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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 4 — Apr. 1, 2013
  • pp: 452–458
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Deactivation of Yb3+ ions due to photodarkening

S. Jetschke, A. Schwuchow, S. Unger, M. Leich, M. Jäger, and J. Kirchhof  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 4, pp. 452-458 (2013)
http://dx.doi.org/10.1364/OME.3.000452


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Abstract

We carefully examined the fluorescence properties of Yb-doped fibers before and after pump-induced photodarkening (PD) and found a significant reduction of Yb3+ fluorescence intensity and lifetime due to the degradation process, obviously correlated with the measured PD loss. To explain these results, a non-radiative energy transfer from excited Yb3+ ions to the atomic defects (color centers) in close vicinity of the Yb3+ ions is suggested and discussed quantitatively as a function of PD loss generated in fibers with different Yb concentrations, by various pump powers or at different photodarkening states. All modifications of fluorescence properties proved to be reversed by thermal bleaching of the PD-induced color centers.

© 2013 OSA

1. Introduction

Owing to lack of evidence of Yb2+ creation, other mechanisms are to be considered to explain color center generation including Yb3+ ions that are doubtlessly involved in the photodarkening process. In [12

12. K. E. Mattsson, “Photo darkening of rare earth doped silica,” Opt. Express 19(21), 19797–19812 (2011). [CrossRef] [PubMed]

], electron transfer between nearest-neighbor non-binding oxygen atoms caused by doping silica with Yb3+ and Al3+ is proposed as origin of the PD process that takes place by the action of two pump photons in collaboration with phonons produced by excited Yb3+ ions.

A direct proof of charge carrier movement between atomic defects acting as precursors or traps is out of the scope of this paper; however, modifications of the environment should have consequences on the Yb3+ ions themselves and can be investigated by the analysis of fluorescence properties. Despite unchanged absorption, a distinct reduction of Yb3+ fluorescence and lifetime after PD is observed. These effects are quantitatively evaluated in this paper for fibers with various Yb concentrations, for different photodarkening states below the equilibrium at room temperature, and also for degradation by different Yb inversions. The results are discussed in correlation to the measured PD loss values.

2. Experimental

Preform samples were prepared by MCVD (Modified Chemical Vapor Deposition) and solution doping and collapsed in an O2/Cl2 atmosphere. For characterization purposes, fiber samples with a cladding diameter of 125 µm and a core diameter of approximately 10 µm were drawn. All fibers were doped with nearly constant amounts of Al2O3 (≈4.5 mol%) and P2O5 (≈0.5 mol%). The concentration of Yb2O3 varied between 500 and 6800 mol-ppm and was restricted to the range without concentration quenching effects [13

13. S. Jetschke, S. Unger, M. Leich, and J. Kirchhof, “Photodarkening kinetics as a function of Yb concentration and the role of Al codoping,” Appl. Opt. 51(32), 7758–7764 (2012). [CrossRef] [PubMed]

].

To analyze PD and to prepare samples for the subsequent fluorescence investigations, the fibers were cladding-pumped at room temperature with up to 16 W at 915 nm (Yb inversion up to 0.7) in the setup described in [5

5. 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. Express 15(22), 14838–14843 (2007). [CrossRef] [PubMed]

]. A tapered fiber bundle (TFB) coupler with signal feed-through was used to couple the pump power into the cladding and the probe light (wavelength 633 nm, power < 1 nW) into the core of short fiber pieces (1 to 2 cm) under test to observe the temporal evolution of PD loss. The steady state values of PD loss achieved after pump switch-off are later on discussed in relation to the observed changes of fluorescence properties.

Three series of fiber samples were prepared in this photodarkening setup and kept for subsequent fluorescence analysis in comparison to the pristine samples:

  • - Series I: fibers with Yb2O3 concentration of 500, 900, 2000, 2700, 3800, 4700, 5900 and 6800 mol-ppm were pumped with 16 W (Yb inversion 0.7) to induce PD loss near the equilibrium values. However, practically no PD effect was found in both fibers with Yb content below 2000 ppm.
  • - Series II: six pristine pieces of one fiber type (Yb content of 5900 mol-ppm) were pumped with 16 W (Yb inversion 0.7) for different times (pump interrupted after 11 s, 25 s, 65 s, 3 min, 7 min, or 37 min) to vary the PD loss below and up to the equilibrium value for this fiber type.
  • - Series III: six pristine pieces of one fiber type (Yb content of 6800 mol-ppm) were pumped with different pump powers (Yb inversion 0.2, 0.3, 0.4, 0.5, 0.6, 0.7) up to PD loss near the equilibrium values [5

    5. 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. Express 15(22), 14838–14843 (2007). [CrossRef] [PubMed]

    ].

By this procedure, fiber samples with final PD loss varying in a wide range were prepared. A linear dependence of PD loss on Yb inversion and a slightly nonlinear increase with Yb concentration (for constant inversion) are known from earlier investigations [5

5. 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. Express 15(22), 14838–14843 (2007). [CrossRef] [PubMed]

,13

13. S. Jetschke, S. Unger, M. Leich, and J. Kirchhof, “Photodarkening kinetics as a function of Yb concentration and the role of Al codoping,” Appl. Opt. 51(32), 7758–7764 (2012). [CrossRef] [PubMed]

].

The characterization of fluorescence intensity and decay was done with continuous-wave (CW) or chopped (pulses of 0.5 ms, repetition rate 10 Hz) excitation, respectively, using a MOPA system emitting at 976 nm. The laser beam is focused perpendicularly to the fiber core to excite a very small volume (fiber core with diameter 10 µm) thus avoiding ASE (amplified spontaneous emission) and reabsorption effects. A collecting fiber (diameter 400 µm), which guides the fluorescence light to the spectrometer or to the detector, is oriented perpendicularly to both, the exciting beam and the fiber axis. This twice-perpendicular measurement method is described more detailed in [14

14. A. Schwuchow, S. Unger, S. Jetschke, and J. Kirchhof (Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany) are preparing a manuscript to be called “Advanced absorption and fluorescence measurements to characterize photodarkening and related properties of Yb fibers”.

]. Yb inversion was kept low by using the same low excitation power with an estimated value of ≈1 mW corresponding to an Yb inversion of ≈0.1 for all CW measurements, whereas the inversion is even lower in chopped operation. Moreover, the measurement time for averaging the spectrum or chopped signal was kept below 10 s. Thus, PD effects in pristine samples or modifications of PD loss in already photodarkened samples are avoided. A quantitative comparison of the Yb3+ fluorescence properties between pristine and photodarkened fibers and between the samples of the three series introduced above is enabled by the advanced adjustment technique, which guarantees good reproducibility of the measurements by the use of a calibration fiber [14

14. A. Schwuchow, S. Unger, S. Jetschke, and J. Kirchhof (Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany) are preparing a manuscript to be called “Advanced absorption and fluorescence measurements to characterize photodarkening and related properties of Yb fibers”.

]. The fluorescence decay after the end of exciting 976 nm pulses is characterized in the near-infrared (NIR) region (λ > 1 µm). The discussed fluorescence intensity values in NIR (1075 nm) and VIS (500 nm) are picked up from the measured spectra of Yb3+ NIR emission and cooperative luminescence.

3. Results and discussion

The evolution of PD loss during pumping with 16 W (Yb inversion 0.7) is depicted in Fig. 1(a)
Fig. 1 (a) PD evolution in the fibers of series I with Yb content ≥ 2000 mol-ppm, (b) NIR fluorescence decay measured on the fibers of series II after preparation by the PD process up to different PD loss values (pumping of pristine fiber pieces was interrupted after different times), compared to the decay curve of a pristine fiber.
for the fibers of series I with Yb content of 2000 mol-ppm or higher. (No measurable PD loss was observed in fibers with Yb content < 2000 mol-ppm for pump duration up to 900 min.) The pump-induced PD process was applied to prepare the fibers for subsequent fluorescence analysis, also in case of the fibers of series II and III.

In Fig. 1(b), the measured decay of normalized NIR fluorescence intensity If (t)/If (0) after the end of 976 nm excitation is shown for the fibers of series II, serving as examples. Similar curves were recorded for all pristine and photodarkened samples of series I and III, too. In all cases, the measured fluorescence decay could be fitted by the single-exponential decay function of Eq. (1) with good accuracy, thus delivering one meaningful time constant τ for each measurement.

If(t)/If(0)=exp(t/τ)
(1)

The VIS fluorescence intensity is also influenced by PD, even to a stronger extent because of the Yb3+ pair effect of cooperative luminescence as shown in Fig. 3(a) for series II. However, the absolute values of measured VIS intensity are more than 3 orders of magnitude below the NIR ones and can thus be neglected in the following discussion.

PD effects on NIR fluorescence lifetime and intensity are not independent from each other. The accelerated but still single-exponential fluorescence decay after PD points to a certain probability of a non-radiative energy transfer concerning all excited Yb3+ ions (homogeneous deactivation) with a rate Rnr. Thereby, the measured lifetime τPD is reduced in comparison to the pristine value τ0 according to

τPD1=τ01+Rnr.
(2)

IPD/I0=n2(PD)/n2(0)=1+c/τ01+c/τPDwithc=AhυPexc(σabs+σem)
(3)

The results of IPD /I0 calculated from Eq. (3) with Pexc = 1 mW are included in Figs. 2(b) and 3(b) for the samples of series I and III, respectively. Similar results were found for series II. It is to be observed that the courses of calculated values show lower fluctuations than the measured ones because of the higher accuracy of temporal analysis compared to intensity measurements. Therefore, only the experimental results of lifetime analysis are used in the following discussion.

Generally, a strong correlation of fluorescence decrease with increasing PD loss, which is supposed to be proportional to the density of generated color centers, is indicated from Fig. 3. This is also valid for the fiber series with increasing Yb content (series I). From these observations we conclude that the non-radiative energy transfer discussed above could take place from Yb3+ ions (donors) excited during fluorescence analysis to the color centers generated by the pump-induced PD process before. The absorption of color centers spectrally overlaps with Yb3+ emission; they can thus be hypothesized as acceptors of transferred energy if they remain in close vicinity to the Yb3+ ions after the PD process. Moreover, the complete recovery of fluorescence intensity and lifetime after thermal bleaching of PD loss (temperature 600°C) points to the color centers as the real acceptors of energy in the photodarkened fibers.

An appropriate measure to quantitatively evaluate this effect is the transfer efficiency η [16

16. M. Laroche, S. Girard, J. K. Sahu, W. A. Clarkson, and J. Nilsson, “Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers,” J. Opt. Soc. Am. B 23(2), 195–202 (2006). [CrossRef]

], which can be calculated in our case from the NIR fluorescence lifetimes measured before and after PD,

η=1τPD/τ0.
(4)

The results are depicted in Fig. 4
Fig. 4 Efficiency of non-radiative energy transfer vs. measured PD loss.
for all three series of fiber samples investigated in this work. An almost linear dependence of transfer efficiency on PD loss (proportional to density of color centers) is observed. All samples of series I and III were pumped up to PD loss near the equilibrium value. The differences in transfer efficiency for PD loss below 800 dB/m can be attributed to the higher Yb content (6800 mol-ppm) in the samples of series III; a stronger energy migration between Yb3+ ions (donors) can enhance the non-radiative transfer to the acceptors [16

16. M. Laroche, S. Girard, J. K. Sahu, W. A. Clarkson, and J. Nilsson, “Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers,” J. Opt. Soc. Am. B 23(2), 195–202 (2006). [CrossRef]

]. The results for series II (Yb content of 5900 mol-ppm) are found between both other curves. An influence of the fiber states below PD equilibrium (PD loss < 700 dB/m), which could be characterized by particular distributions of distances between Yb3+ ions and color centers, cannot be proved by these results.

4. Conclusions

The atomic defects generated during photodarkening in Yb fibers not only operate as absorbing color centers for photons in the wavelength range from UV up to NIR. They are suspected also to take over energy from excited Yb3+ ions directly, because a strong, nearly linear correlation of PD loss (proportional to density of color centers) and reduction of fluorescence was found. From the distinct efficiency of this non-radiative energy transfer we conclude that color centers remain in close vicinity of the Yb3+ ions where they were generated during the pump-induced PD process.

This energy transfer can cause additional loss of laser efficiency by the deactivation of Yb3+ ions that absorb pump power without contributing to the laser process. Further investigations including modeling of Yb fiber lasers will study the impact of both, color center absorption and Yb3+ deactivation, on the performance particularly of high power fiber lasers and amplifiers.

Photodarkening can also occur during fluorescence analysis of Yb fibers, especially in case of high Yb content. As our investigations show, the results will then not correspond to the fluorescence properties of pristine fibers. To avoid this inaccuracy, the exciting intensity or the measurement time should be reduced until no further increase of fluorescence intensity and lifetime are observed. The upper limits of reasonable intensity and measurement time depend on the Yb concentration as well as on the host glass composition.

Acknowledgments

We gratefully acknowledge financial support by the German Federal Ministry of Education and Research (BMBF) under contract 13N 9555 (FALAMAT) and the Thuringian Ministry of Economics, Labor and Technology (TMWAT) under contract 2011 FGR 0104 (FG Faser-Tech) with financial support from the European Social Fund (EFS).

References and links

1.

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

2.

J. J. Koponen, M. J. Söderlund, H. J. Hoffman, and S. K. T. Tammela, “Measuring photodarkening from single-mode ytterbium doped silica fibers,” Opt. Express 14(24), 11539–11544 (2006). [CrossRef] [PubMed]

3.

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 16(2), 1260–1268 (2008). [CrossRef] [PubMed]

4.

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]

5.

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. Express 15(22), 14838–14843 (2007). [CrossRef] [PubMed]

6.

A. D. Guzman Chávez, A. V. Kiryanov, Y. O. Barmenkov, and N. N. Ilichev, “Reversible photodarkening 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]

7.

J. Kirchhof, S. Unger, S. Jetschke, A. Schwuchow, M. Leich, and V. Reichel, “Yb doped silica based laser fibers: correlation of photodarkening kinetics and related optical properties with the glass composition,” Proc. SPIE 7195, 71950S, 71950S-15 (2009). [CrossRef]

8.

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. SPIE 7598, 75980B, 75980B-11 (2010). [CrossRef]

9.

M. N. Zervas, F. Ghiringhelli, M. K. Durkin, and I. Crowe, “Distribution of photodarkening-induced loss in Yb-doped fiber amplifiers,” Proc. SPIE 7914, 79140L, 79140L-8 (2011). [CrossRef]

10.

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]

11.

H. Gebavi, S. Taccheo, D. Tregoat, A. Monteville, and T. Robin, “Photobleaching of photodarkening in ytterbium doped aluminosilicate fibers with 633 nm irradiation,” Opt. Mater. Express 2(9), 1286–1291 (2012). [CrossRef]

12.

K. E. Mattsson, “Photo darkening of rare earth doped silica,” Opt. Express 19(21), 19797–19812 (2011). [CrossRef] [PubMed]

13.

S. Jetschke, S. Unger, M. Leich, and J. Kirchhof, “Photodarkening kinetics as a function of Yb concentration and the role of Al codoping,” Appl. Opt. 51(32), 7758–7764 (2012). [CrossRef] [PubMed]

14.

A. Schwuchow, S. Unger, S. Jetschke, and J. Kirchhof (Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany) are preparing a manuscript to be called “Advanced absorption and fluorescence measurements to characterize photodarkening and related properties of Yb fibers”.

15.

S. Jetschke and U. Röpke, “Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers,” Opt. Lett. 34(1), 109–111 (2009). [CrossRef] [PubMed]

16.

M. Laroche, S. Girard, J. K. Sahu, W. A. Clarkson, and J. Nilsson, “Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers,” J. Opt. Soc. Am. B 23(2), 195–202 (2006). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(160.2290) Materials : Fiber materials
(160.5690) Materials : Rare-earth-doped materials

ToC Category:
Materials for Fiber Optics

History
Original Manuscript: November 26, 2012
Revised Manuscript: February 19, 2013
Manuscript Accepted: March 6, 2013
Published: March 7, 2013

Citation
S. Jetschke, A. Schwuchow, S. Unger, M. Leich, M. Jäger, and J. Kirchhof, "Deactivation of Yb3+ ions due to photodarkening," Opt. Mater. Express 3, 452-458 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-4-452


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References

  1. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibers,” Opt. Commun.136(5-6), 375–378 (1997). [CrossRef]
  2. J. J. Koponen, M. J. Söderlund, H. J. Hoffman, and S. K. T. Tammela, “Measuring photodarkening from single-mode ytterbium doped silica fibers,” Opt. Express14(24), 11539–11544 (2006). [CrossRef] [PubMed]
  3. M. Engholm and L. Norin, “Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass,” Opt. Express16(2), 1260–1268 (2008). [CrossRef] [PubMed]
  4. 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]
  5. 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]
  6. A. D. Guzman Chávez, A. V. Kiryanov, Y. O. Barmenkov, and N. N. Ilichev, “Reversible photodarkening 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]
  7. J. Kirchhof, S. Unger, S. Jetschke, A. Schwuchow, M. Leich, and V. Reichel, “Yb doped silica based laser fibers: correlation of photodarkening kinetics and related optical properties with the glass composition,” Proc. SPIE7195, 71950S, 71950S-15 (2009). [CrossRef]
  8. 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. SPIE7598, 75980B, 75980B-11 (2010). [CrossRef]
  9. M. N. Zervas, F. Ghiringhelli, M. K. Durkin, and I. Crowe, “Distribution of photodarkening-induced loss in Yb-doped fiber amplifiers,” Proc. SPIE7914, 79140L, 79140L-8 (2011). [CrossRef]
  10. 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]
  11. H. Gebavi, S. Taccheo, D. Tregoat, A. Monteville, and T. Robin, “Photobleaching of photodarkening in ytterbium doped aluminosilicate fibers with 633 nm irradiation,” Opt. Mater. Express2(9), 1286–1291 (2012). [CrossRef]
  12. K. E. Mattsson, “Photo darkening of rare earth doped silica,” Opt. Express19(21), 19797–19812 (2011). [CrossRef] [PubMed]
  13. S. Jetschke, S. Unger, M. Leich, and J. Kirchhof, “Photodarkening kinetics as a function of Yb concentration and the role of Al codoping,” Appl. Opt.51(32), 7758–7764 (2012). [CrossRef] [PubMed]
  14. A. Schwuchow, S. Unger, S. Jetschke, and J. Kirchhof (Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany) are preparing a manuscript to be called “Advanced absorption and fluorescence measurements to characterize photodarkening and related properties of Yb fibers”.
  15. S. Jetschke and U. Röpke, “Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers,” Opt. Lett.34(1), 109–111 (2009). [CrossRef] [PubMed]
  16. M. Laroche, S. Girard, J. K. Sahu, W. A. Clarkson, and J. Nilsson, “Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er3+-Yb3+-codoped fibers,” J. Opt. Soc. Am. B23(2), 195–202 (2006). [CrossRef]

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