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Abstract
Owing to its capability to produce large volume of glass while preserving a high degree of purity and homogeneity, the suspension method was employed here to synthesize a Yb3+/Al3+/P5+ doped silica preforms. The glass structure was studied by relying on both nuclear magnetic resonance (NMR) and Raman spectroscopies, confirming the formation of Al(PO)4 units. Thence, photoluminescence emission spectra were acquired, assessing the beneficial impact of the phosphorus addition in Yb/Al doped silica glass to curtail the Yb2+ ions content. The results reported here suggest that alumino-phosphosilicate matrix having an equimolar concentration of Al3+/P5+ co-dopants exhibits significantly weaker concentration in Yb2+ ions than equivalent aluminosilicate matrix. This glass composition is thus shown relevant to look further on circumventing the photodarkening phenomenon occurring into fiber laser.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ytterbium-doped fibers (YDF) have shown to be well suited for the development of high-power lasers, stimulating a remarkable infatuation towards commercial and scientific applications [1–3]. In fact, such fiber lasers bring together several advantages such as a very high conversion efficiency (low thermal load), a high stability and a high beam quality [4,5]. However, operating them under severe conditions such as a high pump power (ultra-violet, visible, infrared) or a harsh radiative environment - typically for spatial purposes - induces the appearance of a harmful darkening [6–8]. This phenomenon, causing a progressive degradation of fiber losses and thus laser efficiency, originates from the glass structure. No less than four different factors trigger the photo-darkening effect (PD). While the color centers, also known as Non-Bridging Oxygen Hole Centers (NBOHC), are inherent to the host glass matrix [9], the other sources of PD stem from the introduction of the rare-earth ions. As heavy glass modifier, their addition in the silica matrix is likely to cause the clustering of Yb3+ ions [10,11], the generation of Oxygen Deficiency Centers (ODC) [12], or a reduction into Yb2+ ions [13,14]. Therefore, to balance this phenomenon, it appears mandatory to increase both the glass homogeneity and the Yb3+ solubility, requiring the use of appropriate manufacturing process and proper co-dopants.
Among the various strategies undertaken to prevent the PD, one consists in using Al co-doping as network modifier [15,16] since it is known to effectively prevent ytterbium clustering [11,16–19]. However, introduction of aluminum into silica leads to the generation of new defects, called aluminum-oxygen hole centers (AlOHC) [20], which are similar to NBOHC centers. Another approach to prevent the formation of rare-earth clusters is through phosphorus co-doping, [21,22]. Nevertheless, the use of a phosphosilicate matrix has several drawbacks, including a reduction of the effective absorption and emission cross-sections of ytterbium [23]. Alternatively, cerium can be added to reduce the photodarkening although no effect on aggregate reduction were reported. It reduces glass sensitivity to both pump photons and ionising radiation [24]. On the other hand, an increase of the fiber temperature of about 200 °C during the pumping process has been observed by Jetschke S. and their group for Yb and Ce concentrations above 0.47 mol% [25]. The increased temperature can cause modal instability (MI) problems, leading to reduced laser efficiency [26]. Furthermore, this temperature increase may lead to degradation of the polymer coating of the fibers to the point of physical rupture. Several researchers have demonstrated that the use of equimolar proportions of Al2O3 and P2O5 can effectively reduce the photo-darkening (PD) owing to an improved miscibility of the ytterbium ions [27–30]. Additionally, it restrains the refractive index of the doped-core glass to a value close to the one of pure silica [31–33]. These characteristics are advantageous to the fabrication of large effective area fibers for high-power lasers, conventionally produced by gas-phase MCVD method [34]. This technique consists in the deposition of multiple fine layers of soot in a pure silica tube, yielding into small volume of rare-earth doped material per batch - typically 15 g. The resulting core preform commonly presents both radial and longitudinal gradients of dopant distribution over its 3 to 5 mm cross section. Heading to larger volume and homogeneous, novel methods based on nanoporous doped silica green bodies have emerged during the last decade [35,36].
In this paper, the focus is made on the glass structure of diverse glass samples synthesized using a simple powder suspension technique based on REPUSIL technology [35]. Compared to conventional chemical vapor deposition methods, the suspension approach offers several advantages: it allows the synthesis of a larger volume of doped silica (200 g at least) while preserving a dopant homogeneity of ± 0.02 mol% across and along the whole preform [36]. The resulting fibers commonly have low background losses (α<0.1 dB/m), a high slope efficiency (up to 80%) and a fine control over the core refractive index contrast: ∼ 1.10−5 [37]. However, it should be noted that the background loss, typically of approximately 0.1 dB/m at 1200 nm [36], still exceed those of MCVD-prepared fibers, without being large enough to significantly reduce the laser efficiency as the fiber remains very short. This approach has thus been employed here to produce Yb3+-doped silica glass fiber preform co-doped with Al2O3 (YAS) and Al2O3/P2O5 (YAPS). Aluminosilicate (AlSi), phosphosilicate (PSi) and alumino-phosphosilicate (APS) were also prepared using the suspension method. Al2O3 and P2O5 were uniformly incorporated into silica glass. Spectroscopic analysis were conducted to study the structural characteristics of the different samples. The influence of Al3+ and P5+ ions incorporation on the redox properties of ytterbium ions was investigated. Furthermore, the effect of P5+ ions insertion on the near-infrared luminescence intensity of Yb3+ ions is also discussed.
2. Experimental section
The suspension method used in this work is inspired from the REPUSIL [36,37]. This technology has already been successfully employed for the synthesis of passive and active materials dedicated to high-power laser applications [37,38]. The starting precursors are high-purity silica nanoparticles (NPs) (Evonik, ≥ 99.8%) and water-soluble compounds of H3PO4 (Alfa Aesar, 85% aq. soln.), AlCl3.6H2O (Sigma-Aldrich, 99.99+ %) and YbCl3.6H2O (Acros Organics, 99.998%). Silica NPs are homogeneously dispersed in ultrapure water containing the dissolved dopant precursors. Subsequently, the doped suspension is concentrated, and the dried oxide granulates are then milled and calcined in a tubular furnace. The resulting fine powders are sintered and vitrified at temperatures above 2000 °C under vacuum to yield a transparent preform (L = 40 cm, D = 15 mm).
To study the benefits of an Al/P co-doping, five preforms of different compositions were prepared: AlSi, PSi, YAS, APS and YAPS. X-ray fluorescence (XRF) analysis were achieved over slices of each preform and summarized into Table 1. The Bruker Tornado M4 spectrometer allowed here to quantify the dopants concentrations with a resolution of ± 0.02 mol%. It is worth noting that strict equimolar concentration of Al and P were found for both APS and YAPS sample. Further on, the glass network was characterized through the acquisition of both Raman and solid-state NMR spectra.
Table 1. Chemical composition of the five investigated samples obtained by X-ray fluorescence. SiO2 indicated as the reference for silicate glasses.
Raman spectra were achieved using a Renishaw InVia Qontor micro-spectrometer based on a 10-mW 355 nm laser. This operating wavelength was chosen to prevent the photoluminescence of ytterbium ions, A 3600 g/mm grating and a 40x microscope objective equip as well this spectrometer, offering a resolution of 2 cm−1 over 60-second of acquisition time.
In parallel, all solid-state NMR experiments were conducted on a Bruker 850 MHz WB spectrometer. 27Al magic angle spinning (MAS) was performed at a rotation frequency of 30 KHz with a magnetic field strength of 20 Tesla. Similarly, MAS of 31P was carried out at a rotation frequency of 10 kHz with a magnetic field strength of 7 Tesla.
Room temperature photoluminescence (PL) properties were measured using a Horiba-Jobin-Yvon Fluorolog 3 spectrofluorimeter, operated in reflective geometry and equipped with a 450W Xe lamp as the excitation source. PL spectra have been recorded to study the spectroscopic properties of both Yb2+ and Yb3+ ions. The infrared emission spectra of Yb3+ ions were recorded using the same spectrophotometer under excitation at 978 nm.
3. Results and discussion
3.1 Raman spectroscopy
To study the global network structure, the Raman spectra of all samples were recorded over the spectral range 170 to 1500 cm−1. All spectra are normalized between the minimum and the peak intensity at 800 cm−1. The spectra have been deliberately shifted in intensity to improve clarity. All spectra, including the pure silica sample, display five characteristic signatures (cf. Figure 1). The short Raman shift region can be deconvoluted into three major peaks: a broad intensity band located around 440 cm−1 (labeled ω1) and attributed to the Si-O-Si deformation vibrations, as well as two peaks at 490 cm−1 (D1) and 600 cm−1 (D2) refers to symmetric ring vibrations consisting of four and three tetrahedra, respectively [39]. The asymmetric band at about 800 cm−1 (ω3) is attributed to a bending movement of the Si-O-Si link [40]. High frequency bands at approximately 1060 cm−1 (TO) and 1200 cm−1 (LO) are assigned to Si-O-Si antisymmetric elongation [41]. In contrast to the pure silica glass, a sharp peak ascribed to the P = O stretching vibration band appears at 1328 cm−1 in P single doped (PSi) silica glass [42,29]. This P = O stretching band is also observed in APS and YAPS samples due to the excess in phosphorus, as confirmed by the XRF results (see Table 1). An additional characteristic band spanning from 1000 to 1250 cm−1 emerges into Al3+/P5+ (APS) and Yb3+/Al3+/P5+ (YAPS) co-doped samples. Its maximum, located at 1148 cm−1, is assigned to the formation of Al-O-P linkages [32,42,29].
Fig. 1. Raman spectra collected for the five investigated samples. The pure silica spectrum in black is presented as a reference.
3.2 Solid state-NMR
To get a deeper and insightful understanding of the glass structure, a Nuclear Magnetic Resonance (NMR) measurement campaign was conducted. Through the acquisition of both 27Al and 31P MAS-NMR spectra, the local environment of Al3+ and P5+ ions was determined for two specific samples: a passively doped APS material as well as an active YAPS glass.
3.2.1 27Al MAS NMR
As depicted on Fig. 2, the 27Al MAS NMR spectra of both samples show an asymmetric peak (sharp left rising side and slowly decaying right side) with a maximum located around 37 ppm, i.e. the typical signature of the aluminum tetrahedral coordination (AlIV) in AlPO4−SiO2 glasses [29,43]. Despite an equivalent Al-concentration in both samples, a 50% loss in intensity is observed on the YAPS spectrum in contrast to the APS one (as measured from the area of the spectra scaled by the sample’s weight). Owing to their paramagnetic nature, the Ytterbium atoms hinder the detection of Al and/or P atoms in their vicinity, i.e., 50% of the Al ions present in the YAPS sample [44]. Symmetric spinning side bands can also be observed on both sides of the central peak. Additionally, a very small shoulder around 10 ppm evidences a minor concentration of the five-coordinated aluminum (AlV). The presence of this complex and its assignment is confirmed on the 27Al 2D MQMAS NMR reported as Fig. 2(b).
Fig. 2. (a) 27Al MAS NMR spectra of APS (blue) and YAPS (green) samples. Spinning sidebands are indicated by asterisks. (b) 27Al 2D MQMAS NMR spectrum of APS glass.
The presence of direct Al-O-P linkage has been further evidenced using Multiple Quantum (MQ) filtering MAS NMR experiments using the 27Al/31P scalar interaction arising from the bonding electrons. Three types of traces are then acquired: a quantitative spectrum, a spectrum arising from aluminum connected to at least one phosphorus and finally a spectrum of aluminum connected to at least two phosphorus atoms. The combination of these 3 information allows to propose a quantification of the different units forming the glass network, away from the ytterbium. The resonances corresponding to each unit, as shown in Fig. 3, allow to deconvolute fairly the entire quantitative spectrum (including the spinning sidebands arising from the external transitions). To perform this numerical deconvolution, the so-called Czjzek line shape model [45], well known to reproduce 27Al MAS NMR spectra of glass, was used. The obtained quantification of all units is summarized in Table 2.
Table 2. δiso (27Al) and fraction of different AlOP units determined by 27Al MAS NMR spectra for APS and YAPS glasses applying numerical deconvolution.
Four distinct signals with isotropic chemical shifts of 12.2 ppm, 41.7 ppm, 45.3 ppm and 55.5 ppm were evidenced from this “deconvolution” procedure of the 27Al MAS NMR spectra. The resonance at 12.2 ppm is only observed in APS glass (Fig. 3(a)) implying the formation of Al(OSi)3(OP)2 units. Peaks at 41.7 ppm and 45.3 ppm have been assigned to Al(OSi)4-x (OP)x with x ≥ 2 and Al(OSi)3(OP) units respectively based on the MQ filtering experiments, while the resonance at 55.5 ppm is assigned to Al(OSi)4. The spectrum of the YAPS composition is complexified by the presence of Yb which broadens the lines, lowering the resolution obtained in the case of the APS sample. An additional very broad component with many associated spinning sidebands seems to show up as well for the YAPS, probably coming from the aluminum nuclei feeling the magnetic field produced by the unpaired electron of nearby Yb3+ (and filtered out from the spectra displayed in Fig. 3(b) due to different acquisition conditions). The deconvolution shown in Fig. 3(b) suffers from the overall lack of resolution and the rather high uncertainty on the calculated proportion. It can nevertheless be inferred that in the absence of Yb, 85% of the aluminum units bind with phosphorus (APS glass) while those environments represent only 37% of all aluminum atoms in the YAPS sample. As the YAPS glass contains only 0.1 mol. % of Yb2O3, such a strong impact on the 27Al signal tends to demonstrate that most of AlOP units are localized in close vicinity to the ytterbium ions.
3.2.2 31P MAS NMR
Then, a focus was set on the environment of phosphorus ions by acquiring 31P MAS NMR spectra of PSi, APS and YAPS glasses (Fig. 4), showing a single P resonance. For phosphosilicate glass (PSi), a single gaussian peak located around -37 ppm, which is assigned to the tetrahedral coordination of phosphorus atoms in a O = P(OP/Si)3 Q3-type configuration is evidenced in Fig. 4(a) together with a set of associated spinning sidebands [46–48]. The introduction of aluminum into the phosphosilicate matrix (APS) leads to a widening of the peak, caused by an increase disorder around phosphorus. In addition, the intensity of the spinning sidebands decreases, suggesting the formation of Al-O-P bonds. As shown in Fig. 4(b), the incorporation of Yb in alumino-phospho-silicate matrix reduces the P signal by 50%. This shows that half of the phosphorus atoms introduced into the YAPS glass are located near the ytterbium, improving their miscibility into the matrix and preventing the formation of clusters. The presence of configurations involving phosphorus with Al-O-P linkages configurations seen here is consistent with the 27Al observation described above.
Fig. 4. (a) 31P MAS NMR spectra of APS (Blue) and YAPS (green) samples, (b) 31P MAS NMR spectra of PSi and APS glasses. Spinning sidebands are indicated by asterisks.
3.3 Photoluminescence properties
Going further, photoluminescence (PL) properties of the YAS and YAPS samples were measured. By acquiring both the absorption and emission spectra, one gets compelling indicators of the RE ions state in crystal or glass matrices. Indeed, while the fluorescence of the Yb3+ ions is exploited for the realization of the laser, its redox Yb2+ form is troublesome since it leads to the UV-absorption around 330 nm through a 3-photons energy transfer, which will then trigger the photo-darkening effect, and lowering the in-core absorption [49]. For this reason, 1 mm-thick slices of the YAS and YAPS preform were first placed onto a UV transilluminator to track the potential presence of Yb2+ ions. Visually, we note that under a UV excitation at 254 nm, the glow of the YAS slice looks clearly more greenish than in the case of the YAPS slice, suggesting then an Yb2+ content significantly higher. (Cf. Figure 5).
The PL normalized (to Yb2O3 content) excitation spectra of Yb2+ doped alumino-silicate glass (YAS) and alumino-phosphosilicate glass (YAPS), with emission wavelength fixed either at 493 nm or at 530 nm (maximum of the emission intensity – see further), respectively, are shown in Fig. 6. A broad band centered at 330 nm and corresponding to the 4f14 → 4f135d transition of the Yb2+ is evidenced [50]. Figure 7 shows the normalized (to Yb2O3 content – Cf. Table 1) emission spectra of Yb2+ ions doped silica glass under the 330 nm UV excitation. The emission band position of Yb2+ ions changes with the glass composition. It is located at approximately 493 nm for YAS and 526 nm for YAPS samples. This emission band is attributed to the 4f5d → 4f transition of Yb2+ ions, which is broadened by the presence of Yb2+ multi-sites in the glass [50]. The shift to longer wavelength for the emission band of the YAPS sample is probably related to a slightly different local environment related to the difference in composition. Furthermore, the emission intensity recorded for the YAPS is drastically - almost two orders of magnitude - lower than that of the YAS sample (Cf. Figure 7). This experimental observation confirms a significant reduction of the Yb2+ ions content through the addition of P5+ to the initial composition of the glass. The oxidation effect of P5+ on Yb2+ can be explained by the strong tendency of P5+ to form a lower oxidation state (P4+) to match that of Si4+ and enhance the stability of the glass. The oxidation reaction has already been described in the work of S. Wang et al. [50] and shown as follows:
This section examines the effect of matrix composition on the valence state of ytterbium ions, specifically the impact of phosphorus on the oxidation process from Yb2+ ions to Yb3+. Phosphorus incorporation into aluminosilicate glass offers a favorable outcome in restricting the formation of Yb2+ ions that cause photodarkening. It should be noted that a reduction in the concentration of Yb2+ ions in the aluminosilicate matrix can be achieved by the use of an oxidizing atmosphere during the manufacturing process. This effect is well known and has been demonstrated in the production of ytterbium-doped fibers by the MCVD process [51]. However, its effectiveness in the prevention of photodarkening is limited by the presence of other defects in the matrix that are contributors to its occurrence [52].
Figure 8 shows the normalized (to Yb2O3 content) emission spectra of Yb3+ ions under excitation at 978.5 nm. The main emission peak at 1020 nm for YAS and at 1023 nm for YAPS sample, respectively, is attributed to the transition of Yb3+ from the excited state 2F5/2 to of the ground state 2F7/2 [53]. In addition, another contribution of the same transition, which implies different Stark sublevels, is evidenced around 1065 nm: the latter is neatly more pronounced for the YAPS sample in comparison with that is observed for the YAS sample. As we have already discussed, the introduction of phosphorus into aluminosilicate matrix leads to a decrease in the concentration of Yb2+ ions. Hence, we could expect to measure a stronger peak emission intensity for the YAPS sample than the YAS sample. This could indeed be explained by an increase of the non-radiative transition probability, in relation with to the higher average phonon energy of the [AlPO4] units and P = O bond when compared with the Si–O bond [50]. But apparently, this is definitely not what is observed here, as the PL intensity roughly reaches the same level for the two samples.
Fig. 8. Normalized PL emission spectra of Yb3+ ions under excitation at λex = 978.5 nm for YAS and YAPS samples.
4. Conclusion
The present study reports on the successful preparation of homogeneous alumino-phosphosilicate preforms using the suspension method. A thorough investigation of the synthesized glasses was carried out using Raman scattering, solid-state nuclear magnetic resonance (NMR), and photoluminescence spectroscopy. By employing these techniques, a systematic exploration of the glass properties was conducted to analyze the specific local environment of Yb3+ ions and examine the network structure of the Yb3+/Al3+/P5+-doped silica glasses.
Samples were investigated using Raman spectroscopy to gain overall insights into the structure of the glass network. The results unveil the presence of Al-O-P bonds in APS and YAPS glasses. This observation was confirmed by NMR spectroscopy. The 27Al NMR spectra have revealed that both APS and YAPS glasses are predominantly composed of AlO4 sites, with a minor presence of AlO5 sites. No AlO6 sites were detected. 31P NMR spectra have disclosed the presence of phosphoryl groups O = P(OP/Si)3 in the studied glasses, indicating the existence of [PO4] tetrahedra linked either to phosphorus atoms or silicon atoms within the glass network. The 27Al multi-quantum magic angle spinning (MQMAS) NMR technique, using the 27Al/31P scalar interaction, confirmed the presence of Al-O-P bonds in the investigated glasses (APS and YAPS). Thus, it was evidenced that these bonds are located near the ytterbium ions to disperse them and prevent clusters formation. In addition, the study of the spectroscopic properties of Yb2+ ions indicated that the introduction of P5+ ions into the aluminosilicate matrix promotes the oxidation of Yb2+ ions to Yb3+ ions. A reduction by two orders of magnitude of the Yb2+ content was demonstrated. In fact, to adapt to Si4+ and improve the stability of the silica glass, P5+ ions have a strong tendency to form a lower oxidation state (P4+). The presence of P ions reduces the amount of Yb2+ ions in the glass, which are responsible for the occurrence of photodarkening in optical fibers. These finding provide valuable insights into the design and development of novel glass compositions with improved optical properties and reduced photodarkening effect, opening up new opportunities for the fabrication of large mode area fibers (LMA) intended for generating high-power lasers.
Acknowledgments
The authors would like to acknowledge the French ANRT for supporting Nadia Tiabi’s PhD thesis. They acknowledge also the financial support of European Union’s Horizon Europe research and innovation programme (grant agreement No 101096317) and Deutsche Forschungsgemeinschaft (project DFG SPP2289 WO 2540/1-1).
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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