We present an analysis of non-bridging oxygen hole center (NBOHC) defects in Yb-doped silica fibers. Red photoluminescence is observed when several fiber samples are irradiated with green light (532nm). Both highly Ge-doped and moderately P-doped Yb fibers exhibit red-shifted NBOHC emission spectra while highly Al-doped Yb fibers seem to exhibit NBOHC spectra closer to that of pure silica. NBOHC centers may play a role in the photodarkening process of Yb doped fibers.
©2008 Optical Society of America
Non-bridging oxygen hole center (NBOHC) defects have been extensively studied in silica glasses and fibers [1–8]. The importance of these centers lies within the context of the transmission properties of silica fibers in the visible and UV.
NBOHC centers are represented chemically as [≡Si-O∙] where the symbol ≡ represents bonding to three oxygens and the symbol ∙ represents an unpaired electron, i.e. a dangling bond. These centers are highly localized, optically active, and have luminescence near 1.9 eV (thus otherwise known as R-centers or red centers), and absorption near 2.0 eV. Photoinduced damage can lead to the exacerbation of absorption in this band [9–14], with long absorption tails that can extend into the near IR . The photoluminescence may be attributed to a charge transfer transition between a non-bonding orbital of the non-bridging oxygen and one of the three ligand oxygens . NBOHC centers arise from the presence of dopants or contaminants that are network modifiers [1,15], such as might be expected from a large concentration of a rare earth dopant and the requisite co-dopants.
With these optically active absorption centers residing in the red and UV wavelength regions, it appears unlikely that NBOHC defects would play a first order role in the performance of Yb fiber lasers, aside from possible nonlinear interactions at very high peak powers. However, the characterization of NBOHC defects may provide some insight into the local bonding structure of Yb doped silica fibers.
We characterize NBOHC centers in several samples of Yb doped silica fibers with differing [Yb], [Al], [P], [F], and [Ge]. In this paper, dopant concentrations will be represented by square brackets surrounding the chemical symbol of the element. Laser light from a CW doubled Nd:YAG laser is injected into each fiber, and the resulting NBOHC spectra are recorded. We find that both highly Ge-doped and moderately P-doped fibers tend to exhibit long-wavelength NBOHC emission peaks, while highly Al-doped fibers exhibit spectra that are more similar to those obtained of pure silica. In all cases the spectra are broad, and in some cases exhibit properties consistent with the superposition of several R-centers. The introduction of a rare earth dopant appears to increase [NBOHC] relative to passive fibers.
Finally, these defect centers may also play a role in an upconversion process in Yb doped fibers that leads to the well known photodarkening phenomenon [16,17]. A possible example of such a process is provided in the Discussion section.
2. Optical Fiber fabrication
Besides the commercially available SMF-28™ fiber and the Nd doped fiber, all others fibers in the table below were developed at INO. The fiber mother preforms were all fabricated by the modified chemical vapor deposition (MCVD) process. The Ge, P, and F were incorporated into the white silica soot composition needed for the solution doping. Their compositions were adjusted by changing their flow during the soot deposition. The Al and all others rare-earths were impregnated into the core glass soot material by the solution doping technique. The various concentrations were adjusted by modifying the doping solutions. The chemical compositions were measured on the mother preforms by the electron probe micro analysis (EPMA) technique. The given data are for the central position of the cores. The core diameters are also provided in Table 1, and we should point out that all fibers were multimode in the visible wavelength range, which may play a significant role in the subsequent spectra.
3. Experimental setup and data
Photoluminescence measurements of the fiber samples described in the previous section were performed using the setup of Fig. 1. The output of a 200 mW, 532 nm CW Nd:YAG laser was coupled into the cores of the fiber samples under test using an aspheric lens and flexure stage. A 3 nm FWHM 532 nm band pass filter was inserted in the free space path prior to fiber coupling to remove any residual 808 nm pump or 1064 nm emission from the laser. It is estimated that the coupling efficiencies for all single clad fibers were similar, about 60% for fibers Yb198, Yb103, and Yb118 and about 75% for fibers Yb125, Yb214, the Al/Ge fiber, and SMF-28™. Close to 100% coupling efficiency was achieved for the double clad fibers.
The green pump light excited the red photoluminescence in the fiber core. The photograph in Fig. 2 shows the Yb 103 spool of fiber under excitation from the 532 nm laser. The photo was taken with a GRISM in front of the camera lens and visibly demonstrates a red photoluminescence signature emanating from the coiled fiber. A GRISM is a dispersive element with a transmission grating that is optically epoxied to one side of a wedge prism.
In order to measure this photoluminescence, the output of the fiber was collimated with an aspheric lens and passed through a Schott glass long pass filter (OG550) to remove the residual pump light at 532 nm. The filtered light was coupled into a fiber connected to an Ocean Optics HR4000 fiber coupled spectrometer. In the spectrometer, the spectra were taken over an integration period of 10 seconds with dark noise removed prior to spectrum acquisition.
The normalized spectra for the five single clad Yb-doped fiber samples are shown in Figs. 3 and 4, and those for the three double-clad fiber samples in Fig. 5. Progressing from left to right in the figures, we observe that some pump bleed-through is apparent at 532 nm. There are emission lines just beyond the pump, which are in some cases masked by the red luminescence. We believe that these lines are Raman spectra since we observe lines at the same energy difference when the fibers are pumped with an Argon-ion laser. These lines are not consistent with the Raman spectrum for silica, and we are trying to deduce their origin. Luminescence from the filter does not contribute to the spectra presented in this paper.
The photoluminescence in the red to very near IR is clearly apparent for the sample fibers. Consistent in some of the data is an increasing red shift in the spectra with increasing [Al]/[Yb] for the Yb-doped fibers. The blue side of the spectra with shorter wavelengths was probably affected by the turn-on edge of the long-pass filter. The P co-doped silica fibers exhibit red-shifted spectra, with a peak near 700nm. However, even though these fibers are doped somewhat differently, their R-emission spectra are nearly indistinguishable.
For comparison with the rare earth-doped fibers, and experimental control, the photoluminescence of two passive fibers (described in the previous section) were also investigated with results provided in Fig. 6. The Al-doped passive fiber has a peak near 900 nm, which based on its spectral width, is currently believed to be originating from a background contaminant.
Finally, in an effort to show that the red luminescence spectra are most likely due to NBOHC, and not due to emission from some other source, such as an anomalous state of Yb, a photoluminescence spectrum is shown for an Nd and P co-doped silica fiber in Fig. 7. All of the usual emission lines from the Nd fiber are labeled in the plot. We see an NBOHC spectrum similar to those in the Yb fibers, with a peak near 650nm. Erbium and thulium were not investigated due to the presence of red emission from the 4F9/2→4I15/2 and 3H6→3F2 transitions, respectively.
The emission spectra observed in the figures are consistent with those of NBOHC centers [1,4]. However, even with the peak absorption of the NBOHC center usually lying near 2.0eV, we were able to excite emission using 2.3eV photons. This would imply that some phonon interaction is taking place within the fiber.
The long-wavelength NBOHC emission peak (~750nm) observed from the high [Ge] fibers is similar to those observed in , explained by the authors as an interaction between the NBOHC and an impurity in the glassy matrix. Interestingly, fibers Yb 103 and Yb 118 exhibit very similar NBOHC spectra. This may imply that the presence of a large [Ge] is driving the NBOHC, since both fibers have different [Yb] and [Al] concentrations. Since NBOHC spectra are normally very Gaussian , the leaning shape of these spectra probably implies a superposition of Gaussians, possibly arising from additional NBOHC sites with higher energies, or draw induced defects (DID) .
Additionally, the highly Al-doped fiber (Yb 198) exhibits a peak NBOHC wavelength that is more similar to that of pure silica . This may be characteristic of Al-doping, but may also suggest that the NBOHC site is one which resembles that of pure silica, and that the Al dopant does not strongly influence the local bonding structure as experienced by the NBOHC .
The P-doped fibers may lead to some interesting conclusions as well. The test segments for these fibers were shorter (40m) than for the Ge-doped fibers (100m). The spectra from both fibers were nearly indistinguishable, with fiber Yb 125 only slightly broader. It should be noted here that both of these fibers exhibited very large scale attenuation near 532nm. In fact, the long-pass filter was not employed in acquiring the spectra for these two fibers, with the outputs of the test fibers sent directly into the spectrometer, employing a reduced integration time of one second. More specifically, the green pump wavelength was observed to be completely attenuated in less than 10m of fiber Yb 214. Fiber Yb 125 transmitted slightly more pump light, which is the contribution near 532nm seen to be off the scale in Fig. 4. The red luminescence signature was much stronger for these fibers.
It seems clear that there is a peak wavelength shift relative to Yb 198, however, there seems to be no significant difference, aside from relative green attenuation, between fibers Yb 125 and Yb 214, despite having different levels of [P], [Al], and [Yb]. This would imply that a changing [P] does not influence the R-center peak wavelength. Instead, it may be driven by the manufacturing process. Alternatively, fibers Yb 125 and Yb 214 have smaller [Al]/[Yb] ratios than Yb 198, or fibers B or C, but larger than that of fiber A. Following this trend, it seems that fibers with higher [Al]/[Yb] (>10) exhibit blue-shifted NBOHC spectra.
Dual-clad fibers B and C exhibit spectra that are very similar to those of fiber Yb 198, each with [Al]/[Yb]>10. Consistent between fibers B, C, and Yb 198, there is a red-shift experienced with an increasing [Al]/[Yb], in some disagreement with the final conclusion of the last paragraph. The wide tails also imply additional NBOHC sites with longer peak wavelengths, perhaps contributed to by the inclusion of the other dopants. The spectrum from fiber A is considered ‘anomalous’ in that the fiber only contains Al and Yb (with a smaller [Al]/[Yb] of about 6), but exhibits a peak NBOHC wavelength close to 650nm, longer than either that of fibers B or C. Additionally, it also has an NBOHC spectrum that is much narrower than the others (expanded in the inset). This may be an effect of the short test segment, leading to a more homogeneous spectrum. More likely, however, is that the dopant distribution in the more lightly doped fiber is more homogeneous than in the other fibers, with no other influencing dopants, therefore implying a more homogeneous NBOHC distribution. It is interesting to note that much less NBOHC luminescence was observed from fiber A than from fibers B or C.
Two passive fibers were also tested to compare with the Yb-doped fibers. In particular, 66m segments of SMF-28™ and an Al and Ge co-doped fiber were investigated, and their resulting spectra are shown in Fig. 6. No NBOHC was observed from the Al-doped passive fiber. Given its spectral width, the small noisy peak near 900nm is believed to be emission from a background contamination. A very weak background luminescence that resembles NBOHC was observed for the SMF-28™, with an expanded spectrum shown in the inset. Consistent among SMF-28™, Yb 103, and Yb 118 is an increasing red-shift with increasing [Ge].
The passive fibers under test were slightly shorter than the Yb fiber lengths, 66m vs. 100m, but we do not believe that this would influence the general observation of NBOHC. Although no NBOHC emission was observed from the Al/Ge passive fiber, this does not mean that the centers are not present, just that their concentrations would fall below our detection limits.
The data presented may provide evidence that the introduction of Yb increases the concentration of NBOHC ([NBOHC]) defect centers. This could lead to the conclusion that these centers are in the vicinity of, or coordinated to, the rare earth dopant [15,20]. This is consistent with the red luminescence observed from the Nd doped fiber. Interestingly, if this is the case, then these centers may play a role in the photodarkening process of Yb doped fibers, possibly through energy transfer and upconversion processes that involve other defect centers.
An example of one such path, involving an upconversion process, is shown in Fig. 8, where a simplified energy level diagram is provided. The value of the energy contribution by each ‘arrow’ is provided in the figure. Phonon emission is represented by the squiggle-lines. Photon emission is denoted by bold lines. The dashed lines represent energy contributions in a cooperative process.
The excitation of an NBOHC center, which has a lifetime in the vicinity of 20µs , would involve energy transfer from two excited Yb atoms (~1.2eV each, up to a level of around 2.4eV), then phonon relaxation, yielding red luminescence. Furthermore, if the NBOHC is in the vicinity of, or coordinated to, an ODC (II) defect [1,14, 21], its energy (~2.0eV) plus that of one additional excited Yb (~1.2eV) atom, may be transferred to that ODC site with excitation to 3.2eV . Then, one excited NBOHC (2.0eV) can contribute to ODC (II) to upconvert it to the 5.2eV level. Upconversion to these UV centers can then lead to photo-induced optical damage [10,12].
The ODC (II) center can also possibly be excited through a two-to-three-photon energy transfer process, where excited Yb atoms transfer their energy to a nearby ODC (II) center. This would result in blue luminescence. Yb fibers have been observed in some cases to luminesce strongly in the blue wavelength range , rather then their usual blue-green, but this may also be due to divalent Yb , or be the result of a Tm or other contamination. Investigations of this defect center in these Yb fiber samples will be presented in Part II.
In this example, energy transfer from several excited Yb atoms contributes to the initiation of an upconversion process. As such, a logical conclusion would be that clustering of Yb atoms would result in a degraded resistance to photodarkening. Therefore, the use of Al doping, in order to form a suitable solvation cell surrounding the Yb atom , would be expected to increase the resistance to photodarkening within this model. This may be due to the reduction of clustering sites  or that the Al may shield the Yb atom from certain aspects of its surroundings, such as other Yb atoms [24, 25] or defect centers.
In conclusion, we have characterized the NBOHC emission spectra for several different Yb doped optical fibers under excitation with 532nm laser light. Generally, high [Ge] and moderate [P] levels shift the NBOHC spectrum to longer wavelengths, while higher [Al]/[Yb] ratios seem to produce an NBOHC spectrum with slightly longer peak wavelengths in fibers when this ratio is greater than ten. No NBOHC emission was observed from a passive Al-doped optical fiber, and very little background luminescence from SMF-28™.
Our data suggests that the introduction of Yb (or rare earths in general) into the fiber increases [NBOHC], and that these centers may be coordinated to the Yb. This would introduce possible energy transfer mechanisms between excited Yb atoms and defect centers leading to energy transfer and upconversion processes that result in photo-induced optical damage.
This work was funded by an MRI grant from the Joint Technology Office High Energy Laser (JTO-HEL) program.
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