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Abstract
For high power fiber lasers, codoping with Al, P or both is necessary to prevent rare earth (RE) clustering in the silica network of the laser active core material. Here, we present a complementary infrared (IR) based multispectral method combined with elemental analysis data on core/cladding to describe the structure of the doped core material as fabricated by chemical vapor deposition with gas phase doping of Al and P. The resulting 2D image and its corresponding 3D visualization of the data enable an alternative and convenient way to characterize the main species of the dopants aside from NMR measurements.
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1. Introduction
Silica represents an important glass matrix for optical fibers because of its high strength, chemical resistance, availability in high purity and low optical loss. Rare earth (RE) doped silica glasses serve as core material for fiber lasers and fiber amplifiers being indispensable in various applications in industry, science and medicine. Incorporation of RE ions in pure silica is limited due to clustering effects at higher RE-ion concentrations. A better distribution of the RE at higher concentration is possible by adding a certain amount of dopants like aluminum (Al3+) and/or phosphorus (P5+) which act as a network modifier in the silica matrix.
There are trendsetting concepts for RE-doped laser fibers, like large-mode area (LMA) geometry featuring a highly RE-doped core at large diameter with a low numerical aperture (NA), which require a very low refractive index (RI) as well [1]. Such fibers enable to achieve high output power in the multi-kW range. However, because of the required high RE- and thus also high Al- or P- concentrations the refractive index of the doped core material increases, whereby a doping for decreasing the overall NA to reach the required value is essential. For the LMA fibers dopants like fluorine or a combination of Al and P with a molar ratio of one are usually chosen to reduce the refractive index [2–4], which poses additional challenges in terms of achieving the desired spatial homogeneity, therefore having led to alternative manufacturing processes [5].
Today, the modified chemical vapor deposition (MCVD) technology in combination with gas phase doping of RE and Al is the preferred technology [6,7] to manufacture large cores with homogenous dopant distribution and high purity. However, due to evaporation and diffusion of the phosphorus species during preparation steps of such performs, it is challenging to realize a dip-free refractive index profile with phosphorus codoping.
The optional codoping of Al/P with a ratio of one is known to result in the formation of AlPO4-tetrahedra in silica glasses, reducing not only the refractive index of the core glass but also the photodarkening losses of such fiber cores as well [8,9].
Nuclear magnetic resonance (NMR) measurements showed that (P2O5)x (SiO2)1-x glasses consist of tetrahedral silicon-oxygen (SiO4)-groups and phosphate-oxygen (O = PO3)-groups being randomly distributed in a three-dimensional network, where the phosphorus is entering the silica network only as Q3-group with one P = O-bond [10].
While the single doping with either Al or P leads to an increase in refractive index, the combined doping at Al/P ratio of one partially results in a decrease in RI relative to that of silica or slightly below [3,11].
Different structures have been identified from NMR in [12]: In phosphorus rich glasses with Al/P <1, sixfold- (Al(6)) and fourfold- (Al(4)) coordinated aluminum units interact with metaphosphate (P(2)) species; and Al(4) units interact with P(4) groups similar to the situation in AlPO4. Three distinct phosphorus environments have been identified in such phosphorus rich glasses with Al/P <1, that are silicon-bonded P(3) units linked to silicon and/or phosphate species, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4 (the superscript denotes the number of bridging oxygen atoms attached to a P atom). In Al-rich glasses (Al/P > 1) the alumina component forms Al(4), Al(5), and Al(6) units linked to the silica. Al is present in Al-O-P, Al-O-Si, and possibly also in Al-O-Al linkages, besides the dominant AlPO4 like structural units where P(4) units constitute the sole phosphorus species.
The interplay of such structural units was reported to also reflect in intermediate-range structural heterogeneity [13], whereby mild additions of alumina to a silica network were found to decrease the characteristic length of fluctuations. Interestingly, codoping with another network former can be used to further tailor this effect, for example, in order to reduce clustering effects of optically active RE dopants. [14]
IR absorption measurements confirmed NMR-results by taking into account the bands of Si-O [15,16] and P = O [17–20]. We have demonstrated in previous papers [21,22], that the IR band area of the P = O-band at 1325 cm−1 correlates with P-concentration as determined from Electron probe micro analysis (EPMA) measurements of pre-sintered P-doped silica soot material prepared by MCVD process. For this work, different Al- and Al/P-doped silicate glasses were prepared by MCVD in combination with gas phase doping of Al, the glasses were analyzed by using IR spectroscopy and evaluating the corresponding IR-bands. Thus, a wavenumber dependent 2D and a corresponding 3D visualization of the species distribution of the core region was constructed based on the IR-map of the samples.
2. Samples and methods
2.1 Sample preparation
The samples were prepared by MCVD in combination with gas phase deposition of AlCl3 in two series. Series A are Al/P-doped samples with one only P-doped sample, while in series B all samples are only doped with Al. For each sample, several glassy core layers were deposited in quartz glass carrier tubes (F300 Heraeus) at a burner temperature of about 1880 °C. To do so, AlCl3 was evaporated at 135 °C, while SiCl4 and POCl3 both were evaporated at room temperature using carrier gases. An additional oxygen flow was used for the direct deposition of the glassy Al/P-doped layers.
Five to eight layers were applied to obtain a core of about 2 mm in diameter. The core of the P-doped preform consisted of 25 layers which were deposited at higher temperatures to create a specific profile for the characterization.
Parameters of variation during deposition were the carrier gas flows VO2(POCl3) and VHe(AlCl3), respectively. SiCl4 flow was set constant. Table 1 summarizes the experimental conditions and the obtained concentrations as determined by electron probe micro analysis (EPMA). All samples were characterized using the methods described in the following sections.
Table 1. Process parameters and dopant concentrations of the different series
2.2 Measurement of the refractive index profile
The preforms were characterized nondestructively by a beam deflection method [23] at different positions of the samples.
2.3 Electron probe micro analysis (EPMA)
The chemical composition of the samples was done using the electron micro probe analyzer Jeol JXA-8800L equipped with a Bruker silicon drift detector. A 1-2 mm slice of the preform was polished, fixed and deposited with a carbon layer to avoid charging effects during the measurement. A vertical and horizontal line scan of the preform core with a step size of 0.02 mm was done for every sample.
2.4 Vibrational spectroscopy
Infrared spectra were acquired with a Fourier transform infrared spectrometer Thermo Fischer Scientific iS10. A 1-2 mm thin polished slice of the preform was placed directly on the diamond of the spectrometer attenuated total reflection (ATR) cuvette and pressed with a stamp for a better sample contact. Each spectrum was taken in a range from 6000 cm−1 to 600 cm−1 with a 2 cm−1 resolution and represents an average of 100 scans without advanced ATR-correction.
In addition, an infrared microscope Thermo Fischer Scientific iN10 in ATR-mode was employed to map the core region in space-resolved manner. The polished preform slices were fixed at the sample holder, and the germanium crystal for the ATR-measurement was pressed on the samples with a defined load. While the infrared spectrometer has a measuring field of around 2-3 mm without lateral resolution, the IR-microscope with its aperture size of 40 × 40 µm2 and step size of 20 µm (x- and y-direction) allows a lateral resolution of about 4000 × 4000 µm2. The MCT detector cooled by liquid nitrogen of the infrared microscope enables mapping of the core region with 16 individual measurements per point in a manageable time period. The software Omnic Picta supplied from Thermo Fischer Scientific allows a space resolved 2D- and 3D-visualization of the absorbance data (mapping; Fig. 1) as well as analysis of single point measurements and line scans.
Fig. 1. Elements of the user interface of Omnic Picta software. The 2D-image (left) includes a color assignment for the absorbance level of the actual wavenumber while the corresponding 3D-visualization (upper right) presents the absorbance directly as height. Also, a single point spectrum at the position 388 µm, −2338 µm; point 4586) is shown.
The extraction of an IR-line scan enables detailed analysis of the different P-related bands, especially estimation of the P2O5-concentration based on the P = O-bond. In Ref. [21,22] the phosphorus band area ratio (PBAR) of P = O-band as measured by ATR-method is correlated with the P2O5-concentration via:
3. Results and discussion
3.1 P-doped sample
All P-doped samples prepared by MCVD technology show a characteristic concentration dip in the center of the preform core because of the evaporation of the P-species caused by tube collapsing at high temperatures. The layer structure of the core can be visible depending on deposition conditions due to partial evaporation of P-species during each layer deposition. Here, the only P-doped sample of series A was also deposited at a higher temperature to enable a visible layer structure of the core. The P2O5-concentration profile of the sample as obtained from EPMA measurements shows smaller dips between each layer (Fig. 2 left). Based on the IR-analysis of an IR-line scan measured by the IR-microscope and the correlation according to Eq. (1), the IR-derived radial P-concentration profile was compared to the one measured by EPMA, see Fig. 2.
Fig. 2. Radial concentration profile as determined by EPMA (left) and comparison to the phosphorus band area ratio (PBAR) of P = O-band as calculated from IR spectra (right) of a P-doped sample.
The average concentration value based on the IR-line scan is lower than the one obtained from EPMA, but the single layers are visible in both cases. This demonstrates the capability to estimate easily the P2O5-concentration from a single line scan with little requirements within a few minutes.
Further, the IR-mapping of the P-doped core material enables a wavenumber dependent presentation of the sample in a 2D- and corresponding 3D-image. Figure 3 shows an IR-spectrum of a P-characteristic region from 1400–700 cm−1 with the significant P = O-band at 1325 cm−1. The related 2D- and 3D-visualizations at each wavenumber illustrate the spatial P-distribution in the preform core material.
Fig. 3. Single IR-spectrum of the P-rich region in the core with 2D- and 3D-visualizations at specific wavenumbers which are significant for different types of P-bonds as indicated. The 2D- and 3D-images are shown only in half due to the radial (process related) symmetry for easier visual comparison with the EPMA concentration distribution as given in Fig. 2.
The software Omnic Picta allows to go step by step from one wavenumber to another along the single spectrum and to observe the changes in the 2D- and corresponding 3D-visualization (see supplementary material: Visualization 1). So, at every point when the 3D-visualization resembles the one at 1325 cm−1, a P-related species is identified. In this manner, three more significant wavenumbers were identified: P-O-Si-band at around 1100 cm−1, P-O-P-band at 970 cm−1, as well as an inverse structure (peak instead of dip) at 1030 cm−1 which belongs to the Si-O-Si-bond remaining after phosphorous had been evaporated.
Such a quick IR-mapping allows to identify the different main species without deconvolution or other methods by just taking the EPMA-derived preform core structure into account. The calculation of the phosphorus band area ratio (PBAR) of P = O-band of such a map enables time saving and convenient assessment of P-concentration distribution at every (x,y)-position of the preform core as well.
3.2 Al-doped samples
The Al-doped samples of series B as prepared by MCVD in combination with high temperature evaporation of AlCl3 show a very homogenous radial dopant distribution profile as verified by EPMA (Fig. 4). Figure 4 displays IR-spectra of the Al-doped samples of series B. They all show a significant band at around 875 cm−1 and some changes in comparison to pure SiO2 at around 1175 cm−1. The bands at around 800 cm−1 and 1100 cm−1 are similar to those of pure SiO2.
Fig. 4. Radial Al2O3 concentration profile of 2.64 mol% Al-doped preform (inserted) and IR-spectra of Al-doped samples of series B. Note that the difference in absorbance level of the samples occurs from different pressure during the measurement at the IR-spectrometer iS10 caused by different sample surface quality and thus different sample contact.
The IR-mapping of an Al-doped sample allows to identify more Al-related bands. Figure 5 shows an IR-spectrum of the Al-doped core and the corresponding 2D- and 3D-visualization of similar core structures only visible at specific wavenumbers. The map includes most of the cylindrical Al-doped core and at the edges the SiO2-cladding as reference. The following characteristic bands could be identified: Al-O- at around 735 cm−1, 875 cm−1 and 1175 cm−1 and as before an inverse (broad valley) structure at 990 cm−1 which is attributed to Si-O-Si that has been substituted by Si-O-Al in the doped core region.
Fig. 5. Single IR-spectrum of an Al-doped preform core with 2D- and corresponding 3D-visualisaztion at specific wavenumbers which are significant for different Al-bonds.
Again, this analysis demonstrates the possibility not only for P-doped SiO2, but also for Al-doped SiO2, to identify the main characteristic species. Here, it has been proven to identify the different Al-species of this binary glass system with an IR-mapping without any deconvolution or other methods.
3.3 Al/P-doped samples
The glass system Al2O3-P2O5-SiO2 shows some special features. Al and P separately doped increase the refractive index of the core material, but with a doping of Al/P at a ratio of one the refractive index is similar to that of silica glass. To investigate the changes in the IR-spectrum of an Al/P-doped silica sample in comparison to only Al- or P-doped silica material, three different preforms have been prepared in series A: one with a P-excess, one with an Al-excess and one with a P/Al ratio of nearly one. Figure 6 shows the measured radial refractive index profiles and the EPMA results on the radial dopant distribution of the three different samples. Because of the strong evaporation of the P-species during the collapsing steps, similar to the P-doped SiO2, a prominent P2O5-concentration dip in the center of the core material is visible for every sample accompanied by local Al-depletion. So, it is of more interest to look at the border area of the core rather than at the center to investigate the effects of Al/P ratio.
Fig. 6. Radial EPMA and refractive index measurement of the preform with P-excess (left), with similar P- and Al-concentration (middle) and with Al-excess (right).
3.3.1 IR-mapping of the Al/P-doped sample with Al-excess
IR-mapping of the Al/P-doped sample with Al-excess is illustrated in Fig. 7. Obviously, another structure is visible compared to only Al-doped samples: the P-O-Si-band at around 1100 cm−1. Although there is a lot of P in the core material, the P = O-band near 1325 cm−1 is missing. It is concluded, that every P-atom is connected with Al as AlPO4-tetrahedra without any P = O-bond. Also, the shape of the 3D-core visualization which is visible at 1100 cm−1 is very similar to the dopant distribution of P2O5 measured by EPMA.
Fig. 7. Single point IR-spectrum of the Al/P-doped preform core with Al-excess together with 2D- and corresponding 3D-visualization at specific wavenumbers which are significant for different Al- and P-bond types and the IR-spectrum of pure silica.
The Al related bands as determined in the binary SiO2-Al2O3 glass samples are observable at the same wavenumbers (around 735 cm−1, 875 cm−1 and 1175 cm−1). Even the complete inverse (valley) structure at around 990 cm−1 can be detected which means that the Al-O-Si-bond replaces this Si-O-Si-bond in the core region of an Al/P-doped sample with Al-excess as well.
The 3D-visualization at 735 cm−1 and 1175 cm−1 are very similar and correspond well with the dopant distribution of Al2O3. However, the 3D-image at 875 cm−1 looks totally different and is (from a visual point of view) more comparable to the radial refractive index curve than to the overall dopant distribution. This indicates, that the structural relations of the Al-species differ within the radius of the core, e.g. there is an enrichment of Al-O-(Al, Si) in the center of the core and rather a depletion of Al-O-(Al, Si) near the border. This is most likely due to evaporated P-species in the center of the core yielding a P-depletion compared to the core border where more P-species are available allowing a high number of Al-atoms to create Al-O-P. In this case, the refractive index is determined by excess of Al which is more pronounced in the center.
This type of IR-mapping of the core and the wavenumber dependent observation of the presented core structures allow for the first time such investigation of structure and refractive index profile.
3.3.2 IR-mapping of the Al/P-doped sample with P-excess
IR-mapping of the Al/P-doped sample with P-excess illustrates many differences as compared to an only P-doped silica material. The single IR-spectrum in Fig. 8 shows the P = O-band at 1325 cm−1 as expected. If the P2O5-concentration would be assessed by this band with the phosphorus band area ratio (PBAR) only the P-excess of around 1.8 mol% P2O5 would be calculated, which underestimates the P-content as determined by EPMA (3.4 mol%).
Fig. 8. Single IR-spectrum of the Al/P-doped preform core with P-excess together with 2D- and 3D-images at specific wavenumbers which are significant for different Al- and P-bonds and the IR-spectrum of pure silica.
The P-O-Si- (1100 cm−1) and the P-O-P-band (970 cm−1) are detectable as well as the inverse structure at 1030 cm−1 as in only P-doped samples. Additional 3D-visualizations appear near 1175 cm−1 and 875 cm−1 which are characteristic for Al-O-bonds. The 3D-image obtained at 1175 cm−1 corresponds well with the radial concentration distribution of Al2O3 with the higher content (yellow) near the dip region. The 2D- and 3D-visualization at 875 cm−1 differ by displaying only a high peak in the center of the core where P-species are evaporated surrounded by a plateau being flatter than the surrounding silica cladding. Because of the P-excess every Al-atom of the core (excluded the dip region) is connected to a P-atom and the specific Al-related bonds such as Al-O-Si- or Al-O-Al-bond are missing in the glass matrix. The reason for the higher IR-signal at 875 cm−1 in the pure silica cladding region is not yet known.
3.3.3 IR-mapping of the Al/P-doped sample with Al/P ratio of nearly one
The sample with a P/Al ratio of nearly one (Fig. 9) shows only a few specific 2D/3D-images in the IR-mapping analysis. As already mentioned, at 1175 cm−1 the Al-O-band (probably Al-O-Si-band) generates a 3D-image which looks very similar to the dopant distribution of Al2O3. The P-O-Si-band at 1100 cm−1 proves to be the same for the P2O5-concentration distribution.
Fig. 9. Single IR-spectrum of the Al/P-doped preform core with an Al/P ratio of nearly one together with 2D- and corresponding 3D-visualization at specific wavenumbers which are significant for different Al- and P-bonds and the IR-spectrum of pure silica.
While the 2D- and corresponding 3D-visualization of the P-O-P (970 cm−1) and the P = O (1325 cm−1) always show a lager depletion (dip) in the center which is on the same level like the pure silica cladding, the structure of the P-O-Si-band at 1100 cm−1 do not reach such a low level in the dip region whilst the P2O5-concentration is not zero in this region. As already observed in case of the Al-excess sample, P-O-P- or P = O-bands are missing for an Al/P ratio nearly one. So, the Al-atom and the P-atom are incorporated as AlPO4-tetrahedra without any P = O-, P-O-P- or Al-O-Al-bonds. The Al-O-band at 875 cm−1 seems to be more characteristic for Al-O-Al-bonds, which could be explained by P being evaporated in the center resulting in more Al-O- being released from AlPO4-tetrahedra. The samples with one dopant or an excess of one dopant always show either P-O-P-bond (970 cm−1) or Al-O-Al-bond (875 cm−1) in the homogenous regions. Hence, absence of one of these bonds in Al/P-doped sample analysis is an evidence of absence of clustering of these single dopants.
4. Conclusion
In this paper an IR-based analysis method for doped preform core structures is presented. The IR-spectroscopy and IR-microscopy enable a band identification and thereby a structural characterization of Al and/or P-doped silica glass cores as prepared by MCVD technique in combination with high temperature evaporation of AlCl3. Taking into account the EPMA derived radial core/cladding structure of a doped silica preform in terms of its elemental composition, an IR-map can be used to conduct a distinct band identification of the 2D- and the corresponding 3D-visualization.
The Al/P-doped samples with Al or P-excess show the same characteristic bands as only Al- or P-doped samples and display always the bands at 1000 cm−1 and 1175 cm−1 where the corresponding 3D-image are representative for the Al- and P-concentration distribution, respectively. The spectra of the P-excess samples in the homogenous regions do not have the Al-O-Al-bands (875 cm−1) while in the spectra of the Al-excess samples no P = O- (1325 cm−1) or P-O-P-bands (990 cm−1) were detected. In both samples displaying either P- or Al-excess, the element in excess determines which IR-band appears as the “inverse” 3D-structure instead of the substituted Si-O-Si-band at 1030 cm−1 and 990 cm−1, respectively. The evidence for the creation of AlPO4- tetrahedra was provided by the appearance of neither P = O, P-O-P nor Al-O-Al-bands in the homogenous core region of Al/P-doped samples with an Al to P ratio of nearly one. The IR-mapping of the complete core region of a doped silica preform and the resulting 2D and corresponding 3D-visualization enable an alternative method and easy way to characterize the glass matrix and the existing main species of the dopants.
Funding
Leibniz-Gemeinschaft; Freistaat Thüringen (2018 FGR 0096).
Acknowledgments
The authors are grateful for the support from the Leibniz Institute of Photonic Technology Jena and the helpful discussions with Dr. F. Froehlich and Dr. S. Unger.
The publication of this article was funded by the State of Thuringia with funds from the European Social Fund (2018 FGR 0096) and the Open Access Fund of the Leibniz Association.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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