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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 6681–6688
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Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers

S. Rydberg and M. Engholm  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 6681-6688 (2013)
http://dx.doi.org/10.1364/OE.21.006681


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Abstract

In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb2+ ions. A concentration decrease in Yb3+ could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.

© 2013 OSA

1. Introduction

Near infrared (NIR) diode laser induced optical losses (photodarkening) in ytterbium (Yb) doped fiber lasers has been under extensive investigation during the last decade. Fiber lasers are commonly used in different industrial applications, where reliability and cost effective solutions are of key importance. Extensive efforts have been made by different research groups to investigate the physical mechanisms related to photodarkening (PD). Several routes to improve the Yb-doped glass material have also been reported including co-doping with phosphorus [1

1. A. Shubin, M. Yashkov, M. Melkumov, S. Smirnov, I. Bufetov, and E. Dianov, “Photodarkening of aluminosilicate and phosphosilicate Yb-doped fibers,” CLEO Europe Conference pp. CJ3–1–THU (2007).

], a combination of phosphorus and aluminum [2

2. 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. Express 16, 15540–15545 (2008) [CrossRef] [PubMed] .

] or cerium [3

3. M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in Yb-doped fiber lasers by Cerium co-doping,” Opt. Lett. 34, 1285–1287 (2009) [CrossRef] [PubMed] .

], post processing fibers with hydrogen [4

4. M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al based fiber lasers,” Proc. of SPIE 6873, 68731E–1 (2008).

] or oxygen [5

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

]. Still, the physical origin of the pump induced losses is a subject of discussion. So far, essentially two different models have been presented in the literature, which address the fundamental kinetic processes responsible for the pump induced losses. The first model implies that the valence instability of the Yb-ion is the origin of PD where a valence state change is believed to occur through a charge transfer (CT) process [6

6. M. Engholm, L. Norin, and D. Åberg, “Strong UV-absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV-excitation,” Opt. Lett. 32, 3352–3354 (2007) [CrossRef] [PubMed] .

, 7

7. M. Engholm and L. Norin, “Comment on “photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett. 33, 1216–1216 (2008) [CrossRef] [PubMed] .

]. This will result in pair generation of divalent Yb ions together with bound hole centers, which most likely can be assigned to aluminum oxygen hole centers (Al-OHCs) [8

8. T. Arai, K. Ichii, S. Tanigawa, and M. Fujimaki, “Gamma-radiation-induced photodarkening in ytterbium-doped silica glasses,” Proc. of SPIE 7914, 79140K (2011) [CrossRef] .

]. The second model is related to intrinsic defects such as Oxygen Deficiency Centers (ODCs), which are believed to form during the silica preform and fiber fabrication. The ODCs are believed to act as precursors for the generated hole related color centers [5

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

, 9

9. P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express 16, 4688–4697 (2008) [CrossRef] [PubMed] .

, 10

10. P. D. Dragic, Y.-S. Liu, T. C. Galvin, and J. G. Eden, “Ultraviolet absorption and excitation spectroscopy of rare-earth-doped glass fibers derived from glassy and crystalline preforms,” Proc. of SPIE 8237 (2012) [CrossRef] .

]. In addition, Liu et al. [11

11. Y.-S. Liu, T. C. Galvin, T. Hawkins, J. Ballato, L. Dong, P. Foy, P. Dragic, and J. G. Eden, “Linkage of oxygen deficiency defects and rare earth concentrations in silica glass optical fiber probed by ultraviolet absorption and laser excitation spectroscopy,” Opt. Express 20, 14494–14507 (2012) [CrossRef] [PubMed] .

] reported on the existence of an energy transfer between the ODC(II) center and the Yb3+ ion.

As the physical origin of PD is still an open question, our objective in this paper is to address the issue above concerning the valence instability of the Yb ion versus the ODC model as the primary cause of PD. Jasapara et al. [12

12. J. Jasapara, M. Andrejco, D. DiGiovanni, and R. Windeler, “Effect of heat and H2 gas on the photo-darkening of Yb3+ fibers,” Conf. Digest of CLEO p. CTuq5 (2005).

] was one of the first to suggest that Yb2+ ions are part of the PD process. Thermoluminescence measurements made by Mady et al. [13

13. F. Mady, M. Benabdesselam, and W. Blanc, “Termoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers,” Opt. Lett. 35, 3542–3543 (2010) [CrossRef] .

] on photodarkened Yb-doped silica preform samples also implied that Yb2+ ions play an important role in the PD process. Still, conclusive experimental evidence have so far not been presented for the presence of divalent ytterbium in photodarkened near infrared (NIR) diode pumped Yb/Al doped fibers. We have performed new spectroscopic experiments to investigate the presence of Yb2+ ions in UV-irradiated Yb/Al doped silica preforms as well as on NIR diode pumped photodarkened Yb/Al silica fibers. As will be shown the results are in favor for the first model above and we are indeed able to present experimental evidence for generation of divalent ytterbium in the PD process of Yb-doped fiber lasers.

2. Experimental

All investigated samples were fabricated by using the standard modified chemical vapor deposition (MCVD) technique as described in [14

14. M. Engholm and L. Norin, “Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass,” Opt. Express 2, 1260–1268 (2008) [CrossRef] .

] and doped with Yb- and Al (0.2 at% / 2.18 at%). Normal (oxidizing) conditions were used during the sintering and the collapse phase of the preform fabrication process. One small piece of the preform was also post processed in a reducing atmosphere (N2/H2) at 900 °C for 5 hours. The oxidized preform was drawn into a fiber with a 20/150 μm core and cladding diameter and coated with a low index polymer cladding (Luvantix PC-373).

UV absorption was measured on a thin piece of Yb/Al silica preform by using a deuterium light source and an Ocean Optics Maya Pro spectrometer. UV irradiation experiments were performed by focusing the output of a laser driven light source (LDLS) from Energetiq and monochromator set to 210 nm with an irradiation intensity of ∼0.11mW/mm2. The LDLS was also used as an excitation source in the luminescence and excitation experiments, where the emission from the preform samples was collected at right angles by using a 115 μm core optical fiber connected to an Ocean Optics USB2000+ spectrometer. Excitation spectra were measured by scanning the excitation wavelength and measuring the intensity of the emission from the preform sample by using the lock-in technique. Luminescence experiments on photodarkened fibers were made by using a single mode (SM) fiber coupled (1 mW, 405 nm) laser diode as an excitation source. The oxidized Yb/Al fiber was cladding pumped for several hours until PD saturation was reached by using a 20 W, 920nm, laser diode from Oclaro (MU20-915). A short piece of the photodarkened fiber was striped, carefully cleaned and spliced to the SM fiber. The emission was measured perpendicular to the Yb/Al fiber by using a 115 μm core optical fiber connected to the Ocean Optics USB2000+ spectrometer.

3. Results and discussion

Fig. 1 CT absorption band of the 0.2 at% Yb/Al doped silica glass preform (blue solid line). The black dashed lines show a Gaussian deconvolution of CT absorption band and the red dashed-dotted line shows the absorption spectrum of a non-Yb-doped reference preform.

To measure the potential occurrence of Yb2+ ions through photoluminescence on UV irradiated samples is challenging. The fraction of Yb3+ ions that will change its valency by UV irradiation is expected to be low. Hence, the Yb2+ emission is weak and difficult to measure. The fraction of Yb3+ ions that will change its valency in a NIR diode pumped fiber is expected to be even less, making the luminescence even more challenging to measure. Still, by careful optimization of the experimental setup and by using long integration times, we could detect the presence of Yb2+ through emission.

Figure 2 shows a normalized luminescence spectra for the reduced Yb/Al preform sample and the oxidized Yb/Al preform sample irradiated for 6 hours at 210 nm. Although they are not identical, there is a strong similarity between the characteristic Yb2+ luminescence observed for the post-processed Yb/Al preform and the UV irradiated oxidized preform. The most apparent discrepancy is a small shift to longer wavelengths for the emission band of the 210 nm irradiated preform sample. The origin of this shift is most likely related to a slightly different local environment for the irradiation induced Yb2+ ions compared to the Yb2+ ions in the post-processed (heat treated) Yb/Al preform. A different local environment will change the crystal field, which in turn affects the energy level structure for the Yb2+ ion. Hence, a minor discrepancy, e.g. shift of the emission band between the irradiated and post-processed Yb/Al samples is not unexpected. Further investigations revealed that the center wavelength of the emission band is actually shifting to longer wavelengths during irradiation, see inset of Fig. 2. The greatest change of the center wavelength occurs during the first hour after which the center wavelength appears to stabilize around 534 nm. Figure 3 displays the integrated intensity for the Yb2+ luminescence as function of irradiation time. As observed in the inset of Fig. 3 a small decrease is also observed for the characteristic Yb3+ emission indicating a decrease in Yb3+ concentration, which is consistent with relation (1).

Fig. 2 Normalized luminescence from the reduced Yb/Al preform (blue solid line) and the oxidized Yb/Al preform irradiated at 210 nm for 6 hours (red dashed line). The excitation wavelength is 300 nm for both samples. The center wavelength as a function of irradiation time is shown in the inset.
Fig. 3 Integrated intensity in the 400 – 800 nm range as a function of time for the Yb2+ luminescence. The integrated intensity for the Yb3+ emission near 1 μm is displayed in the inset.

Fig. 4 Excitation spectra for the reduced Yb/Al preform (blue solid line) and UV irradiated Yb/Al preform (red dashed line) monitored at 530 nm. The characteristic Yb2+ absorption spectrum is also shown for comparison (black dash-dotted line).

From the results above it can be concluded that Yb2+ ions are formed in Yb/Al doped silica glass through a CT process according to relation (1). A major question still remain: are Yb2+ ions also formed in the PD process of NIR diode pumped Yb/Al doped fibers? To investigate this, a short piece of the oxidized Yb/Al doped fiber was photodarkened to reach saturation by using a 920 nm, 20 W laser diode as a pump source. Due to the small dimension of the fiber core we where unsuccessful in using the LDLS as the excitation source. Instead, we used a SM fiber coupled 405 nm laser diode for these excitation experiments. According to the excitation spectrum in Fig. 4, the Yb2+ ions can be excited at this wavelength although it will be more experimentally challenging. UV- and visible pump sources have earlier been reported to cause photobleaching [18

18. A. Guzman Chávez, A. Kir’yanov, Y. Barmenkov, and 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, 734–739 (2007) [CrossRef] .

, 19

19. I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. B. Doua, and F. Sallin, “Photodarkening and photobleaching of an ytterbium-doped silica double clad LMA fiber,” Opt. Express 15, 1606–1611 (2007) [CrossRef] [PubMed] .

], which means that any possible presence of Yb2+ in the photodarkened fiber rapidly decreases when the fiber is exposed to UV- or visible radiation. Hence, special care has to be taken in selecting the excitation power of the 405 nm laser diode in relation to the integration time of the spectrometer. Figure 5 shows the 405 nm excited luminescence from the photodarkened fiber (black dashed line). No emission was detected from the pristine oxidized Yb/Al fiber (blue dash-dotted line). As observed there is a very good correlation between the NIR diode pumped Yb/Al fiber and the reduced Yb/Al preform, which provides evidence for the formation of Yb2+ in the PD process. A rapid decrease in intensity is observed due to the photobleaching effect, see the inset of Fig. 5. Unlike the case of the UV irradiated preform, no shift of the emission band is observed for the NIR diode pumped Yb/Al fiber. We interpret this result as only a small fraction of the Yb3+ ions are converted into Yb2+ in the photodarkened NIR diode pumped Yb/Al fiber as compared to the UV irradiated preform.

Fig. 5 Normalized luminescence from the NIR diode pumped photodarkened Yb/Al fiber excited by a 405 nm SM fiber coupled laser (black dash-dotted line) and the pristine Yb/Al fiber (blue dashed-dotted line). The reduced Yb/Al preform excited by 300 nm is shown for comparison (red solid line). The inset shows the decrease in intensity as a function of time.

Our results provides evidence that Yb2+ ions are formed in photodarkened Yb/Al doped silica fibers through a CT process. Yet a few more questions need to be considered for a comprehensive understanding of the PD phenomenon. In particular, what is the transfer route of the low energy NIR photons to the CT absorption band in the UV spectral range? It appears now that there is common acceptance that NIR photons are transferred to higher energy states (CT-band) through some still unidentified excitation channel. Peretti et al. [20

20. R. Peretti, A.-M. Jurdyc, B. Jacquier, C. Gonnet, A. Pastouret, E. Burov, and O. Cavani, “How do traces of thulium explain photodarkening in Yb doped fibers?,” Opt. Express 18, 20455–20460 (2010) [CrossRef] [PubMed] .

, 21

21. R. Peretti, C. Gonnet, and A.-M. Jurdyc, “A new vision of photodarkening in Yb3+-doped fibers,” J. of Appl. Phys. 112, 093511 (2012).

] have recently observed that small impurity levels of thulium (Tm3+) constitute a path to the color center generating CT bands. In contrast, Jetschke et al. [22

22. S. Jetschke, M. Leich, S. Unger, A. Schwuchow, and J. Kirchhof, “Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers,” Opt. Express 19, 14473–14478 (2011) [CrossRef] [PubMed] .

] implied that Tm3+ impurities below 1 mol-ppm should not influence the typical PD behavior in Yb/Al doped fibers. Yet another possible transfer route could be excited state absorption (ESA) from the 2F5/2 level [23

23. M. Engholm and S. Rydberg, “Strong excited state absorption (ESA) in Yb-doped fiber lasers,” Proc. of SPIE 8601, (2013).

]. The onset of a strong ESA in the visible range was observed in Yb/Al doped silica fibers with an inversion dependent absorption of several tens of dB per meter. Surely, more investigations are needed to identify the energy transfer route from the NIR range to the color center generating CT absorption bands and for a complete understanding of the PD mechanisms.

4. Conclusions

To conclude, we have shown that Yb2+ ions are formed in a 210 nm irradiated Yb/Al doped silica preform and a NIR diode pumped Yb/Al doped silica fiber through a CT process. The presented results show a strong support for the model that the valence stability of the Yb ion play a key role for the PD kinetics in Yb-doped fiber lasers. Furthermore, the Yb2+ ions can act as electron traps, which can charge compensate the Al-OHCs that has earlier been identified to be responsible for the induced absorption bands in the visible- and NIR range [8

8. T. Arai, K. Ichii, S. Tanigawa, and M. Fujimaki, “Gamma-radiation-induced photodarkening in ytterbium-doped silica glasses,” Proc. of SPIE 7914, 79140K (2011) [CrossRef] .

].

Acknowledgments

The Swedish Agency for Economic and Regional Growth, Fiber Optic Valley, The County Administrative Board in Västernorrland and the Regional Development Council of Gävleborg are gratefully acknowledged for the financial support.

References and links

1.

A. Shubin, M. Yashkov, M. Melkumov, S. Smirnov, I. Bufetov, and E. Dianov, “Photodarkening of aluminosilicate and phosphosilicate Yb-doped fibers,” CLEO Europe Conference pp. CJ3–1–THU (2007).

2.

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. Express 16, 15540–15545 (2008) [CrossRef] [PubMed] .

3.

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

4.

M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al based fiber lasers,” Proc. of SPIE 6873, 68731E–1 (2008).

5.

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

6.

M. Engholm, L. Norin, and D. Åberg, “Strong UV-absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV-excitation,” Opt. Lett. 32, 3352–3354 (2007) [CrossRef] [PubMed] .

7.

M. Engholm and L. Norin, “Comment on “photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett. 33, 1216–1216 (2008) [CrossRef] [PubMed] .

8.

T. Arai, K. Ichii, S. Tanigawa, and M. Fujimaki, “Gamma-radiation-induced photodarkening in ytterbium-doped silica glasses,” Proc. of SPIE 7914, 79140K (2011) [CrossRef] .

9.

P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express 16, 4688–4697 (2008) [CrossRef] [PubMed] .

10.

P. D. Dragic, Y.-S. Liu, T. C. Galvin, and J. G. Eden, “Ultraviolet absorption and excitation spectroscopy of rare-earth-doped glass fibers derived from glassy and crystalline preforms,” Proc. of SPIE 8237 (2012) [CrossRef] .

11.

Y.-S. Liu, T. C. Galvin, T. Hawkins, J. Ballato, L. Dong, P. Foy, P. Dragic, and J. G. Eden, “Linkage of oxygen deficiency defects and rare earth concentrations in silica glass optical fiber probed by ultraviolet absorption and laser excitation spectroscopy,” Opt. Express 20, 14494–14507 (2012) [CrossRef] [PubMed] .

12.

J. Jasapara, M. Andrejco, D. DiGiovanni, and R. Windeler, “Effect of heat and H2 gas on the photo-darkening of Yb3+ fibers,” Conf. Digest of CLEO p. CTuq5 (2005).

13.

F. Mady, M. Benabdesselam, and W. Blanc, “Termoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers,” Opt. Lett. 35, 3542–3543 (2010) [CrossRef] .

14.

M. Engholm and L. Norin, “Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass,” Opt. Express 2, 1260–1268 (2008) [CrossRef] .

15.

S. Rydberg and M. Engholm, “Charge transfer processes and UV induced absorption in Yb:YAG single crystal laser materials,” To be submitted (2013).

16.

J. Kirchhof, S. Unger, A. Schwuchow, S. Grimm, and V. Reichel, “Materials for high-power fiber lasers,” J. Non. Cryst. Solids 352, 2399–2403 (2006) [CrossRef] .

17.

J. Kirchhof, S. Unger, A. Schwuchow, S. Jetschke, V. Reichel, M. Leich, and A. Scheffel, “The influence of Yb2+ ions on optical properties and power stability of ytterbium-doped laser fibers,” Proc. of SPIE 7598, 75980B (2010) [CrossRef] .

18.

A. Guzman Chávez, A. Kir’yanov, Y. Barmenkov, and 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, 734–739 (2007) [CrossRef] .

19.

I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. B. Doua, and F. Sallin, “Photodarkening and photobleaching of an ytterbium-doped silica double clad LMA fiber,” Opt. Express 15, 1606–1611 (2007) [CrossRef] [PubMed] .

20.

R. Peretti, A.-M. Jurdyc, B. Jacquier, C. Gonnet, A. Pastouret, E. Burov, and O. Cavani, “How do traces of thulium explain photodarkening in Yb doped fibers?,” Opt. Express 18, 20455–20460 (2010) [CrossRef] [PubMed] .

21.

R. Peretti, C. Gonnet, and A.-M. Jurdyc, “A new vision of photodarkening in Yb3+-doped fibers,” J. of Appl. Phys. 112, 093511 (2012).

22.

S. Jetschke, M. Leich, S. Unger, A. Schwuchow, and J. Kirchhof, “Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers,” Opt. Express 19, 14473–14478 (2011) [CrossRef] [PubMed] .

23.

M. Engholm and S. Rydberg, “Strong excited state absorption (ESA) in Yb-doped fiber lasers,” Proc. of SPIE 8601, (2013).

OCIS Codes
(140.3510) Lasers and laser optics : Lasers, fiber
(160.5690) Materials : Rare-earth-doped materials
(260.5210) Physical optics : Photoionization
(140.3615) Lasers and laser optics : Lasers, ytterbium

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 11, 2013
Revised Manuscript: March 3, 2013
Manuscript Accepted: March 4, 2013
Published: March 11, 2013

Citation
S. Rydberg and M. Engholm, "Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers," Opt. Express 21, 6681-6688 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-6681


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References

  1. A. Shubin, M. Yashkov, M. Melkumov, S. Smirnov, I. Bufetov, and E. Dianov, “Photodarkening of aluminosilicate and phosphosilicate Yb-doped fibers,” CLEO Europe Conference pp. CJ3–1–THU (2007).
  2. 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, 15540–15545 (2008). [CrossRef] [PubMed]
  3. M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in Yb-doped fiber lasers by Cerium co-doping,” Opt. Lett.34, 1285–1287 (2009). [CrossRef] [PubMed]
  4. M. Engholm and L. Norin, “Reduction of photodarkening in Yb/Al based fiber lasers,” Proc. of SPIE6873, 68731E–1 (2008).
  5. S. Yoo, C. Basu, A. Boyland, C. Sones, J. Nilsson, J. Sahu, and D. Payne, “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett.32, 1626–1628 (2007). [CrossRef] [PubMed]
  6. M. Engholm, L. Norin, and D. Åberg, “Strong UV-absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV-excitation,” Opt. Lett.32, 3352–3354 (2007). [CrossRef] [PubMed]
  7. M. Engholm and L. Norin, “Comment on “photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation,” Opt. Lett.33, 1216–1216 (2008). [CrossRef] [PubMed]
  8. T. Arai, K. Ichii, S. Tanigawa, and M. Fujimaki, “Gamma-radiation-induced photodarkening in ytterbium-doped silica glasses,” Proc. of SPIE7914, 79140K (2011). [CrossRef]
  9. P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express16, 4688–4697 (2008). [CrossRef] [PubMed]
  10. P. D. Dragic, Y.-S. Liu, T. C. Galvin, and J. G. Eden, “Ultraviolet absorption and excitation spectroscopy of rare-earth-doped glass fibers derived from glassy and crystalline preforms,” Proc. of SPIE8237 (2012). [CrossRef]
  11. Y.-S. Liu, T. C. Galvin, T. Hawkins, J. Ballato, L. Dong, P. Foy, P. Dragic, and J. G. Eden, “Linkage of oxygen deficiency defects and rare earth concentrations in silica glass optical fiber probed by ultraviolet absorption and laser excitation spectroscopy,” Opt. Express20, 14494–14507 (2012). [CrossRef] [PubMed]
  12. J. Jasapara, M. Andrejco, D. DiGiovanni, and R. Windeler, “Effect of heat and H2 gas on the photo-darkening of Yb3+ fibers,” Conf. Digest of CLEO p. CTuq5 (2005).
  13. F. Mady, M. Benabdesselam, and W. Blanc, “Termoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers,” Opt. Lett.35, 3542–3543 (2010). [CrossRef]
  14. M. Engholm and L. Norin, “Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass,” Opt. Express2, 1260–1268 (2008). [CrossRef]
  15. S. Rydberg and M. Engholm, “Charge transfer processes and UV induced absorption in Yb:YAG single crystal laser materials,” To be submitted (2013).
  16. J. Kirchhof, S. Unger, A. Schwuchow, S. Grimm, and V. Reichel, “Materials for high-power fiber lasers,” J. Non. Cryst. Solids352, 2399–2403 (2006). [CrossRef]
  17. J. Kirchhof, S. Unger, A. Schwuchow, S. Jetschke, V. Reichel, M. Leich, and A. Scheffel, “The influence of Yb2+ ions on optical properties and power stability of ytterbium-doped laser fibers,” Proc. of SPIE7598, 75980B (2010). [CrossRef]
  18. A. Guzman Chávez, A. Kir’yanov, Y. Barmenkov, and 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, 734–739 (2007). [CrossRef]
  19. I. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. B. Doua, and F. Sallin, “Photodarkening and photobleaching of an ytterbium-doped silica double clad LMA fiber,” Opt. Express15, 1606–1611 (2007). [CrossRef] [PubMed]
  20. R. Peretti, A.-M. Jurdyc, B. Jacquier, C. Gonnet, A. Pastouret, E. Burov, and O. Cavani, “How do traces of thulium explain photodarkening in Yb doped fibers?,” Opt. Express18, 20455–20460 (2010). [CrossRef] [PubMed]
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