Abstract

In the present study, we examined photodarkening loss reduction in Yb3+ doped aluminosilicate fibers by utilizing 633 nm light irradiation. It is demonstrated that the final photobleaching value is intensity dependent, but the percentage of photodarkening reduction does not depend on dopant concentrations for settled bleaching intensity. Further examination was committed to explore the photobleaching starting from different photodarkening loss levels while keeping the same dopant concentration and subsequently the other way around. This approach related photobleaching with the number of photodarkening induced color centers and also showed that photobleaching was dopant concentration dependent. In all experiments an unbleachable residual loss for the bleaching powers up to ~100 W/mm2 was found. To the best of our knowledge this is the most extensive investigation of photobleaching effect.

© 2012 OSA

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  1. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
    [CrossRef]
  2. M. Leich, U. Röpke, S. Jetschke, S. Unger, V. Reichel, and J. Kirchhof, “Non-isothermal bleaching of photodarkened Yb-doped fibers,” Opt. Express17(15), 12588–12593 (2009).
    [CrossRef] [PubMed]
  3. J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
    [CrossRef]
  4. G. R. Atkins and F. Ouellette, “Reversible photodarkening and bleaching in Tb3+-doped optical fibers,” Opt. Lett.19(13), 951–953 (1994).
    [CrossRef] [PubMed]
  5. M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett.34(8), 1285–1287 (2009).
    [CrossRef] [PubMed]
  6. K. E. Mattsson and J. Broeng, “Alleviate photo darkening by single-mode RMO fiber design,” Proc. SPIE7580, 758024 (2010).
    [CrossRef]
  7. S. Jetschke, S. Unger, U. Röpke, and J. Kirchhof, “Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power,” Opt. Express15(22), 14838–14843 (2007).
    [CrossRef] [PubMed]
  8. S. Yoo, C. Basu, A. J. Boyland, C. Sones, J. Nilsson, J. K. Sahu, and D. Payne, “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett.32(12), 1626–1628 (2007).
    [CrossRef] [PubMed]
  9. M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al-doped fiber lasers,” Proc. SPIE6873, 68731E (2008).
    [CrossRef]
  10. A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
    [CrossRef]
  11. I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. Bello Doua, and F. Salin, “Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber,” Opt. Express15(4), 1606–1611 (2007).
    [CrossRef] [PubMed]
  12. N. Inoue, A. Shirakawa, and K. Ueda, “Photodarkening and photobleaching of Yb-doped fibers by laser diodes,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMGG5, http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-CMGG5 .
  13. P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
    [CrossRef]
  14. S. Taccheo, H. Gebavi, A. Monteville, O. Le Goffic, D. Landais, D. Mechin, D. Tregoat, B. Cadier, T. Robin, D. Milanese, and T. Durrant, “Concentration dependence and self-similarity of photodarkening losses induced in Yb-doped fibers by comparable excitation,” Opt. Express19(20), 19340–19345 (2011).
    [CrossRef] [PubMed]
  15. H. Gebavi, S. Taccheo, D. Milanese, A. Monteville, O. Le Goffic, D. Landais, D. Mechin, D. Tregoat, B. Cadier, and T. Robin, “Temporal evolution and correlation between cooperative luminescence and photodarkening in ytterbium doped silica fibers,” Opt. Express19(25), 25077–25083 (2011).
    [CrossRef] [PubMed]
  16. M. J. Söderlund, J. J. Montiel i Ponsoda, and S. Honkanen, “Measurement of thermal binding energy of photodarkening-induced color centers in ytterbium-doped silica fibers,” in Conference on Lasers and Electro-Optics-European Quantum Electronics Conference CLEO/EUROPE-EQEC (Optical Society of America, 2009), paper CE3.3.
  17. S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, “Efficient Yb laser fibers with low photodarkening by optimization of the core composition,” Opt. Express16(20), 15540–15545 (2008).
    [CrossRef] [PubMed]
  18. A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
    [CrossRef]

2011

2010

K. E. Mattsson and J. Broeng, “Alleviate photo darkening by single-mode RMO fiber design,” Proc. SPIE7580, 758024 (2010).
[CrossRef]

2009

M. Leich, U. Röpke, S. Jetschke, S. Unger, V. Reichel, and J. Kirchhof, “Non-isothermal bleaching of photodarkened Yb-doped fibers,” Opt. Express17(15), 12588–12593 (2009).
[CrossRef] [PubMed]

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett.34(8), 1285–1287 (2009).
[CrossRef] [PubMed]

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

2008

2007

1997

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

1994

Aleshkina, S. S.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Atkins, G. R.

Barber, P. R.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Barmenkov, Y. O.

A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
[CrossRef]

Basu, C.

Bello Doua, R.

Boullet, J.

Boyland, A. J.

Broeng, J.

K. E. Mattsson and J. Broeng, “Alleviate photo darkening by single-mode RMO fiber design,” Proc. SPIE7580, 758024 (2010).
[CrossRef]

Bubnov, M. M.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Cadier, B.

Caplen, J. E.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Cardinal, T.

Deladurantaye, P.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Desbiens, L.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Dianov, E. M.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Drolet, M.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Durrant, T.

Engholm, M.

Ermeneux, S.

Gebavi, H.

Guillen, F.

Gur'yanov, A. N.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Guzman Chávez, A. D.

A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
[CrossRef]

Hanna, D. C.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Honkanen, S.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Iho, A.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Il’ichev, N. N.

A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
[CrossRef]

Jelger, P.

Jetschke, S.

Kir’yanov, A. V.

A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
[CrossRef]

Kirchhof, J.

Koplow, J.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Koponen, J.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Landais, D.

Laperle, P.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Laurell, F.

Le Foulgoc, K.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Le Goffic, O.

Leich, M.

Likhachev, M. E.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Manek-Hönninger, I.

Mattsson, K. E.

K. E. Mattsson and J. Broeng, “Alleviate photo darkening by single-mode RMO fiber design,” Proc. SPIE7580, 758024 (2010).
[CrossRef]

Mechin, D.

Milanese, D.

Monteville, A.

Montiel i Ponsoda, J. J.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Nilsson, J.

S. Yoo, C. Basu, A. J. Boyland, C. Sones, J. Nilsson, J. K. Sahu, and D. Payne, “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett.32(12), 1626–1628 (2007).
[CrossRef] [PubMed]

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Norin, L.

Ouellette, F.

Paschotta, R.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Payne, D.

Podgorski, M.

Proulx, A.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Reichel, V.

Robin, T.

Röpke, U.

Rybaltovsky, A. A.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Sahu, J. K.

Salin, F.

Schwuchow, A.

Söderlund, M.

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

Sones, C.

Taccheo, S.

Taillon, Y.

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Tregoat, D.

Tropper, A. C.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Umnikov, A. A.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Unger, S.

Yashkov, M. V.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Yoo, S.

Laser Phys. Lett.

A. D. Guzman Chávez, A. V. Kir’yanov, Y. O. Barmenkov, and N. N. Il’ichev, “Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation,” Laser Phys. Lett.4(10), 734–739 (2007).
[CrossRef]

Opt. Commun.

R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun.136(5–6), 375–378 (1997).
[CrossRef]

Opt. Express

M. Leich, U. Röpke, S. Jetschke, S. Unger, V. Reichel, and J. Kirchhof, “Non-isothermal bleaching of photodarkened Yb-doped fibers,” Opt. Express17(15), 12588–12593 (2009).
[CrossRef] [PubMed]

I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. Bello Doua, and F. Salin, “Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber,” Opt. Express15(4), 1606–1611 (2007).
[CrossRef] [PubMed]

S. Jetschke, S. Unger, U. Röpke, and J. Kirchhof, “Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power,” Opt. Express15(22), 14838–14843 (2007).
[CrossRef] [PubMed]

S. Taccheo, H. Gebavi, A. Monteville, O. Le Goffic, D. Landais, D. Mechin, D. Tregoat, B. Cadier, T. Robin, D. Milanese, and T. Durrant, “Concentration dependence and self-similarity of photodarkening losses induced in Yb-doped fibers by comparable excitation,” Opt. Express19(20), 19340–19345 (2011).
[CrossRef] [PubMed]

H. Gebavi, S. Taccheo, D. Milanese, A. Monteville, O. Le Goffic, D. Landais, D. Mechin, D. Tregoat, B. Cadier, and T. Robin, “Temporal evolution and correlation between cooperative luminescence and photodarkening in ytterbium doped silica fibers,” Opt. Express19(25), 25077–25083 (2011).
[CrossRef] [PubMed]

S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, “Efficient Yb laser fibers with low photodarkening by optimization of the core composition,” Opt. Express16(20), 15540–15545 (2008).
[CrossRef] [PubMed]

Opt. Lett.

Proc. SPIE

K. E. Mattsson and J. Broeng, “Alleviate photo darkening by single-mode RMO fiber design,” Proc. SPIE7580, 758024 (2010).
[CrossRef]

J. J. Montiel i Ponsoda, M. Söderlund, J. Koplow, J. Koponen, A. Iho, and S. Honkanen, “Combined photodarkening and thermal bleaching measurement of an ytterbium-doped fiber,” Proc. SPIE7195, 71952D (2009).
[CrossRef]

M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al-doped fiber lasers,” Proc. SPIE6873, 68731E (2008).
[CrossRef]

P. Laperle, L. Desbiens, K. Le Foulgoc, M. Drolet, P. Deladurantaye, A. Proulx, and Y. Taillon, “Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers,” Proc. SPIE7195, 71952C (2009).
[CrossRef]

Quantum Electron.

A. A. Rybaltovsky, S. S. Aleshkina, M. E. Likhachev, M. M. Bubnov, A. A. Umnikov, M. V. Yashkov, A. N. Gur'yanov, and E. M. Dianov, “Luminescence and photoinduced absorption in ytterbium-doped optical fibres,” Quantum Electron.41(12), 1073–1079 (2011).
[CrossRef]

Other

M. J. Söderlund, J. J. Montiel i Ponsoda, and S. Honkanen, “Measurement of thermal binding energy of photodarkening-induced color centers in ytterbium-doped silica fibers,” in Conference on Lasers and Electro-Optics-European Quantum Electronics Conference CLEO/EUROPE-EQEC (Optical Society of America, 2009), paper CE3.3.

N. Inoue, A. Shirakawa, and K. Ueda, “Photodarkening and photobleaching of Yb-doped fibers by laser diodes,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMGG5, http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-CMGG5 .

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Figures (4)

Fig. 1
Fig. 1

Bleaching of 1.1 wt% Yb3+ doped fiber for different HeNe laser (633 nm) powers. Initial PD excess loss was 170 ± 5dB/m.

Fig. 2
Fig. 2

Percentage of the photodarkening bleached loss as the function of laser power at 633 nm.

Fig. 3
Fig. 3

(a) Bleaching with 0.8 mW light at 633 nm on the same fiber doped with 1.8 wt% Yb3+. Each curve presents bleaching which started from different PD loss as it is assigned in the label. (b) The bleaching amplitude as the function of initial PD loss amplitude.

Fig. 4
Fig. 4

(a) Bleaching of 1.8 wt% and 1.1 wt% Yb3+ fibers with 0.8 mW of 633 nm light irradiation. (b) Bleaching of 1.8 wt % and 1.35 wt% Yb3+ fibers with 0.8 mW light. The initial PD loss level was similar.

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