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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 7148–7155
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A high efficiency architecture for cascaded Raman fiber lasers

V. R. Supradeepa, Jeffrey W. Nichsolson, Clifford E. Headley, Man F. Yan, Bera Palsdottir, and Dan Jakobsen  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 7148-7155 (2013)
http://dx.doi.org/10.1364/OE.21.007148


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Abstract

We demonstrate a new high efficiency architecture for cascaded Raman fiber lasers based on a single pass cascaded amplifier configuration. Conversion is seeded at all intermediate Stokes wavelengths using a multi-wavelength seed source. A lower power Raman laser based on the conventional cascaded Raman resonator architecture provides a convenient seed source providing all the necessary wavelengths simultaneously. In this work we demonstrate a 1480nm laser pumped by an 1117nm Yb-doped fiber laser with maximum output power of 204W and conversion efficiency of 65% (quantum-limited efficiency is ~75%). We believe both the output power and conversion efficiency (relative to quantum-limited efficiency) are the highest reported for cascaded Raman fiber lasers.

© 2013 OSA

1. Introduction

Cascaded Raman fiber lasers provide a convenient way to obtain high powers at wavelengths which may not be accessible through rare-earth doped fiber lasers [1

1. S. G. Grubb, T. Erdogan, V. Mizrahi, T. Strasser, W. Y. Cheung, W. A. Reed, P. J. Lemaire, A. E. Miller, S. G. Kosinski, G. Nykolak, and P. C. Becker, “High power, 1.48 µm cascaded Raman laser in germanosilicate fibers,” OSA Topic. Meeting, Optic. Amp. and Their Applications (1994).

7

7. D. Georgiev, V. P. Gapontsev, A. G. Dronov, M. Y. Vyatkin, A. B. Rulkov, S. V. Popov, and J. R. Taylor, “Watts-level frequency doubling of a narrow line linearly polarized Raman fiber laser to 589nm,” Opt. Express 13(18), 6772–6776 (2005). [CrossRef] [PubMed]

]. The principle is to wavelength convert the output of a rare-earth doped fiber laser to the required output wavelength using a series of Raman Stokes shifts. Conventionally, wavelength conversion over two or more Stokes shifts is performed through the use of a cascaded Raman resonator (as shown schematically in Fig. 1(a)
Fig. 1 (a) Schematic of a cascaded Raman laser, RIG – Raman input grating set, ROG – Raman output grating set, (b) Components of a cascaded Raman resonator converting 1117nm input to 1480nm output, HR – High reflectivity grating (> 99%), OC – output coupler, low reflectivity (< 10%) grating
). It is comprised of nested cavities at each of the intermediate wavelengths made with fiber Bragg gratings (referred to as the Raman input and output grating sets) and a low effective area (high nonlinearity) fiber (Raman fiber). Each intermediate wavelength in the resonator is chosen to be close to the peak of the Raman gain of the wavelength preceding it. A low reflectivity output coupler terminates the wavelength conversion. At the output most of the light is at the desired final wavelength with small fractions at the intermediate wavelengths.

Figure 1(b) shows an implementation of the cascaded Raman resonator performing five Stokes shifts from 1117nm to 1480nm. High power sources at 1.5 micron provide significantly higher eye safety than at the Yb wavelength region which is attractive for a variety of high power applications like material processing. Another interesting application for high power 1.5micron Raman lasers utilizes their ability to emit at the in-band absorption region of Erbium doped media. They can provide convenient high brightness and low quantum defect pump sources for Erbium-doped fiber amplifiers resulting in high efficiency performance and low thermal load. Shorter amplifier lengths made possible with high brightness pumping results in reduced non-linearity in the amplifiers, compared to cladding pumping with multi-mode 9xx diode lasers. This is particularly attractive for pulsed or single frequency amplifiers. This method has been used to pump large-mode area (LMA) Er-doped fiber amplifiers [8

8. J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. E. Headley, and D. J. DiGiovanni, “Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber,” Opt. Lett. 32(16), 2429–2431 (2007). [CrossRef] [PubMed]

], higher-order mode (HOM), Er-doped fiber amplifiers [9

9. J. W. Nicholson, J. M. Fini, A. M. DeSantolo, X. Liu, K. Feder, P. S. Westbrook, V. R. Supradeepa, E. Monberg, F. DiMarcello, R. Ortiz, C. Headley, and D. J. DiGiovanni, “Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers,” Opt. Express 20(22), 24575–24584 (2012). [CrossRef] [PubMed]

] and conventional Er fibers at high powers [10

10. V. R. Supradeepa, J. W. Nicholson, and K. Feder, “Continuous wave Erbium-doped fiber laser with output power of >100 W at 1550 nm in-band core-pumped by a 1480nm Raman fiber laser,” in CLEO: Science and Innovations, OSA Technical Digest (online) (Optical Society of America, 2012), paper CM2N.8.

]. Due to high transparency of the earth’s atmosphere at 1550nm, such sources are attractive for free space applications like LIDAR and directed energy.

The primary competing technologies for high power 1.5 micron fiber lasers pumped at 975nm (where mature, high power diode technology is available) are cladding pumped ErYb codoped fibers [17

17. Y. Jeong, S. Yoo, C. A. Codemard, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, P. W. Turner, L. M. B. Hickey, A. Harker, M. Lovelady, and A. Piper, “Erbium:ytterbium codoped large-core fiber laser with 297 W continuous-wave output power,” IEEE J. Sel. Top. Quantum Electron. 13(3), 573–579 (2007). [CrossRef]

] and cladding pumped Er fibers [18

18. V. Kuhn, D. Kracht, J. Neumann, and P. Wessels, “Er-doped photonic crystal fiber amplifier with 70 W of output power,” Opt. Lett. 36(16), 3030–3032 (2011). [CrossRef] [PubMed]

]. Apart from parasitic lasing issues and beam quality problems, the efficiencies achieved at comparable power levels were significantly smaller than demonstrated here. Furthermore, the output power in our system was limited by total pump power and significant power scaling (limited only by thermal considerations and laser damage threshold) is possible. From the results demonstrated here we believe this is a very efficient and scalable approach to high power fiber lasers in the 1.5micron wavelength region.

2. Architecture

The primary sources responsible for the reduced efficiency in conventional cascaded Raman lasers can be identified as-

  • 1. Transmission loss in the Raman input grating set and output grating set [19

    19. W. A. Reed, A. J. Stentz, and T. A. Strasser, “Article comprising a cascaded raman fiber laser,” U.S. Patent 5,815,518 (1998).

    ]
  • 2. Two intra-cavity splices between low effective area (possibly dissimilar) fibers constituting the grating sets and the Raman gain fiber.
  • 3. Linear loss in the Raman fiber.
  • 4. Enhanced backward and forward light at the intermediate Stokes wavelengths due to their bandwidth being higher than the grating bandwidths [20

    20. C. Headley and G. Agrawal, “Raman amplification in fiber optical communication systems,” (Academic Press, 2005).

    22

    22. S. D. Jackson and P. H. Muir, “Theory and numerical simulation of nth-order cascaded Raman fiber lasers,” J. Opt. Soc. Am. B 18(9), 1297–1306 (2001). [CrossRef]

    ].
  • 5. Splice loss between the Yb-doped fiber laser output and the low effective area Raman fiber.

A number of loss components are associated with the cascaded Raman resonator assembly. Here we intend to eliminate the cascaded Raman resonator and use a single pass cascaded amplifier scheme. At higher powers this is expected to work very well as long as it is seeded at all the intermediate Stokes wavelengths with sufficient power. Physically, the seed powers at all the intermediate wavelengths are essential since they reduce the gain requirement, provide wavelength selectivity and preferential forward Raman scattering. The idea of using a pump separated by more than one Stokes shift from the signal with the wavelength conversion mediated by intermediate wavelengths has been used previously used in optical communications for distributed Raman amplifiers [23

23. S. B. Papernyi, V. I. Karpov, and W. R. L. Clements, “Third-order cascaded Raman amplification,” in Optical Fiber Communication Conference (OFC) 2002, FB4–1.IEEE, (2002). [CrossRef]

, 24

24. S. Papernyi, V. Karpov, and W. Clements, “Cascaded pumping system and method for producing distributed Raman amplification in optical fiber telecommunication systems,” U. S. Patent 6,480,326 (2002).

].

3. Experimental results

Simulation studies to understand the optimal operating conditions for the new architecture as a function of input power, seed power, amplifier length etc are in progress. It is interesting however to point out that the difference in efficiency in the current scheme from the quantum limited efficiency can be mostly accounted for. The primary sources are splice loss between the output of the Yb-fiber to the much smaller Raman filter fiber (~5%) and residual power at all the intermediate Stokes wavelengths (~5%). This indicates that by reducing the splice loss through further optimization or novel splicing methods and better wavelength conversion through engineering the power and spectrum of the seed source, we should be able to further enhance the conversion efficiency.

4. Summary

Specifically for high power 1.5micron fiber lasers, the optical to optical efficiency from multi-mode 975nm pumps to 1480 nm laser demonstrated here (43%) is significantly higher than competing technologies at similar power levels based on cladding pumped Er and ErYb co-doped fibers. In this comparison, we do have to account for the difference in quantum defect between 975nm to 1480nm (this work) or to 1550nm (Er, ErYb). However this difference is quite small (~3%) and not significant compared to efficiency enhancements demonstrated here. We believe this is the most efficient and scalable approach to high power fiber lasers in the 1.5micron wavelength region. Another advantage specific to Raman lasers is their ability to act as high power, high brightness pump sources for pumping Er-doped media (like core-pumping large mode area Er-doped fiber amplifiers). This is an attractive option to reduce nonlinearity in high peak power pulsed amplifiers and single frequency amplifiers.

References and links

1.

S. G. Grubb, T. Erdogan, V. Mizrahi, T. Strasser, W. Y. Cheung, W. A. Reed, P. J. Lemaire, A. E. Miller, S. G. Kosinski, G. Nykolak, and P. C. Becker, “High power, 1.48 µm cascaded Raman laser in germanosilicate fibers,” OSA Topic. Meeting, Optic. Amp. and Their Applications (1994).

2.

S. K. Sim, H. C. Lim, L. W. Lee, L. C. Chia, R. F. Wu, I. Cristiani, M. Rini, and V. Degiorgio, “High-power cascaded Raman fibre laser using phosphosilicate fiber,” Electron. Lett. 40(12), 738–739 (2004). [CrossRef]

3.

Z. Xiong, N. Moore, Z. G. Li, and G. C. Lim, “10-W Raman fiber lasers at 1248 nm Using phosphosilicate fibers,” J. Lightwave Technol. 21(10), 2377–2381 (2003). [CrossRef]

4.

Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highly-efficient Raman fiber laser,” Opt. Express 17(26), 23678–23683 (2009). [CrossRef] [PubMed]

5.

R. Vallee, E. Belanger, B. Dery, M. Bernier, and D. Faucher, “Highly efficient and High-power Raman fiber laser based on broadband chirped fiber Bragg gratings,” J. Lightwave Technol. 24(12), 5039–5043 (2006). [CrossRef]

6.

C. Headley and G. P. Agrawal, Raman Amplification in Fiber Optical Communication Systems (Elsevier, 2005).

7.

D. Georgiev, V. P. Gapontsev, A. G. Dronov, M. Y. Vyatkin, A. B. Rulkov, S. V. Popov, and J. R. Taylor, “Watts-level frequency doubling of a narrow line linearly polarized Raman fiber laser to 589nm,” Opt. Express 13(18), 6772–6776 (2005). [CrossRef] [PubMed]

8.

J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. E. Headley, and D. J. DiGiovanni, “Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber,” Opt. Lett. 32(16), 2429–2431 (2007). [CrossRef] [PubMed]

9.

J. W. Nicholson, J. M. Fini, A. M. DeSantolo, X. Liu, K. Feder, P. S. Westbrook, V. R. Supradeepa, E. Monberg, F. DiMarcello, R. Ortiz, C. Headley, and D. J. DiGiovanni, “Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers,” Opt. Express 20(22), 24575–24584 (2012). [CrossRef] [PubMed]

10.

V. R. Supradeepa, J. W. Nicholson, and K. Feder, “Continuous wave Erbium-doped fiber laser with output power of >100 W at 1550 nm in-band core-pumped by a 1480nm Raman fiber laser,” in CLEO: Science and Innovations, OSA Technical Digest (online) (Optical Society of America, 2012), paper CM2N.8.

11.

Y. Emori, K. Tanaka, C. Headley, and A. Fujisaki, “High-power cascaded Raman fiber laser with 41-W output power at 1480-nm band,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (OSA, 2007), paper CFI2. [CrossRef]

12.

J. W. Nicholson, M. F. Yan, P. Wisk, J. Fleming, F. DiMarcello, E. Monberg, T. Taunay, C. Headley, and D. J. DiGiovanni, “Raman fiber laser with 81 W output power at 1480 nm,” Opt. Lett. 35(18), 3069–3071 (2010). [CrossRef] [PubMed]

13.

M. A. Arbore, Y. Zhou, G. Keaton, and T. Kane, “36dB gain in S-band EDFA with distributed ASE suppression,” in Optical Amplifiers and Their Applications, J. Nagel, S. Namiki, and L. Spiekman, eds., Vol. 77 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper PD4.

14.

P. D. Dragic, “Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss,” Opt. Commun. 250(4-6), 403–410 (2005). [CrossRef]

15.

J. Kim, P. Dupriez, C. Codemard, J. Nilsson, and J. K. Sahu, “Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off,” Opt. Express 14(12), 5103–5113 (2006). [CrossRef] [PubMed]

16.

V. R. Supradeepa, J. W. Nicholson, C. E. Headley, Y. Lee, B. Palsdottir, and D. Jakobsen, “Cascaded Raman fiber faser at 1480nm with output power of 104W,” no. 8237–48, SPIE photonics west 2012.

17.

Y. Jeong, S. Yoo, C. A. Codemard, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, P. W. Turner, L. M. B. Hickey, A. Harker, M. Lovelady, and A. Piper, “Erbium:ytterbium codoped large-core fiber laser with 297 W continuous-wave output power,” IEEE J. Sel. Top. Quantum Electron. 13(3), 573–579 (2007). [CrossRef]

18.

V. Kuhn, D. Kracht, J. Neumann, and P. Wessels, “Er-doped photonic crystal fiber amplifier with 70 W of output power,” Opt. Lett. 36(16), 3030–3032 (2011). [CrossRef] [PubMed]

19.

W. A. Reed, A. J. Stentz, and T. A. Strasser, “Article comprising a cascaded raman fiber laser,” U.S. Patent 5,815,518 (1998).

20.

C. Headley and G. Agrawal, “Raman amplification in fiber optical communication systems,” (Academic Press, 2005).

21.

M. Rini, I. Cristiani, and V. Degiorgio, “Numerical modeling and optimization of cascaded CW Raman fiber lasers,” IEEE J. Quantum Electron. 36(10), 1117–1122 (2000). [CrossRef]

22.

S. D. Jackson and P. H. Muir, “Theory and numerical simulation of nth-order cascaded Raman fiber lasers,” J. Opt. Soc. Am. B 18(9), 1297–1306 (2001). [CrossRef]

23.

S. B. Papernyi, V. I. Karpov, and W. R. L. Clements, “Third-order cascaded Raman amplification,” in Optical Fiber Communication Conference (OFC) 2002, FB4–1.IEEE, (2002). [CrossRef]

24.

S. Papernyi, V. Karpov, and W. Clements, “Cascaded pumping system and method for producing distributed Raman amplification in optical fiber telecommunication systems,” U. S. Patent 6,480,326 (2002).

25.

http://ofscatalog.specialityphotonics.com/category/high-power-products-and-cladding-pumped-fibers.

26.

Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12(25), 6088–6092 (2004). [CrossRef] [PubMed]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers
(140.3550) Lasers and laser optics : Lasers, Raman

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 22, 2013
Revised Manuscript: March 6, 2013
Manuscript Accepted: March 8, 2013
Published: March 14, 2013

Citation
V. R. Supradeepa, Jeffrey W. Nichsolson, Clifford E. Headley, Man F. Yan, Bera Palsdottir, and Dan Jakobsen, "A high efficiency architecture for cascaded Raman fiber lasers," Opt. Express 21, 7148-7155 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7148


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References

  1. S. G. Grubb, T. Erdogan, V. Mizrahi, T. Strasser, W. Y. Cheung, W. A. Reed, P. J. Lemaire, A. E. Miller, S. G. Kosinski, G. Nykolak, and P. C. Becker, “High power, 1.48 µm cascaded Raman laser in germanosilicate fibers,” OSA Topic. Meeting, Optic. Amp. and Their Applications (1994).
  2. S. K. Sim, H. C. Lim, L. W. Lee, L. C. Chia, R. F. Wu, I. Cristiani, M. Rini, and V. Degiorgio, “High-power cascaded Raman fibre laser using phosphosilicate fiber,” Electron. Lett.40(12), 738–739 (2004). [CrossRef]
  3. Z. Xiong, N. Moore, Z. G. Li, and G. C. Lim, “10-W Raman fiber lasers at 1248 nm Using phosphosilicate fibers,” J. Lightwave Technol.21(10), 2377–2381 (2003). [CrossRef]
  4. Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highly-efficient Raman fiber laser,” Opt. Express17(26), 23678–23683 (2009). [CrossRef] [PubMed]
  5. R. Vallee, E. Belanger, B. Dery, M. Bernier, and D. Faucher, “Highly efficient and High-power Raman fiber laser based on broadband chirped fiber Bragg gratings,” J. Lightwave Technol.24(12), 5039–5043 (2006). [CrossRef]
  6. C. Headley and G. P. Agrawal, Raman Amplification in Fiber Optical Communication Systems (Elsevier, 2005).
  7. D. Georgiev, V. P. Gapontsev, A. G. Dronov, M. Y. Vyatkin, A. B. Rulkov, S. V. Popov, and J. R. Taylor, “Watts-level frequency doubling of a narrow line linearly polarized Raman fiber laser to 589nm,” Opt. Express13(18), 6772–6776 (2005). [CrossRef] [PubMed]
  8. J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. E. Headley, and D. J. DiGiovanni, “Picosecond pulse amplification in a core-pumped large-mode-area erbium fiber,” Opt. Lett.32(16), 2429–2431 (2007). [CrossRef] [PubMed]
  9. J. W. Nicholson, J. M. Fini, A. M. DeSantolo, X. Liu, K. Feder, P. S. Westbrook, V. R. Supradeepa, E. Monberg, F. DiMarcello, R. Ortiz, C. Headley, and D. J. DiGiovanni, “Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers,” Opt. Express20(22), 24575–24584 (2012). [CrossRef] [PubMed]
  10. V. R. Supradeepa, J. W. Nicholson, and K. Feder, “Continuous wave Erbium-doped fiber laser with output power of >100 W at 1550 nm in-band core-pumped by a 1480nm Raman fiber laser,” in CLEO: Science and Innovations, OSA Technical Digest (online) (Optical Society of America, 2012), paper CM2N.8.
  11. Y. Emori, K. Tanaka, C. Headley, and A. Fujisaki, “High-power cascaded Raman fiber laser with 41-W output power at 1480-nm band,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (OSA, 2007), paper CFI2. [CrossRef]
  12. J. W. Nicholson, M. F. Yan, P. Wisk, J. Fleming, F. DiMarcello, E. Monberg, T. Taunay, C. Headley, and D. J. DiGiovanni, “Raman fiber laser with 81 W output power at 1480 nm,” Opt. Lett.35(18), 3069–3071 (2010). [CrossRef] [PubMed]
  13. M. A. Arbore, Y. Zhou, G. Keaton, and T. Kane, “36dB gain in S-band EDFA with distributed ASE suppression,” in Optical Amplifiers and Their Applications, J. Nagel, S. Namiki, and L. Spiekman, eds., Vol. 77 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper PD4.
  14. P. D. Dragic, “Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss,” Opt. Commun.250(4-6), 403–410 (2005). [CrossRef]
  15. J. Kim, P. Dupriez, C. Codemard, J. Nilsson, and J. K. Sahu, “Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off,” Opt. Express14(12), 5103–5113 (2006). [CrossRef] [PubMed]
  16. V. R. Supradeepa, J. W. Nicholson, C. E. Headley, Y. Lee, B. Palsdottir, and D. Jakobsen, “Cascaded Raman fiber faser at 1480nm with output power of 104W,” no. 8237–48, SPIE photonics west 2012.
  17. Y. Jeong, S. Yoo, C. A. Codemard, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, P. W. Turner, L. M. B. Hickey, A. Harker, M. Lovelady, and A. Piper, “Erbium:ytterbium codoped large-core fiber laser with 297 W continuous-wave output power,” IEEE J. Sel. Top. Quantum Electron.13(3), 573–579 (2007). [CrossRef]
  18. V. Kuhn, D. Kracht, J. Neumann, and P. Wessels, “Er-doped photonic crystal fiber amplifier with 70 W of output power,” Opt. Lett.36(16), 3030–3032 (2011). [CrossRef] [PubMed]
  19. W. A. Reed, A. J. Stentz, and T. A. Strasser, “Article comprising a cascaded raman fiber laser,” U.S. Patent 5,815,518 (1998).
  20. C. Headley and G. Agrawal, “Raman amplification in fiber optical communication systems,” (Academic Press, 2005).
  21. M. Rini, I. Cristiani, and V. Degiorgio, “Numerical modeling and optimization of cascaded CW Raman fiber lasers,” IEEE J. Quantum Electron.36(10), 1117–1122 (2000). [CrossRef]
  22. S. D. Jackson and P. H. Muir, “Theory and numerical simulation of nth-order cascaded Raman fiber lasers,” J. Opt. Soc. Am. B18(9), 1297–1306 (2001). [CrossRef]
  23. S. B. Papernyi, V. I. Karpov, and W. R. L. Clements, “Third-order cascaded Raman amplification,” in Optical Fiber Communication Conference (OFC) 2002, FB4–1.IEEE, (2002). [CrossRef]
  24. S. Papernyi, V. Karpov, and W. Clements, “Cascaded pumping system and method for producing distributed Raman amplification in optical fiber telecommunication systems,” U. S. Patent 6,480,326 (2002).
  25. http://ofscatalog.specialityphotonics.com/category/high-power-products-and-cladding-pumped-fibers .
  26. Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express12(25), 6088–6092 (2004). [CrossRef] [PubMed]

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