fiber format,” in European Conference on Lasers and Electro-Optics and the International Quantum Electronics Conference. CLEOE-IQEC 2007, (IEEE, 2007), pp. CP2.

11.

Y. Tang, L. Xu, Y. Yang, and J. Xu, “High-power gain-switched Tm3+-doped fiber laser,” Opt. Express 18(22), 22964–22972 (2010). [CrossRef] [PubMed]

12.

E. Stiles, “New developments in IPG fiber laser technology,” in Proceedings of the 5th International Workshop on Fiber Lasers (2009).

13.

S. D. Jackson, F. Bugge, and G. Erbert, “High-power and highly efficient diode-cladding-pumped Ho3+-doped silica fiber lasers,” Opt. Lett. 32(22), 3349–3351 (2007). [CrossRef] [PubMed]

14.

A. Shirakawa, H. Maruyama, K. Ueda, C. B. Olausson, J. K. Lyngsø, and J. Broeng, “High-power Yb-doped photonic bandgap fiber amplifier at 1150-1200 nm,” Opt. Express 17(2), 447–454 (2009). [CrossRef] [PubMed]

15.

S. D. Jackson, “Midinfrared holmium fiber lasers,” IEEE J. Quantum Electron. 42(2), 187–191 (2006). [CrossRef]

16.

A. S. Kurkov, V. V. Dvoyrin, and A. V. Marakulin, “All-fiber 10 W holmium lasers pumped at λ=1.15 microm,” Opt. Lett. 35(4), 490–492 (2010). [CrossRef] [PubMed]

17.

J. W. Kim, A. Boyland, J. K. Sahu, and W. A. Clarkson, “Ho-doped silica fibre laser in-band pumped by a Tm-doped fibre laser,” in CLEO/Europe and EQEC 2009 Conference Digest, (Optical Society of America, 2009), paper CJ6_5.

18.

S. D. Jackson, A. Sabella, A. Hemming, S. Bennetts, and D. G. Lancaster, “High-power 83 W holmium-doped silica fiber laser operating with high beam quality,” Opt. Lett. 32(3), 241–243 (2007). [CrossRef] [PubMed]

19.

A. S. Kurkov, E. M. Sholokhov, V. B. Tsvetkov, A. V. Marakulin, L. A. Minashina, O. I. Medvedkov, and A. F. Kosolapov, “Holmium fibre laser with record quantum efficiency,” Quantum Electron. 41(6), 492–494 (2011). [CrossRef]

20.

S. Nagel, J. MacChesney, and K. Walker, “An overview of the modified chemical vapor deposition (MCVD) process and performance,” IEEE Trans. Microw. Theory Tech. 30(4), 305–322 (1982). [CrossRef]

21.

O. Humbach, H. Fabian, U. Grzesik, U. Haken, and W. Heitmann, “Analysis of OH absorption bands in synthetic silica,” J. Non-Cryst. Solids 203, 19–26 (1996). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3460) Lasers and laser optics : Lasers
(140.3510) Lasers and laser optics : Lasers, fiber

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: November 22, 2012
Revised Manuscript: January 24, 2013
Manuscript Accepted: February 1, 2013
Published: February 14, 2013

Citation
Alexander Hemming, Shayne Bennetts, Nikita Simakov, Alan Davidson, John Haub, and Adrian Carter, "High power operation of cladding pumped holmium-doped silica fibre lasers," Opt. Express 21, 4560-4566 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-4-4560


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References

  1. J. Wu, Z. Yao, J. Zong, A. Chavez-Pirson, N. Peyghambarian, and J. Yu, “Single frequency fiber laser at 2.05 µm based on Ho-doped germanate glass fiber,” Proc. SPIE7195, 71951K, 71951K-7 (2009). [CrossRef]
  2. J.-P. Cariou, B. Augere, and M. Valla, “Laser source requirements for coherent lidars based on fiber technology,” Compt. Rend. Phys.7(2), 213–223 (2006). [CrossRef]
  3. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron.15(1), 85–92 (2009). [CrossRef]
  4. N. Simakov, A. Davidson, A. Hemming, S. Bennetts, M. Hughes, N. Carmody, P. Davies, and J. Haub, “Mid-infrared generation in ZnGeP2 pumped by a monolithic, power scalable 2-µm source,” Proc. SPIE8237, 82373K, 82373K-6 (2012). [CrossRef]
  5. C. Kieleck, A. Hildenbrand, M. Eichhorn, D. Faye, E. Lallier, B. Gerard, and S. D. Jackson, “OP-GaAs OPO pumped by 2 µm Q-switched lasers: Tm:Ho:silica fiber laser and Ho:YAG laser,” Proc. SPIE7836, 783607, 783607-8 (2010). [CrossRef]
  6. D. Creeden, P. A. Ketteridge, P. A. Budni, S. D. Setzler, Y. E. Young, J. C. McCarthy, K. Zawilski, P. G. Schunemann, T. M. Pollak, E. P. Chicklis, and M. Jiang, “Mid-infrared ZnGeP2 parametric oscillator directly pumped by a pulsed 2µm Tm-doped fiber laser,” Opt. Lett.33(4), 315–317 (2008). [CrossRef] [PubMed]
  7. S. D. Jackson and T. King, “High-power diode-cladding-pumped Tm-doped silica fiber laser,” Opt. Lett.23(18), 1462–1464 (1998). [CrossRef] [PubMed]
  8. A. Carter, J. Farroni, B. Sampson, D. Machewirth, N. Jacobson, W. Torruellas, Y. Chen, M.-Y. Cheng, A. Galvanauskas, and A. Sanchez, “Robustly single-mode polarization maintaining Er/Yb co-doped LMA fiber for high power applications,” in Conference on Lasers and Electro-Optics, CLEO, (IEEE, 2007), pp. 1–2.
  9. S. U. Alam, A. T. Harker, R. J. Horley, F. Ghiringhelli, M. P. Varnham, P. W. Turner, M. N. Zervas, and S. R. Norman, “All-fibre, high power, cladding-pumped 1565 nm MOPA pumped by high brightness 1535 nm pump sources,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD) (Optical Society of America, 2008), paper CWJ4.
  10. M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, “415 W single-mode CW thulium fiber laser in all-fiber format,” in European Conference on Lasers and Electro-Optics and the International Quantum Electronics Conference. CLEOE-IQEC 2007, (IEEE, 2007), pp. CP2.
  11. Y. Tang, L. Xu, Y. Yang, and J. Xu, “High-power gain-switched Tm3+-doped fiber laser,” Opt. Express18(22), 22964–22972 (2010). [CrossRef] [PubMed]
  12. E. Stiles, “New developments in IPG fiber laser technology,” in Proceedings of the 5th International Workshop on Fiber Lasers (2009).
  13. S. D. Jackson, F. Bugge, and G. Erbert, “High-power and highly efficient diode-cladding-pumped Ho3+-doped silica fiber lasers,” Opt. Lett.32(22), 3349–3351 (2007). [CrossRef] [PubMed]
  14. A. Shirakawa, H. Maruyama, K. Ueda, C. B. Olausson, J. K. Lyngsø, and J. Broeng, “High-power Yb-doped photonic bandgap fiber amplifier at 1150-1200 nm,” Opt. Express17(2), 447–454 (2009). [CrossRef] [PubMed]
  15. S. D. Jackson, “Midinfrared holmium fiber lasers,” IEEE J. Quantum Electron.42(2), 187–191 (2006). [CrossRef]
  16. A. S. Kurkov, V. V. Dvoyrin, and A. V. Marakulin, “All-fiber 10 W holmium lasers pumped at λ=1.15 microm,” Opt. Lett.35(4), 490–492 (2010). [CrossRef] [PubMed]
  17. J. W. Kim, A. Boyland, J. K. Sahu, and W. A. Clarkson, “Ho-doped silica fibre laser in-band pumped by a Tm-doped fibre laser,” in CLEO/Europe and EQEC 2009 Conference Digest, (Optical Society of America, 2009), paper CJ6_5.
  18. S. D. Jackson, A. Sabella, A. Hemming, S. Bennetts, and D. G. Lancaster, “High-power 83 W holmium-doped silica fiber laser operating with high beam quality,” Opt. Lett.32(3), 241–243 (2007). [CrossRef] [PubMed]
  19. A. S. Kurkov, E. M. Sholokhov, V. B. Tsvetkov, A. V. Marakulin, L. A. Minashina, O. I. Medvedkov, and A. F. Kosolapov, “Holmium fibre laser with record quantum efficiency,” Quantum Electron.41(6), 492–494 (2011). [CrossRef]
  20. S. Nagel, J. MacChesney, and K. Walker, “An overview of the modified chemical vapor deposition (MCVD) process and performance,” IEEE Trans. Microw. Theory Tech.30(4), 305–322 (1982). [CrossRef]
  21. O. Humbach, H. Fabian, U. Grzesik, U. Haken, and W. Heitmann, “Analysis of OH absorption bands in synthetic silica,” J. Non-Cryst. Solids203, 19–26 (1996). [CrossRef]

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