1. Introduction

High-power eye-safe erbium-doped silica fiber (EDF) laser operated at 1.5µm is attractive for its wide application in the fields of free-space telecommunications, remote sensing, and range finding [1–4]. So far, the maximum output power (∼656 W) from a single EDF has lagged far behind that of ytterbium doped silica fiber (> 10 KW) [5,6]. There are two key factors that limit the power scaling of EDF [5,7,8]: small absorption cross section at 980 nm and serious concentration quenching effect.

Laser diodes (LDs) with wavelengths of 980, 1480 and 1530 nm are usually used as pump sources of EDF [1]. Compared with 1480 and 1530 nm LDs, the 980 nm LD has higher output power and electrical-to-optical efficiency as well as lower price. However, the absorption cross section of Er³⁺ ion at 980 nm is one order of magnitude smaller than that of Yb³⁺ ion [1]. In order to increase the absorption coefficient of EDF at 980 nm and suppress the nonlinear effect of EDF due to the long fiber length, Yb³⁺ ions are always co-doped into EDF to sensitize Er³⁺ ions. The Yb³⁺ ions can absorb most of the pump light at 980 nm and transfer absorbed energy to the adjacent Er³⁺ ions via cross relaxation. Furthermore, co-doping with Yb³⁺ ions can reduce Er³⁺ clusters and inhibit concentration quenching effect [9].

Early studies show that the rare earth (RE) ions tend to form clusters and micro-phase separation even at a low doping levels (∼1000 ppm by weight), due to their poor solubility in pure silica glass [7,8,10–12]. Such RE clustering gives rise to drastically reduce fluorescence lifetime of RE ions, significantly increase laser threshold and substantially reduce slope efficiency of RE-doped silica fibers. This phenomenon is called concentration quenching effect [8]. Co-doping with Al₂O₃ or P₂O₅ can effectively improve the solubility of RE ions and prevent clustering of RE ions in silica glass [12–14]. On the other hand, in order to improve the energy transfer efficiency from Yb³⁺ to Er³⁺, high concentrations of phosphorus are always doped into silica matrix. However, single-doping with large amounts of phosphorus (>10 mol% P₂O₅) has following disadvantages, such as a detrimental central dip in the refractive index profile, a high core numerical aperture (N_A) and a high core background loss [15]. Early studies [16,17] show that Al and P tend to bond with each other in Al₂O₃-P₂O₅-SiO₂ glass, and its global network structure is strongly dominated by the excessed Al₂O₃ or P₂O₅ component. Co-doping with equimolar Al₂O₃ and P₂O₅ can decrease the refractive index of fiber core due to the formation of AlPO₄-like unit [17,18]. In addition, co-doping with Al and P can effectively disperse RE ion clusters and inhibit the central refractive index dip of fiber core [19–21].

The spectral properties of Er³⁺ ions are closely correlated to their local environment. The spectral properties and local environment of Er³⁺ ions in different host glasses (such as silicate, aluminate, phosphate and fluoride) have been systematically studied using optical absorption and photoluminescence (PL) as well as x-ray absorption fine structure (XAFS) methods [22–24]. Effect of Al₂O₃ content on the spectral properties and local environment of Er³⁺ ions in Er³⁺/Al³⁺ co-doped silica glasses have been also systematically studied using PL and XAFS methods [25–27]. The PL spectral shape and peak cross section (σ_em) of Er³⁺ ions at 1.53 µm in Er³⁺/Al³⁺, Er³⁺/Ge⁴⁺, Er³⁺/P⁵⁺ doped silica glasses were comparatively studied by Wang et al. [28], the σ_em of the studied glasses follows the sequence of Er³⁺/Ge⁴⁺ > Er³⁺/Al³⁺ > Er³⁺/P⁵⁺. Using pulse EPR spectroscopy, Saitoh et al. [29] confirmed that Er³⁺ ions preferentially coordinate to phosphorus rather than aluminum in Er³⁺/Al³⁺ or Er³⁺/P⁵⁺-doped silica glasses. However, the effects of P and Al co-doping ratio (P/Al ratio) on the spectral properties and local environment of Er³⁺ ions as well as the energy transfer mechanism between Yb³⁺ and Er³⁺ ions in Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ co-doped silica glasses are not fully understood.

In this work, Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ co-doped silica glasses with different P/Al ratio were fabricated by sol-gel method combined with high temperature sintering. The doping concentrations of Yb³⁺ and Er³⁺ ions were fixed. The effects of P/Al ratio on the spectral properties of Yb³⁺ and Er³⁺ ions and energy transfer efficiency between them were systematically studied. The local environment of RE ions was probed using pulse EPR. The global glass network structure was studied by Raman spectroscopy. Based on the above experiments, the relationships between glass structure and spectroscopic properties in Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ co-doped silica glasses were established. Due to the influence of fiber drawing process, there will be a little difference between the performance of optical fiber and that of fiber preform [30,31]. However, to some extent, this work can provide theoretical guidance for the optimization of fiber core composition.

2. Experimental details

In this study, Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ (denoted as EYAPS) doped silica glasses were fabricated using the sol-gel method combined with high temperature sintering. TEOS, ErCl₃·6H₂O, YbCl₃·6H₂O, AlCl₃·6H₂O, H₃PO₄, and C₂H₅OH were used as precursors. Pure water was used to sustain the hydrolysis reaction. All precursors were mixed and stirred at 25°C to form homogeneous doped gel. Then the sol was heated from 30 to 1000 under oxygen atmosphere to decompose the hydroxyl and organics. The obtained dry sol powder was then melted at 1650-1750 °C for 2 h in vacuum state to obtain transparent bulk glass. The details of the fabrication process are given in Ref. [32]. In order to calculate the energy transfer efficiency from ²F_5/2 level of Yb³⁺ to ⁴I_11/2 level of Er³⁺ ions, the corresponding Er-free Yb³⁺/Al³⁺/P⁵⁺ doped silica (denoted as YAPS) glass samples were prepared using the same method.

Table 1 shows mean theoretical compositions of EYAPS and YAPS series glass samples. The doping concentrations of Er₂O₃ and Yb₂O₃ were fixed at 0.1 mol% and 0.5 mol%, respectively. P/Al mole ratio increases from zero to infinity. Inductively coupled plasma atomic emission spectrometer (ICP-AES) test indicated that the actual Yb₂O₃, Er₂O₃ and Al₂O₃ contents are very close to their theoretical values, while the actual P₂O₅ content is slightly lower than its theoretical content due to the volatilization of phosphorus during high temperature sintering.

Table 1. Mean compositions of EYAPS and YAPS glass samples (in mol%)

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Bulk glasses were cut and polished to small chips with a thickness of 2 mm and a diameter of about 15 mm for the spectroscopic tests. Powder samples with a weight of about 200 mg were used for EPR tests.

The optical absorption spectra were tested via a Lambda 950 UV–Vis–NIR spectrophotometer. The PL spectra excited at 896 nm for Yb³⁺ and Er³⁺ ions with Xe lamp, and the fluorescence decay curves of Er³⁺: ⁴I_13/2 and Yb³⁺: ²F_5/2 energy levels pumped with a pulsed 980 nm laser diode (LD) were recorded on a high-resolution Edinburgh Instruments, FLS 920 spectrofluorometer. The testing step lengths of absorption and PL spectra are 1 nm, which are much smaller than the peak widths of absorption and PL spectra. The Raman measurement using a 488 nm argon ion laser as exciting source was carried out by a Renishaw InVia Raman Microscope.

Pulse EPR experiments were performed using an X-band EPR spectrometer (E-580 BRUKER ELEXSYS) at the temperature of 4 K. In order to probe the effect of P/Al ratio on the local environment of RE ions, the two-pulsed (π/2-τ-π-τ-echo) echo detected field-swept (EDFS) and four-pulsed (π/2-τ-π/2-T₁-π-T₂-π/2-τ-echo) two-dimensional hyperfine sublevel correlation (2D-HYSCORE) EPR spectra were recorded. The π/2, π, and τ values in these two experiments corresponded to 16, 32, and 136 ns, respectively.

3. Results and discussion

3.1 Effect of P/Al ratio on the spectral properties

Figure 1 shows the absorption spectra of EYAPS series samples in the ultraviolet and visible (UV-VIS) region. The UV absorption edges show a blue shift and the UV absorption intensities show a decrease with increasing P/Al ratio. These changes are mainly correlated to the presence of Yb³⁺ ions, because the content of Yb³⁺ ions (1mol%) are much higher than that of Er³⁺ ions (0.2mol%) and the matrix glass has not absorption in UV region. According to early studies [33–35], the strong UV absorption in the range of 190–290 nm were primarily due to the charge-transfer (CT) transition Yb³⁺ ions and the position of CT bands shifted to a shorter wavelength with increasing electronegativity of the next nearest neighbor atoms (Al/Si/P) of Yb³⁺ ions. With increasing P/Al, Yb³⁺ ions gradually moved from silicon rich and aluminum rich environment to phosphorus rich environment, that is, Yb-O-Al and Yb-O-Si linkages are gradually replaced by Yb-O-P linkage (See Fig. 5). The absorption band at 330 nm is primarily due to the 4f-5d transition of Yb²⁺ ions. With the increase of P/Al ratio, the absorption intensity of Yb²⁺ ion decreases. Since the absorption of Yb²⁺ ion covers the range of 190-600nm [33], the absolute absorption intensities of ⁴G_11/2 (379 nm) and ⁴F_11/2 (520 nm) levels of Er³⁺ ions decrease with the decrease of Yb²⁺ ion absorption intensity. When P/Al > 1, the absorption peak of Yb²⁺ is almost invisible as shown in EYAPS2 and EYAPS3 samples. It indicates that the reduction of Yb³⁺ to Yb²⁺ can be effectively inhibited when P/Al > 1. Similar results have been reported in our previous studies in Yb³⁺/Al³⁺/P⁵⁺ doped silica glasses [18,34].

 

Fig. 1. UV-VIS absorption spectra of EYAPS series samples, the inset photographs show the large-sized and transparent EYAPS0, EYAPS1, EYAPS2 and EYAPS3 samples, respectively.

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The inset photographs in Fig. 1 show the large-sized and transparent EYAPS0, EYAPS1, EYAPS2 and EYAPS3 samples, respectively. With increasing P/Al ratio, the color of glass gradually changes from light yellow to light pink. Light yellow is mainly caused by Yb²⁺ absorption, while light pink is mainly caused by Er³⁺ absorption. The change of glass color also indicates that Yb²⁺ ions were inhibited gradually with the increase of P/Al ratio.

Figure 2(a) shows the near-infrared (NIR) absorption spectra of EYAPS series samples near 1 µm, showing a typical ²F_7/2→²F_5/2 absorption peak of Yb³⁺ ions. Because of low doping concentration and low absorption cross section at 980 nm of Er³⁺ ions, the absorption peak intensities of Er³⁺ ions in this band are very weak, which is completely covered by the absorption peak of Yb³⁺ ions. The absorption spectra of Yb³⁺ ions show two different types of line-shape in Fig. 2(a). When P/Al < 1, the absorption peak widths of EYAPS0 and EYAPS1 samples at 915 nm are relatively fuller. When P/Al > 1, the absorption peak intensities of EYAPS2 and EYAPS3 samples at 915 nm are obviously weakened, and the peak width is narrowed. The absorption peak intensities at 975 nm are also decreased in EYAPS2 and EYAPS3 samples which have P/Al ratio larger than 1. Similar result has been reported in our earlier work [18]. Therefore, the effect of P/Al ratio on spectral properties of Yb³⁺ ions will not be discussed in detail below.

 

Fig. 2. NIR absorption spectra of EYAPS series samples near 1 µm (a) and 1.5 µm (b).

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Figure 2(b) shows the NIR absorption spectra of EYAPS series samples near 1.5 µm, showing a typical ⁴I_15/2→⁴I_13/2 absorption peak of Er³⁺ ions. The absorption peaks also show two different types of line-shape in case of P/Al < 1 and P/Al > 1. Compared with P/Al < 1 samples, the absorption peak intensity of P/Al > 1 samples at 1480 nm decreases significantly, and the strongest absorption peak near 1530 nm appears redshifted. When P/Al < 1, the strongest absorption peak locates at 1528 nm and its full width at half maximum (FWHM) is about 40 nm; when P/Al > 1, the strongest absorption peak is at 1532 nm and its FWHM is about 18 nm (See Table 2 for more detail).

Table 2. The peak absorption (λ _abs) and emission (λ _em) wavelength, full width at half maximum (FWHM) of absorption and emission bands (∼ 1.53 µm), and peak absorption (σ_abs) and emission (σ_em) cross sections for the transition between ⁴I_13/2 and ⁴I_15/2 of Er³⁺ ions with different P/Al ratio.

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Figure 3(a) shows the PL spectra of glass samples excited by 896 nm Xe lamp. All PL spectra are normalized to the same intensity at 1.5 µm. The PL bands at 1 µm and 1.5 µm are due to the ²F_5/2→²F_7/2 of Yb³⁺ and ⁴I_13/2→⁴I_15/2 of Er³⁺ ions, respectively. With the increase of P/Al ratio, the PL intensity at 1 µm decreases. It suggests that high P/Al ratio will reduce the PL intensity of Yb³⁺ ions. Compared with P/Al < 1 samples, the widths of 1.5 µm PL band in P/Al > 1 samples are obviously narrowed. At the same time, the strongest PL peak appears redshifted. For P/Al < 1 samples, the strongest PL peak is located at 1530 nm, and the FWHM of 1.5 µm PL band is about 46 nm; For P/Al > 1 samples, the strongest PL peak is redshifted to 1535 nm, and the FWHM of 1.5 µm PL band is narrowed to 21 nm (See Table 2 for more detail). The narrow FWHM (∼21 nm) in P/Al > 1 sample implies that the Er-doped silica fiber with P/Al > 1 is not suitable for broadband amplification in C-band (1530 ∼ 1565 nm). However, the PL band above 1580 nm is relatively flat in P/Al > 1 samples, which is conducive to improve the gain flatness of Er³⁺-doped fiber in L-band (1565-1625 nm).

 

Fig. 3. (a) Normalized PL spectra, and (b) Emission intensity ratio of Er³⁺ ions at 1530 and Yb³⁺ ions at 975 nm (I₁₅₃₀/I₉₇₅) of EYAPS series samples.

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Figure 3(b) shows the ratio of the strongest PL intensity (1530 nm) of Er³⁺:⁴I_13/2→⁴I_15/2 to that (975 nm) of Yb³⁺:²F_5/2→²F_7/2 (denoted as I₁₅₃₀/I₉₇₅). Increasing P/Al ratio from zero to infinity, the I₁₅₃₀/I₉₇₅ ratio increases from four to fourteen. It implies that increasing P/Al ratio can improve the energy transfer efficiency from ²F_5/2 level of Yb³⁺ to ⁴I_11/2 level of Er³⁺ ions.

Table 2 lists the peak wavelength and full width at half maximum (FWHM) of absorption and emission bands (∼ 1.53 µm) for the transition between ⁴I_13/2 and ⁴I_15/2 of Er³⁺ ions in EYAPS series glasses. The peak absorption (σ_abs) and emission (σ_em) cross sections of Er³⁺ ions at about 1.53 µm are also added into Table 2. σ_abs and σ_em are calculated using the Beer–Lambert law and McCumber theory respectively as follows [36]:

Where, OD(λ) is optical density obtained directly from the absorption tests, N₀ is the Er³⁺ concentration (ions/cm³) determined by ICP and density measurements, and L is the glass sample thickness. Z_U and Z_L are the partition functions of the upper and lower energy levels of Er³⁺ ions, respectively. The ratio of Z_L and Z_U (Z_L/Z_U) is approximately 8/7 at room temperature for Er³⁺ ions. The letters h, c, K, T represent the Planck constant, light speed, Boltzmann constant and Kelvin temperature, respectively. λ₀ is the wavelength of the zero-line energy. Here, λ₀ is taken as the wavelength corresponding to the intersection of absorption and emission bands at 1.5 µm.

Table 3 presents the fluorescence lifetime of Yb³⁺:²F_5/2 level (${\tau _{Yb}}$) and Er³⁺:⁴I_13/2 level (${\tau _{Er}}$) in Er-containing EYAPS series samples, as well as the fluorescence lifetime of Yb³⁺:²F_5/2 level ($\tau _{Yb}^0$) in the corresponding Er-free YAPS series samples. The energy transfer efficiency (η) from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_13/2 level is also added into Table 3.

Table 3. Lifetime of Er³⁺:⁴I_13/2 level (${{\boldsymbol {\tau} }_{{\boldsymbol {Er}}}}$) and Yb³⁺:²F_5/2 level (${{\boldsymbol {\tau} }_{{\boldsymbol {Yb}}}}$) in EYAPS series samples; Lifetime of Yb³⁺:²F_5/2 level (${\boldsymbol {\tau} }_{{\boldsymbol {Yb}}}^0$) in Er³⁺-free YAPS series samples; Energy transfer coefficients (${\bf{\mathrm{\eta}} }$) from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_13/2 level

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The η value can be calculated using Eq. (3):

With the increase of P/Al ratio, the lifetime of Er³⁺:⁴I_13/2 level (${\tau _{Er}}$) in EYAPS samples shows a slight decline from 11.98 to 10.6 ms. The lifetime of Yb³⁺:²F_5/2 level (${\tau _{Yb}}$) in Er-containing EYAPS series samples has no obvious change. It is within the allowable range of measurement uncertainties (≤ 10µs). The lifetime of Yb³⁺:²F_5/2 level ($\tau _{Yb}^0$) in Er-free YAPS series samples shows a significant increase from 1253 to 2011 µs. Compared with YAPS series samples, the shorter lifetime of Yb³⁺:²F_5/2 level (130∼180µs) in EYAPS series samples implies a strong energy transfer from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_13/2 level. With the increase of P/Al ratio, the energy transfer efficiency increases from 86.3% (in EYAPS0) to 92.1% (in EYAPS3) as shown in Table 3. It indicates that increasing P/Al ratio in ETAPS series glasses promotes the forward energy transfer from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_13/2 level. At the same time, it inhibits the reverse energy transfer from Er³⁺:⁴I_13/2 level to Yb³⁺:²F_5/2 level.

3.2 Effect of P/Al ratio on the glass structure

Figure 4 shows EDFS spectra. The EDFS line-shape and its strongest peak position are related to the site heterogeneity and g-anisotropy of RE³⁺ ions [33,37]. All EDFS spectra consist of broad asymmetric curves, which are correlated with the inherent topological disorder of glass. With the increasing of P/Al ratio, the strongest EDFS peak shifts to a lower magnetic field. It indicates that the local environment of RE³⁺ ions has changed in EYPAS series samples.

 

Fig. 4. EDFS spectra of EYPAS series samples.

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In order to probe the local environment of RE³⁺ ions, 2D-HYSCORE spectra were recorded as shown in Fig. 5. At magnetic field of 350 mT, three patterns located at 6.0, 3.9 and 3.0 MHz correspond to the Larmor frequencies of the nuclides ³¹P (natural abundance ∼ 100%), ²⁷Al (natural abundance ∼ 100%), and ²⁹Si (natural abundance ∼ 4.68%), respectively. With increasing P/Al ratio, the magnitude of ³¹P pattern increases gradually, but the magnitude of ²⁹Si pattern decreases until it disappears. The magnitude of ²⁷Al pattern increases first, and then it decreases until disappears. This result is well consistent with the early studies in Yb³⁺/Al³⁺/P⁵⁺-doped silica glasses [16,35,38]. It suggests that the introduction of a small number of Er₂O₃ (∼0.1mol%) does not significantly change the local environment of RE ions (Yb³⁺ and Er³⁺). In fact, the atomic radius and charge of Er³⁺ and Yb³⁺ ions are very close, indicating that they are chemically equivalent to each other. No significant difference can be detected in the HYSCORE tests for chemically equivalent paramagnetic ions. For P/Al < 1 samples, the RE³⁺ ions are mainly located in silicon rich and aluminum rich environment. For P/Al > 1 samples, the RE³⁺ ions are mainly located in phosphorus rich environment. Only phosphorus pattern is detected in EYAPS3 sample. It indicates that almost all RE³⁺ ions in EYAPS3 sample are surrounded by phosphorus solvent shell. While a strong ³¹P pattern and a weak ²⁷Al pattern can be observed in EYAPS2 sample. It suggests that most of RE³⁺ ions are in phosphorus rich environment, but a small part of RE³⁺ ions are still located in aluminum rich environment in EYAPS2 sample. The change of the coordination environment of RE³⁺ ions is responsible for the change of EDFS EPR spectra as shown in Fig. 4. Oxygen atoms that coordinate to the first shell of RE ions are not detected, owing to the low natural abundance (∼0.038%) of magnetic nucleus ¹⁷O.

 

Fig. 5. 2D-HYSCORE spectra of EYAPS0 (a), EYAPS1 (b), EYAPS2 (c) and EYAPS3 (d) samples.

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In order to study the effect of P/Al ratio on global glass network structure, the Raman spectra of EYAPS series samples were recorded as shown in Fig. 6. The Raman spectrum of undoped silica glass (α-SiO₂) is also added into Fig. 6 for comparison. A broad and strong band at 440 cm⁻¹ as well as five weak bands peaked at 490, 606, 800, 1160 and 1200 cm⁻¹ can be observed in α-SiO₂. All these Raman peaks in α-SiO₂ also can be observed in EYAPS series samples. Among them, the 490 and 600 cm⁻¹ bands are ascribed to the planar four-fold (D₁) ring and planar three-fold (D₂) ring, respectively [39]. The 440, 800, 1060 and 1200 cm⁻¹ bands are ascribed to symmetric bending (ω₁), symmetric stretching (ω₃), TO and LO asymmetric stretching (TO-ω₄ and LO-ω₄) vibration of Si-O-Si bonds, respectively [40,41]. The Raman spectra of P/Al < 1 samples are very similar to that of α-SiO₂, the maximum phonon energy of P/Al < 1 samples is about 1200 cm⁻¹, and it is primarily from the LO-ω₄ vibration modes of Si-O-Si bonds. Compared with Raman spectra of P/Al < 1 samples, a narrow and sharp peak peaked at 1326 cm⁻¹ can be observed in P/Al > 1 samples, this peak provides maximum phonon energy for P/Al > 1 samples, and it is due to the stretching vibration of P = O bond in P⁽³⁾ unit (Its structural model is O = P-O_3/2) [17,40]. In addition, a broad and strong band at 1000-1250 cm⁻¹ can only be observed in EYAP2 sample, but not in EYAP3 sample. This broad band peaked at 1145 cm⁻¹ is usually seen as the evidence of the existence of AlPO₄-like units [19]. Sample EYAP0 is only co-doped with aluminum and but not phosphorus, while EYAP3 is only co-doped with phosphorus and but not aluminum. No Al-O-P linkages were formed in EYAP0 and EYAP3 samples, so no AlPO₄-like units are observed in the Raman spectra of these two samples. For EYAP1 sample, the signal of AlPO₄-like units is very weak owing to its low doping amount of phosphorus.

 

Fig. 6. Raman spectra of pure silica (α-SiO₂) and EYAPS series glasses.

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3.3 Correlation between spectral properties and glass structure

As described in Section 3.1, the spectral properties of Er³⁺ and Yb³⁺ ions and the energy transfer efficiency between them in EYAPS series samples, are highly dependent on the P/Al ratio. They should be correlated to the structures of these glasses as revealed in Section 3.2.

From Tables 2 and 3, increasing P/Al ratio, the peak wavelength of absorption and emission bands of Er³⁺ ions at 1.53 µm is redshifted, FWHM is narrowed, and the peak absorption and emission cross sections are increased. At the same time, the energy transfer efficiency of ²F_5/2 level of Yb³⁺ to ⁴I_11/2 level of Er³⁺ ions gradually increase. The changes of these spectral parameters are correlated to the changes of local environment of Er³⁺ ions and phonon energy of glass matrix.

The spectral broadening of RE ions includes homogeneous broadening and inhomogeneous broadening. Among them, the homogeneous broadening is correlated to the interactions between RE and RE as well as RE and glass matrix. The inhomogeneous broadening primarily originates from the heterogeneity of RE sites [1,42]. In silicate glasses, the contribution of inhomogeneous broadening is greater than that of homogeneous broadening [42]. For P/Al < 1 samples, pulsed EPR (See Fig. 5) shows that the RE³⁺ ions are mainly surrounded by Si and Al atoms. Among them, Si mainly exists in the form of SiO_4/2, and Al consists of three structural states (four, five and six coordinated aluminum) as evidenced by nuclear magnetic resonance (NMR) [16,17]. The various coordination environments of RE ions lead to the larger inhomogeneous broadening of absorption and PL bands of Er³⁺ ions at 1.5µm. While for P/Al > 1 samples, pulsed EPR (See Fig. 5) shows that the RE³⁺ ions are mainly surrounded by P atoms. When P/Al > 1, excessive P mainly exists in the form of P⁽³⁾ structure (Its structural model is O = P-O_3/2) as evidenced by Raman (See Fig. 6) and NMR [16,17]. The monotonous coordination environment of RE ions leads to the narrower absorption and PL bands of Er³⁺ ions at 1.5µm. Generally speaking, the absorption and emission cross sections of Er³⁺ ions at 1.5 µm are inversely proportional to their FWHMs. Therefore, increasing P/Al ratio causes the FWHMs of Er³⁺ ions at 1.5 µm bands become narrow, while both absorption and emission cross sections increase (See Table 2).

Figure 7 shows the schematic diagram of forward and reverse energy transfer from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_11/2 level. Early studies show that the lifetimes of Er³⁺:⁴I_11/2 in germanate, aluminosilicate, and phosphate glasses are about 15, 4, and 0.1 µs, respectively [43,44]. While the maximum phonon energy of these three types of glasses increases gradually, which are 820, 1200, 1326 cm⁻¹, respectively. This result suggests that the maximum phonon energy of glass matrix has a great impact on the lifetime of Er³⁺: ⁴I_11/2. The high phonon energy of glass matrix will speed up the non-radiation transition rate of Er³⁺ ions from ⁴I_11/2 level to ⁴I_13/2 level. Therefore, it decreases the fluorescent lifetime of Er³⁺:⁴I_11/2 level, and reduces the number of Er³⁺ ion population in ⁴I_11/2 level. As a result, it inhibits the reverse energy transfer from Er³⁺:⁴I_11/2 level to Yb³⁺:²F_5/2 level, and improves the forward energy transfer from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_11/2 level.

 

Fig. 7. Schematic diagram of energy transfer between Yb³⁺:²F_5/2 level and Er³⁺:⁴I_11/2 level.

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Raman spectra show that the maximum phonon energy of EYAPS series samples gradually increases from 1200 to 1326 cm⁻¹ (See Fig. 6). The forward energy transfer efficiency from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_11/2 level in EYAPS series samples increases gradually from 86.3% (in EYAPS0) to 92.1% (in EYAPS3) with increasing P/Al ratio (See Table 3). When P/Al < 1, the relatively lower maximum phonon energy (∼1200 cm⁻¹) of glass matrix decreases the non-radiative transition rate (∼1/4 µs⁻¹) from ⁴I_11/2 level to ⁴I_13/2 level, and it results in the increased probability of reverse energy transfer from Er³⁺:⁴I_11/2 level to Yb³⁺:²F_5/2 level. At the same time, the forward energy transfer efficiency from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_11/2 level relatively decreases. When P/Al > 1, the relatively higher maximum phonon energy (∼1326 cm⁻¹) of glass matrix increases the non-radiative transition rate (∼1/0.1 µs⁻¹) from ⁴I_11/2 level to ⁴I_13/2 level, and it leads to the decreased reverse energy transfer probability from Er³⁺:⁴I_11/2 level to Yb³⁺:²F_5/2 level. At the same time, the forward energy transfer efficiency from Yb³⁺:²F_5/2 level to Er³⁺:⁴I_11/2 level greatly increases.

4. Conclusions

In this work, Er³⁺/Yb³⁺/Al³⁺/P⁵⁺-doped silica glasses with different P/Al ratios ranging from zero to infinity were fabricated using the sol-gel method combined with high-temperature sintering. Combining with the optical absorption, photoluminescence (PL), fluorescence decay curves, Raman, and pulse electron paramagnetic resonance (EPR) spectroscopies, the effect of P/Al ratio on the spectroscopic properties of Er³⁺ and Yb³⁺ ions and their local environment as well as the global glass network structure were studied systematically, and the relationships between spectral properties and glass structures were established.

The results show that the spectroscopic properties of Er³⁺ ions as well as their local environment present two distinct states with P/Al = 1 as the boundary. When P/Al < 1, the Er³⁺ ions are mainly surrounded by Si and Al atoms, the absorption and fluorescence peaks of Er³⁺ ions at 1.5 µm are wider. When P/Al > 1, the Er³⁺ ions are mainly surrounded by P atoms, the absorption and fluorescence peaks of Er³⁺ ions at 1.5 µm get narrower. Raman spectra show that the maximum phonon energy of Er³⁺/Yb³⁺/Al³⁺/P⁵⁺-doped silica glasses gradually increases from 1200 to 1326 cm⁻¹ with increasing P/Al ratio due to the formation of AlPO₄-like unit and P = O double bonds. Due to these structural changes, the peak absorption and emission cross sections of Er³⁺ ions at 1.5 µm as well as the energy transfer efficiency of ²F_5/2 level of Yb³⁺ to ⁴I_11/2 level of Er³⁺ ions gradually increase with increasing P/Al ratio, at the expense of a slight decrease in fluorescence lifetime of Er³⁺ ions at 1.5 µm. Furthermore, the formation of Yb²⁺ ions can be effectively inhibited when P/Al > 1, which leads to the UV transmittance increasing of the glass.

This work demonstrates that the local environment and spectral properties in Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ co-doped silica glass with P/Al ratio more than one is similar to that of Er³⁺/Yb³⁺/P⁵⁺ co-doped silica glass. In Er³⁺/Yb³⁺/P⁵⁺ co-doped fibers, the central dip of refractive index profile and high refractive index of fiber core are the common problems, which may be solved by an optimized Er³⁺/Yb³⁺/Al³⁺/P⁵⁺ co-doping composition.