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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 13 — Jun. 18, 2012
  • pp: 14494–14507
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Linkage of oxygen deficiency defects and rare earth concentrations in silica glass optical fiber probed by ultraviolet absorption and laser excitation spectroscopy

Y.-S. Liu, T. C. Galvin, T. Hawkins, J. Ballato, L. Dong, P.R. Foy, P.D. Dragic, and J. G. Eden  »View Author Affiliations


Optics Express, Vol. 20, Issue 13, pp. 14494-14507 (2012)
http://dx.doi.org/10.1364/OE.20.014494


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Abstract

Ultraviolet absorption measurements and laser excitation spectroscopy in the vicinity of 248 nm provide compelling evidence for linkages between the oxygen deficiency center (ODC) and rare earth concentrations in Yb and Er-doped glass optical fibers. Investigations of YAG-derived and solution-doped glass fibers are described. For both Yb and Er-doped fibers, the dependence of Type II ODC absorption on the rare earth number density is approximately linear, but the magnitude of the effect is greater for Yb-doped fibers. Furthermore, laser excitation spectra demonstrate unambiguously the existence of an energy transfer mechanism coupling an ODC with Yb3+. Photopumping glass fibers with a Ti:sapphire laser/optical parametric amplifier system, tunable over the 225-265 nm region, or with a KrF laser at 248.4 nm show: 1) emission features in the 200-1100 nm interval attributable only to the ODC (Type II) defect or Yb3+, and 2) the excitation spectra for ODC (II) emission at ~280 nm and Yb3+ fluorescence (λ ~1.03 μm) to be, within experimental uncertainty, identical. The latter demonstrates that, when irradiating Yb-doped silica fibers between ~240 and 255 nm, the ODC (II) defect is at least the primary precursor to Yb3+ emission. Consistent with previous reports in the literature, the data show the ODC (II) absorption spectrum to have a peak wavelength and breadth of ~246 nm and ~19 nm (FWHM). Experiments also reveal that, in the absence of Yb, incorporating either Al2O3 or Y2O3 into glass fibers has a negligible impact on the ODC concentration. Not only do the data reported here demonstrate the relationship between the ODC (II) number density and the Yb doping concentration, but they also suggest that the appearance of ODC defects in the fiber is associated with the introduction of Yb and the process by which the fiber is formed.

© 2012 OSA

1. Introduction

Ytterbium-doped silica optical fiber is now an essential element of many fiber laser systems but is known to suffer from photodarkening [1

1. 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]

,2

2. 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. Express 15(4), 1606–1611 (2007). [CrossRef] [PubMed]

], a photodegradation effect that manifests itself as a gradual reduction in output laser power over time. Although several mechanisms and species potentially responsible for photodarkening, such as charge transfer mechanisms [3

3. M. Engholm and L. Norin, “Comment on “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation”,” Opt. Lett. 33(11), 1216, discussion 1217–1218 (2008). [CrossRef] [PubMed]

,4

4. M. Engholm, L. Norin, and D. Åberg, “Strong UV absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV excitation,” Opt. Lett. 32(22), 3352–3354 (2007). [CrossRef] [PubMed]

] and intrinsic defects [5

5. 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]

], as well as mitigation approaches [6

6. 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]

,7

7. 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(20), 15540–15545 (2008). [CrossRef] [PubMed]

], have been proposed, a definitive explanation for the photodarkening process remains a subject of debate. Furthermore, while photodarkening has generally been characterized in drawn fiber [1

1. 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]

], investigations of its precursors have focused on the fiber preforms and not the optical fiber itself.

It has long been understood that the photosensitivity of germanosilicate glasses in the ultraviolet (UV), upon which the fabrication of fiber Bragg gratings is based, is attributable to the existence of intrinsic point defects in the silica matrix such as the oxygen deficiency centers (ODCs) [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

,9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

]. Specifically, Amossov and Rybaltovsky [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

] note that: “As is generally known, the absorption band in the spectral range of 242-248 nm is connected with one of the most widespread defects in the silica glass network, namely, ODCs formed either on the basis of silicon itself or germanium…” It has also been demonstrated previously [10

10. J. W. Lee, G. H. Sigel Jr, and J. Li, “Processing-induced defects in optical waveguide materials,” J. Non-Cryst. Solids 239(1-3), 57–65 (1998). [CrossRef]

] that the process of drawing fibers is capable of introducing defects such as the ODCs, two of which appear to be dominant [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

,9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

,11

11. H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen-deficient centers in synthetic silica glass,” Phys. Rev. B Condens. Matter 38(17), 12772–12775 (1988). [CrossRef] [PubMed]

].

The first of these, known as ODC (I), consists of an oxygen vacancy lying between two bonded silicon atoms and is represented chemically as ≡Si-Si≡ (also known as the ‘relaxed’ vacancy). The other prominent oxygen vacancy defect, known as ODC (II) (B2 band), has two plausible configurations [9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

]. One is the silylene-type defect (represented as: Si = ) in which two oxygen vacancies reside at a single silicon atom, whereas the second is represented as an oxygen vacancy on two unbounded Si sites (≡Si…Si≡, the ‘unrelaxed’ vacancy). ODCs are produced when one or more oxygen bonds in the glass are cleaved, resulting in defects that are able to profoundly impact the optical properties of the fiber. In this work, we focus on the ODC (II) variant since it has been suggested as a precursor to the E’- center and the formation of the non-bridging oxygen hole center (NBOHC) [9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

,11

11. H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen-deficient centers in synthetic silica glass,” Phys. Rev. B Condens. Matter 38(17), 12772–12775 (1988). [CrossRef] [PubMed]

,12

12. H. Imai, K. Arai, J. Isoya, H. Hosono, Y. Abe, and H. Imagawa, “Generation of E’ centers and oxygen hole centers in synthetic silica glasses by γ irradiation,” Phys. Rev. B Condens. Matter 48(5), 3116–3123 (1993). [CrossRef] [PubMed]

], both of which are participants in a UV-induced defect interconversion process that provides one potential avenue for photodarkening.

To date, few studies characterizing the optical and kinetic properties of optically-active, intrinsic defects in rare earth-doped silica fibers have appeared in the literature. To that end, the experiments reported here were designed and conducted in an effort to explore a potential linkage between Yb ions and ODCs in doped silica fibers that was proposed in Ref. 13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

. The subjects of the present study are silica fibers co-doped with Al and Yb (at Yb number densities [Yb] ≤ 5 × 1019 cm−3) and produced by an industry standard solution-doping method, and Yb:YAG-derived [14

14. J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T. M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, and M. J. Matthewson, “On the fabrication of all-glass optical fibers from crystals,” J. Appl. Phys. 105(5), 053110 (2009). [CrossRef]

], heavily-doped ([Yb] < 2.4 × 1020 cm−3) silica fibers.

Experiments are described in which the characteristic absorption band of ODC (II) in silica, exhibiting maximum absorption at ~246 nm [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

] and a spectral width of 15-18 nm [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

,9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

], is probed by laser excitation spectroscopy. Spectra acquired by photoexciting Yb-doped silica fibers at discrete wavelengths in the 225-265 nm interval while monitoring fluorescence at 978 (Yb3+) or 282 nm (ODC (II)) are found to be identical to within experimental uncertainty, demonstrating unambiguously that the photoexcited ODC (II) defect is a precursor to Yb3+ emission. This result confirms the recent proposal [13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

] of ODC → Yb3+ excitation transfer in Yb-doped silica fibers irradiated at λ ~250 nm, and provides further support for the premise that Yb3+ sites and ODC defects are coordinated. Measurements of the ODC (II) absorption spectrum in a set of fibers, in which the Yb concentration is varied, demonstrate that the ODC (II) number density is linearly proportional to the Yb concentration ([Yb], expressed in units of cm-3). Specifically, the ODC (II) defect density is ~1% of [Yb]. Furthermore, fluorescence lifetime measurements detect no precipitate-like (fast-decay clustering) regions in the most heavily-doped fibers examined to date. Finally, experiments show that two very different performs (solution-doped, modified chemical vapor deposition glass, and crystalline ytrrium aluminum garnet (YAG)) both yield glass fibers having similar absorption and emission spectra when photoexcited near 250 nm in the UV.

II. Experimental arrangement and data acquisition

Eight Yb- or Er-doped silica fibers were investigated in this study and Table 1

Table 1. Physical parameters for the Er or Yb-doped silica fibers investigated in these experiments. The measured peak wavelength (λp) and spectral breadth (FWHM) for the ODC (II) absorption band are also indicated for each fiber. The fluorescence decay constant τ is discussed in in Sect. IIIC.

table-icon
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provides both the physical dimensions and dopant concentrations for each fiber. Fibers A, B, and C are Al-Yb co-doped and were manufactured by Nufern (East Granby, CT) with a solution doping process. Shaped claddings were provided for both Fibers B and C, but as described in Sect. III, this geometry was found to have no impact on the spectroscopic measurements. Fibers D and E are Yb:YAG-derived fibers, fabricated at Clemson University. Details regarding the fabrication and general characteristics of these fibers can be found in [14

14. J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T. M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, and M. J. Matthewson, “On the fabrication of all-glass optical fibers from crystals,” J. Appl. Phys. 105(5), 053110 (2009). [CrossRef]

]. However, it should be noted that while the precursors to these fibers were crystalline, the resulting drawn fibers are fully vitrified. Fibers F and H are Er:YAG-derived fibers, while Fiber G is an Er-doped fiber also produced by a solution doping process. Fiber I is OFS coreless termination fiber (produced from silica) and Fiber J is an undoped YAG-derived fiber. Fibers I and J serve as references in the analysis to follow for the background absorption occurring in pure silica claddings. Microanalysis finds the composition of Fiber J to include 16.4 wt% of alumina and 23.0 wt% of yttria.

Measurements of the fiber cladding absorption coefficient in the deep-UV (230 ≲ λ ≲ 265 nm) were conducted with the experimental arrangement shown schematically in panel (a) of Fig. 1
Fig. 1 Schematic diagrams of experimental arrangements for the following measurements: (a) Fiber absorption in the ~230-265 nm region, obtained with a UV LED (λpeak ~243 nm); (b) Photoluminescence or excitation spectra acquired with an ultrafast Ti:sapphire laser and optical parametric amplifier (OPA) system in which β-BaB2O4 (BBO) frequency-doubles blue pulses into the ultraviolet. Several experiments were also conducted with a KrF excimer laser (248 nm) as the excitation source; (c) Apparatus for Yb3+ lifetime measurements.
. A light-emitting diode (LED), radiating in the UV and producing a ~12.5 nm FWHM (Δλ≃24 nm at the e−2 points of the spectral profile) continuum peaking at ~243 nm, serves as the optical source. The recent development of efficient, GaN-based LEDs in the UV now provides intense sources of broadband radiation from the visible to wavelengths below 250 nm. The spectral brightness (mW-sr−1-nm−1) of these new sources surpasses that of conventional UV lamps (such as deuterium) by several orders of magnitude. Optical fluence is of particular value when the number density of the absorber under study is high (i.e., N > 2/σL where N, σ, and L are the absorber number density, cross-section, and fiber length, respectively). Consequently, absorption (and emission) experiments in fibers can now be conducted readily throughout much of the UV with one or a series of UV LEDs. For the present studies, the diode described above was chosen from a set of five, each emitting at a different peak wavelength below 350 nm. A monolithic ball lens, pre-mounted onto the LED and having a focal length of 15-20 mm, produces a minimum spot size of 1.5-2.0 mm in diameter. In order to obtain cladding absorption measurements, each of the fibers tested was first treated in an acetone bath so as to remove completely the coating material. One end of the fiber was subsequently inserted into a fiber positioner and aligned with the ball lens on the UV LED (no additional lenses were necessary). The far end of the fiber was connected to a spectrometer/diode array (Ocean Optics HR 4000) by means of an SMA fitting. Owing to the exceptionally large absorption coefficients for the Yb:YAG-derived fibers, it was necessary to restrict the fiber lengths to ~7 cm. For the sake of convenience in working with such short lengths, the Yb:YAG-derived fibers (and only these fibers) were spliced to reference OFS termination fibers (such as Fiber I) with a Vytran GPX glass processing system. The reference fiber has a measured loss of 22 dB/m at 243 nm (see Table 1).

If the degree of mode mixing during the propagation of an optical wave in a fiber is sufficient, then absorption in the fiber core can be determined by simply multiplying the measured cladding absorption by the ratio of the cladding cross-sectional area to that for the core. Inadequate mode mixing can result in severely under-estimating the core absorption, and measurements of cladding absorption coefficients that are dependent upon sample lengths. Fibers with circular claddings often suffer from this difficulty [15

15. C. Pare, “Influence of inner cladding shape and stress-applying parts on the pump absorption of a double-clad fiber amplifier,” Proc. SPIE 5260, 272–277 (2003). [CrossRef]

] whereas non-circular claddings generally exhibit mode mixing sufficient to yield length-independent loss measurements. Having confidence, therefore, in the cladding absorption measurements described above demands that fiber length-dependent experiments be conducted and the results are presented in Fig. 2
Fig. 2 Dependence of the cladding absorption coefficients (dB/m) on fiber length for Fibers D and E (cf. Table 1). For clarity, data for Fiber D were intentionally reduced uniformly by 100 dB/m. All measurements were recorded for a probe wavelength of nominally 243 nm.
for Fibers D and E. Despite varying the lengths of both fibers by more than an order of magnitude, the measured absorption coefficient remains constant to within experimental error and the largest fluctuations in the data are observed with the shortest fibers. These results, as well as those of Table 1 and data to be presented later, demonstrate that shaped claddings (such as those of Fibers B and C) are not necessary for obtaining reliable absorption and emission data.

Photoluminescence data and laser excitation spectra were obtained with the experimental arrangement of Fig. 1(b). An optical parametric amplifier (OPA), pumped by a Ti: sapphire ultrafast laser system (~2.7 mJ, ~100 fs pulses, ~1 kHz pulse repetition frequency), produces ~120 fs pulses tunable throughout the visible and near-infrared. The output of the OPA is frequency-doubled in β – BaB2O4 (BBO, 10 mm x 10 mm x 0.5 mm, cut at 39° with respect to the crystal’s optical axis). This system generates pulses tunable in wavelength from ~225 nm to 265 nm. After this radiation was launched into the fiber under test with a UV-grade lens having a focal length of 15 cm, the fluorescence produced within the doped fiber was recorded with a fiber-coupled spectrometer. Several experiments were also conducted in which the ultrafast laser system and doubling crystal were replaced by a KrF excimer laser (248.4 nm, ħω ≈5 eV, ~25 ns pulses). To our knowledge, laser excitation of optically-active defects in fibers has not been reported prior to Ref. 13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

but the narrow bandwidth, temporal resolution, and brightness afforded by these coherent sources are of considerable value in clarifying the identity of the defect, dopant, or impurity under study.

Panel (c) of Fig. 1 is a diagram of the apparatus with which the temporal decays of Yb3+ fluorescence in the near-infrared and green (owing to Yb3+- Yb3+ interactions) were recorded for several fibers. A pulse-modulated pump laser diode (JDSU 27-2601), operating at 975 nm, photoexcited short segments of Yb-doped fiber spliced between two passive fibers. The fiber length and pump power levels were chosen such that the observed amplified spontaneous emission (ASE) was negligible. A monochromator or a bandpass (interference) filter selected the wavelength region of interest and the Yb3+ fluorescence waveforms were detected and recorded with a Si photon-counting avalanche photodiode (APD) and a scaler/averager.

III. Experimental results and conclusions

A. Absorption Spectra and Coefficients

Measurements of the cladding absorption spectra in the ~228 – 270 nm wavelength region for two fibers (C and E, Table 1) are presented in Fig. 3
Fig. 3 Measured cladding absorption spectra for Fiber C (green) and Fiber E (red). The least-squares fit of a Gaussian to the Fiber E spectrum yields the dashed black curve having a peak wavelength and spectral breadth of λ0 = 245 nm and Δλ = 19 nm, respectively.
. Acquired with the UV LED described in Sect. II, these spectra have been normalized to the LED emission profile and are representative of those obtained for all of the fibers studied. As indicated by the dashed black curve of Fig. 3, fitting a Gaussian to the experimental Fiber E spectrum yields an absorption peak and breadth of 245 nm and 19 nm, respectively. Both characteristics are consistent with the known parameters of the deep-UV absorption of the ODC (II) defect in silica [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

,9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

]. Table 1 summarizes the results of cladding absorption measurements for all of the fibers examined in these experiments. As discussed previously, the absorption coefficient in the core for each fiber (αcore) is determined by scaling the cladding coefficient (αclad) by the ratio of the cladding-to-core cross-sectional areas. Furthermore, the entries of Table 1 assume that the ODC (II) background absorption of 22 dB/m for the coreless silica fiber (Fiber I, Table 1) is identical to that for the silica claddings of the doped fibers [10

10. J. W. Lee, G. H. Sigel Jr, and J. Li, “Processing-induced defects in optical waveguide materials,” J. Non-Cryst. Solids 239(1-3), 57–65 (1998). [CrossRef]

]. Consequently, 22 dB/m was subtracted from αclad for each fiber prior to calculating αcore. The measurement of cladding absorption for the undoped, YAG-derived fiber (Fiber J, Table 1) demonstrates that the presence of Y and/or Al contributes to an insignificant degree to the optical loss. Also, the difference of 2 dB between αclad for Fibers I and J lies within the estimated measurement uncertainty of ± 10%. Figure 4
Fig. 4 Dependence of fiber core absorption on the rare earth number density. All of the data were recorded with the LED source operating with a peak emission wavelength of ~243 nm [see text]. Linear least-squares fits to both the Yb and Er data of Table 1 are shown, as are estimated uncertainties for several of the measurements.
illustrates the absorption measurements of Table 1 for both Yb- and Er-doped fibers. Data showing the dependence of αcore on [Yb] and [Er] are represented in the figure by the solid circles (●) and open circles (○), respectively and, to within experimental error, αcore varies linearly with the Yb concentration in the fiber. Although the data of Fig. 4 are limited to five fibers and further measurements at additional [Yb] values are warranted, these results suggest that the ODC (II) number density is linearly proportional to the Yb doping concentration.

We note in Table 1 an apparent, slight blueshift of ~3 nm in the peak absorption as the Yb concentration is increased from [Yb] ~9 × 1018cm−3 to 2.4 × 1020cm−3. Such a trend, if confirmed by further studies, is statistically significant and is attributable to the rising influence of other absorbers such as the peroxy radical having an absorption band peaking at ħω ≈5.4 eV (λ ~230 nm) [9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

,16

16. H. Hosono and R. A. Weeks, “Bleaching of peroxy radical in SiO2 glass with 5 eV light,” J. Non-Cryst. Solids 116(2-3), 289–292 (1990). [CrossRef]

18

18. D. L. Griscom and M. Mizuguchi, “Determination of the visible range optical absorption spectrum of peroxy radicals in gamma-irradiated fused silica,” J. Non-Cryst. Solids 239(1-3), 66–77 (1998). [CrossRef]

]. Other potential contributors to absorption in this spectral region include E’ centers and NBOHC defects having absorption maxima lying at ~215 nm and ~258 nm, respectively. However, because the absorption spectra associated with E’ centers and NBOHC defects are considerably broader than that of ODC (II), we conclude that the contribution of E’ and NBOHC defects to the data of Fig. 3 can be neglected.

If the peak absorption cross-section for the ODC (II) continuum of Fig. 3 is assumed to be 2 × 10−17cm2 [11

11. H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen-deficient centers in synthetic silica glass,” Phys. Rev. B Condens. Matter 38(17), 12772–12775 (1988). [CrossRef] [PubMed]

], then the ODC number density for Fiber E (for example) is estimated (assuming Beer-Lambert absorption) to be ~2.6 × 1018 cm−3, or approximately 1% of [Yb]. Furthermore, the ratio of the ODC number density to that for Er3+ in fibers such as G and H (Table 1) is an order of magnitude lower (i.e., ~0.1%) than the corresponding ratio for the Yb-doped fibers. On this basis, it is reasonable to assume that if the ODC (II) defect is found to be the primary cause of (or contributor to) photodarkening, Er-doped fibers can be expected to be considerably less susceptible than their Yb-doped counterparts to photodegradation effects.

B. Photoluminescence and Laser Excitation Spectra

The direct connection between the populations of ODC (II) defects and Yb3+ ions in doped fibers, implied by the data of Fig. 4, is corroborated by emission spectra recorded in the UV, visible, and near-infrared regions. Figure 5
Fig. 5 Emission spectrum recorded over the 200-1100 nm region when a Yb-doped fiber (Fiber C, cf. Table 1) is pumped by a KrF excimer laser (λL = 248.4 nm, ħω ≈5 eV). Representative of the emission observed when any of the Yb-doped silica fibers of this study are photoexcited at 248 nm, this spectrum comprises fluorescence generated by optically-active defects in silica (such as ODC (II)) and Yb3+. Faint emission from NBOHC defects is detected, and all of the prominent features in the spectrum are identified. Note that the λ > 380 nm portion of the spectrum has been magnified in intensity by a factor of two.
is representative of the panoramic spectra acquired over the 200-1100 nm wavelength interval when Yb-doped fibers are photoexcited with UV pulses from the KrF excimer laser (λ = 248.4 nm, ħω ≈5 eV). The specific spectrum shown in the figure is that for Fiber C of Table 1 but it is obvious that pumping the absorption band of Fig. 3, associated with the ODC (II) defect, does indeed result in intense fluorescence that is known to originate from the ODC. Two prominent continua, peaking at ~280 nm (ħω = 4.4 eV) and ~480 nm (ħω = 2.6 eV), are observed in Fig. 5 and both have been studied extensively previously [8

8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

,9

9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

,13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

]. Weak emission from the NBOHC defect in silica [13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

,19

19. P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express 16(7), 4688–4697 (2008). [CrossRef] [PubMed]

] is also evident. Of greatest interest, however, is the presence of Yb3+ emission lying between ~900 nm and 1050 nm. Similar Yb3+ emission profiles are observed when the Yb-doped fibers are illuminated with the UV LED at λ ~243 nm. These results demonstrate that a fraction of the energy absorbed by ODC (II) defects via the ~19 nm FWHM continuum of Fig. 3 appears as Yb3+ luminescence, thus indicating the existence of ODC (II)→ Yb3+ excitation transfer as proposed in Ref. 13

13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

.

C. Yb3+ Fluorescence Decay

Further insight into the origin of the spectra of Figs. 5 and 6, and the local environment in which the emitters reside, is provided by measurements of the temporal decay of Yb3+ fluorescence. When Yb3+ is photoexcited at 975 nm, spontaneous emission in the near-infrared and visible is generated, as illustrated in Fig. 8
Fig. 8 Spontaneous emission generated in a Yb-doped fiber (Fiber C, Table 1): (a) the near-infrared (~950 – 1100 nm), and (b) the blue-green region of the spectrum (460-560 nm) by photoexcitation of a Yb-doped fiber (Fiber E, Table 1) at 975 nm. Both spectra were recorded at 90° to the fiber axis and, as a reference, the spectrum of the 975 nm pump for these experiments is shown in red in panel (a).
. Panel (a) of the figure shows the characteristic continuum of Yb3+ peaking near 1.03 µm and the blue-green fluorescence of Fig. 8(b) is the result of a cooperative (ion-ion), upconversion process [20

20. S. Magne, Y. Ouerdane, M. Druetta, J. P. Goure, P. Ferdinand, and G. Monnom, “Cooperative luminescence in a ytterbium-doped silica fibre,” Opt. Commun. 111(3-4), 310–316 (1994). [CrossRef]

]. With the apparatus of Fig. 1(c), the temporal history of Yb3+ fluorescence, produced in response to 1 µs laser pulses at 975 nm, was recorded for several fibers and representative results are presented in a semilog format in Fig. 9
Fig. 9 Temporal decay of Yb3+ fluorescence following the photoexcitation of three fibers at 975 nm (cf. Figure 1 (c)). Measurements of the Yb3+ spontaneous emission at ~1.06 µm are indicated by the red profiles for Fibers A, D, and E of Table 1. The green curve reflects the temporal history of emission produced near 530 nm by cooperative ion processes (upconversion) in Fiber E. Note that the ordinate in logarithmic.
for Fibers A, D, and E. These data were chosen to illustrate the monotonically increasing decay rate that is observed as the Yb3+ number density is raised. All of the red curves were acquired by monitoring the temporal history of the Yb ion fluorescence in the long-wavelength tail of the 1.03 µm continuum (Fig. 8) at ~1.06 µm. It is evident that the near-infrared fluorescence decays exponentially over a range in intensity of at least two orders of magnitude. This behavior indicates that the ODCs are not formed at devitrification sites, nor are they the result of precipitate-like clustering [21

21. K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, and T. Handa, “Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass,” J. Appl. Phys. 59(10), 3430–3436 (1986). [CrossRef]

]. This conclusion is reinforced by similar data obtained for the blue-green spectrum of Fig. 8(b). Recording the temporal dependence of the Yb3+ - Yb3+ cooperative emission (monitored near 530 nm) yields the green curve of Fig. 9 for Fiber E. Although the declining fluorescence is best fit with a double exponential, the initial (fast) decay constant is 294 ± 10 µs or almost precisely 50% of the decay constant for the ~1.03 µm waveforms for Fiber E (τ = 584 ± 5 µs; cf. Table 1).

It is also instructive to consider the dependence of the near-infrared exponential decay constant (τ) on the Yb3+ number density, [Yb3+]. Measurements for the five Yb-doped fibers examined in these experiments are shown in Fig. 10
Fig. 10 Measured dependence of the Yb3+ radiative lifetime τ on the Yb number density of Yb-doped silica fibers. The solid curve represents the best fit of Eq. (2) to the data, which yields the quenching number density, [Yb]q, of 4.1 × 1020 cm−3. Solid circles (●) represent data obtained for the Al/Yb co-doped fibers fabricated by a solution doping process (Fibers A-C, Table 1) whereas the two open circles (○) denote measurements for the Yb:YAG-derived fibers (Fibers D and E, Table 1).
in which the Al/Yb co-doped fibers (Table 1, Fibers A-C) are represented by solid circles (●). The two open circles (○) in the figure denote the measured lifetimes for the Yb:YAG-derived fibers of Table 1. The curve in Fig. 10 is the best fit to the data of the relation [22

22. W. J. Miniscalco, “Erbium-doped glasses for fiber amplifiers at 1500 nm,” J. Lightwave Technol. 9(2), 234–250 (1991). [CrossRef]

]
τ=τ01+([Yb]/[Yb]q)2
(2)
where [Yb]q (the quenching concentration) and τo (the Yb3+ excited state lifetime extrapolated to [Yb] = 0) are found to be 4.1 × 1020 cm−3 and 785 ± 10 µs, respectively. However, because these experiments employed fibers having two different host compositions, the interpretation of the lifetime data of Fig. 10 and the applicability of the concentration quenching model [22

22. W. J. Miniscalco, “Erbium-doped glasses for fiber amplifiers at 1500 nm,” J. Lightwave Technol. 9(2), 234–250 (1991). [CrossRef]

] in the present circumstances, in particular, must be approached cautiously. Nevertheless, at least one conclusion is warranted. Since Fibers D, E, and J are all YAG-derived fibers (differing only in Yb concentration), the decrease in the Yb3+ lifetime observed for Fiber E ([Yb] = 2.4∙1026 m−3) relative to Fiber D ([Yb] = 1.4∙1026 m−3) can be attributed to Yb alone. Therefore, the addition of 1∙1026 m−3 of Yb to the glass host is responsible for an increase of 300 s−1 in the Yb3+ excited state decay rate. Furthermore, recognizing that the Yb concentrations of Fibers C and D also differ by ~1∙1026 m−3 (Table 1), it is clear that the difference in the Yb3+ decay rate between the two fibers (120 s−1) can also be accounted for by the dopant alone. Given that the data of Fig. 10 obey the functional form of Eq. (2), therefore, it appears that the impact of both Y2O3 and Al2O3 (constituents of the YAG-derived fibers) on the excited state kinetics of Yb3+ can, to first order, be neglected. More extensive data, acquired with a set of fibers having a specific host composition but a broad range in [Yb], will be necessary to confirm this conclusion.

IV. Summary and conclusions

Absorption measurements and laser excitation spectroscopy of Yb-doped fibers, solution-doped or YAG-derived, have been described. Absorption spectra in the deep-UV (~228 – 280 nm) show consistently the ODC (II) absorption continuum peaking near 248 nm and measurements demonstrate that the ODC (II) absorption coefficient is directly proportional to the number density of the Yb dopant for both solution-doped and YAG-derived fibers. Not only do these data provide evidence directly linking the ODC (II) number density to that for Yb but they also suggest that the appearance of the ODC defect is associated with the introduction of Yb to the fiber and, possibly, the process by which the fiber itself is formed [10

10. J. W. Lee, G. H. Sigel Jr, and J. Li, “Processing-induced defects in optical waveguide materials,” J. Non-Cryst. Solids 239(1-3), 57–65 (1998). [CrossRef]

]. Furthermore, the ODC (II) number densities in Er-doped fibers are an order of magnitude lower than those observed for their Yb-doped counterparts.

Photoexcitation of the ODC (II) absorption spectrum with a KrF (248 nm) laser indicates the existence of ODC (II) → Yb3+ excitation transfer. Unambiguous confirmation of this energy transfer mechanism is provided by laser excitation spectroscopy experiments in which the ODC (II) absorption band was pumped at several wavelengths in the ~225 – 265 nm region by a tunable Ti:sapphire/OPA/BBO laser system. Monitoring ODC (II) and Yb3+ fluorescence at 282 nm and 978 nm, respectively, reveals a variation with the UV laser wavelength that is virtually identical for both emitters. Furthermore, both the Yb3+ and ODC (II) excitation spectra in this UV region match precisely the ODC (II) absorption spectrum, thereby reinforcing the conclusion that electronically-excited Yb3+ is formed directly in these experiments by ODC (II) → Yb3+ excitation transfer.

Acknowledgments

The support of this work by the Joint Technology Office, through the High Energy Laser Multidisciplinary Research Initiative (HEL – MRI) program, and the U. S. Air Force Office of Scientific Research (H.R. Schlossberg) is gratefully acknowledged. Also, the authors are indebted to Nufern for fiber samples and the Department of Defense for providing the glass processing system under DURIP ARO grant no. W911NF-07-1-0325.

References and links

1.

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]

2.

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. Express 15(4), 1606–1611 (2007). [CrossRef] [PubMed]

3.

M. Engholm and L. Norin, “Comment on “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation”,” Opt. Lett. 33(11), 1216, discussion 1217–1218 (2008). [CrossRef] [PubMed]

4.

M. Engholm, L. Norin, and D. Åberg, “Strong UV absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV excitation,” Opt. Lett. 32(22), 3352–3354 (2007). [CrossRef] [PubMed]

5.

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]

6.

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]

7.

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(20), 15540–15545 (2008). [CrossRef] [PubMed]

8.

A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]

9.

L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]

10.

J. W. Lee, G. H. Sigel Jr, and J. Li, “Processing-induced defects in optical waveguide materials,” J. Non-Cryst. Solids 239(1-3), 57–65 (1998). [CrossRef]

11.

H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen-deficient centers in synthetic silica glass,” Phys. Rev. B Condens. Matter 38(17), 12772–12775 (1988). [CrossRef] [PubMed]

12.

H. Imai, K. Arai, J. Isoya, H. Hosono, Y. Abe, and H. Imagawa, “Generation of E’ centers and oxygen hole centers in synthetic silica glasses by γ irradiation,” Phys. Rev. B Condens. Matter 48(5), 3116–3123 (1993). [CrossRef] [PubMed]

13.

C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]

14.

J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T. M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, and M. J. Matthewson, “On the fabrication of all-glass optical fibers from crystals,” J. Appl. Phys. 105(5), 053110 (2009). [CrossRef]

15.

C. Pare, “Influence of inner cladding shape and stress-applying parts on the pump absorption of a double-clad fiber amplifier,” Proc. SPIE 5260, 272–277 (2003). [CrossRef]

16.

H. Hosono and R. A. Weeks, “Bleaching of peroxy radical in SiO2 glass with 5 eV light,” J. Non-Cryst. Solids 116(2-3), 289–292 (1990). [CrossRef]

17.

L. Skuja, M. Hirano, H. Hosono, and K. Kajihara, “Defects in oxide glasses,” Phys. Stat. Sol. C 1, 15–24 (1998).

18.

D. L. Griscom and M. Mizuguchi, “Determination of the visible range optical absorption spectrum of peroxy radicals in gamma-irradiated fused silica,” J. Non-Cryst. Solids 239(1-3), 66–77 (1998). [CrossRef]

19.

P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express 16(7), 4688–4697 (2008). [CrossRef] [PubMed]

20.

S. Magne, Y. Ouerdane, M. Druetta, J. P. Goure, P. Ferdinand, and G. Monnom, “Cooperative luminescence in a ytterbium-doped silica fibre,” Opt. Commun. 111(3-4), 310–316 (1994). [CrossRef]

21.

K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, and T. Handa, “Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass,” J. Appl. Phys. 59(10), 3430–3436 (1986). [CrossRef]

22.

W. J. Miniscalco, “Erbium-doped glasses for fiber amplifiers at 1500 nm,” J. Lightwave Technol. 9(2), 234–250 (1991). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2290) Fiber optics and optical communications : Fiber materials
(160.5690) Materials : Rare-earth-doped materials
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 27, 2012
Revised Manuscript: May 30, 2012
Manuscript Accepted: June 5, 2012
Published: June 14, 2012

Citation
Y.-S. Liu, T. C. Galvin, T. Hawkins, J. Ballato, L. Dong, P.R. Foy, P.D. 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)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-13-14494


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References

  1. 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]
  2. 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. Express 15(4), 1606–1611 (2007). [CrossRef] [PubMed]
  3. M. Engholm and L. Norin, “Comment on “Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation”,” Opt. Lett. 33(11), 1216, discussion 1217–1218 (2008). [CrossRef] [PubMed]
  4. M. Engholm, L. Norin, and D. Åberg, “Strong UV absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV excitation,” Opt. Lett. 32(22), 3352–3354 (2007). [CrossRef] [PubMed]
  5. 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]
  6. 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]
  7. 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(20), 15540–15545 (2008). [CrossRef] [PubMed]
  8. A. V. Amossov and A. O. Rybaltovsky, “Oxygen deficient centers in silica glasses: a review of their properties and structure,” J. Non-Cryst. Solids 179, 75–83 (1994). [CrossRef]
  9. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]
  10. J. W. Lee, G. H. Sigel, and J. Li, “Processing-induced defects in optical waveguide materials,” J. Non-Cryst. Solids 239(1-3), 57–65 (1998). [CrossRef]
  11. H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen-deficient centers in synthetic silica glass,” Phys. Rev. B Condens. Matter 38(17), 12772–12775 (1988). [CrossRef] [PubMed]
  12. H. Imai, K. Arai, J. Isoya, H. Hosono, Y. Abe, and H. Imagawa, “Generation of E’ centers and oxygen hole centers in synthetic silica glasses by γ irradiation,” Phys. Rev. B Condens. Matter 48(5), 3116–3123 (1993). [CrossRef] [PubMed]
  13. C. G. Carlson, K. E. Keister, P. D. Dragic, A. Croteau, and J. G. Eden, “Photoexcitation of Yb-doped aluminosilicate fibers at 250 nm: evidence for excitation transfer from oxygen deficiency centers to Yb3+,” J. Opt. Soc. Am. B 27(10), 2087–2094 (2010). [CrossRef]
  14. J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T. M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, and M. J. Matthewson, “On the fabrication of all-glass optical fibers from crystals,” J. Appl. Phys. 105(5), 053110 (2009). [CrossRef]
  15. C. Pare, “Influence of inner cladding shape and stress-applying parts on the pump absorption of a double-clad fiber amplifier,” Proc. SPIE 5260, 272–277 (2003). [CrossRef]
  16. H. Hosono and R. A. Weeks, “Bleaching of peroxy radical in SiO2 glass with 5 eV light,” J. Non-Cryst. Solids 116(2-3), 289–292 (1990). [CrossRef]
  17. L. Skuja, M. Hirano, H. Hosono, and K. Kajihara, “Defects in oxide glasses,” Phys. Stat. Sol. C 1, 15–24 (1998).
  18. D. L. Griscom and M. Mizuguchi, “Determination of the visible range optical absorption spectrum of peroxy radicals in gamma-irradiated fused silica,” J. Non-Cryst. Solids 239(1-3), 66–77 (1998). [CrossRef]
  19. P. D. Dragic, C. G. Carlson, and A. Croteau, “Characterization of defect luminescence in Yb doped silica fibers: part I NBOHC,” Opt. Express 16(7), 4688–4697 (2008). [CrossRef] [PubMed]
  20. S. Magne, Y. Ouerdane, M. Druetta, J. P. Goure, P. Ferdinand, and G. Monnom, “Cooperative luminescence in a ytterbium-doped silica fibre,” Opt. Commun. 111(3-4), 310–316 (1994). [CrossRef]
  21. K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, and T. Handa, “Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass,” J. Appl. Phys. 59(10), 3430–3436 (1986). [CrossRef]
  22. W. J. Miniscalco, “Erbium-doped glasses for fiber amplifiers at 1500 nm,” J. Lightwave Technol. 9(2), 234–250 (1991). [CrossRef]

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