1. Introduction

Multicomponent phosphate glasses are promising materials for photonics because of their high solubility of rare earth ions without quenching [1]. This enables appropriately doped phosphate glass single mode fibers to provide higher optical gain with shorter fiber length compared to silica single mode fibers. For example, a net gain per unit length of up to 5.2 dB/cm at 1535 nm and 5.7 dB/cm at 1064 nm obtained from a 2-cm-length Er³⁺/Yb³⁺-codoped and a 0.8-cm-length Yb³⁺-doped phosphate glass fiber has been demonstrated, respectively [2, 3 ]. These single frequency fiber lasers from higher gain phosphate glass fibers are attractive for some applications in astrospace, such as high resolution sensing, free-space optical communication, and LIDAR [4–6 ].

However, when the compact single frequency fiber lasers operate in high output power, high power density will be generated in the fiber core due to the thin single mode core and the short resonance cavity configuration. As power output levels continue to increase, a degradation phenomenon - commonly referred to as photodarkening (PD) - often occurs. This will lead to a continuous decrease in the output of a fiber laser system, which may not stabilize over time [7]. Besides, the single frequency fiber lasers will be exposed to high energy radiation when used in a space environment. It has been proven that the irradiated phosphate glasses demonstrate a decrease in ultra-violet and visible transparency, even in near-infrared region, i.e. radiation-induced darkening (RD) [8], which restricts the use of these devices in space.

The PD and RD problems in rare-earth-doped silicate glass fibers have been extensively studied [9–16 ]. A comprehensive understanding of the radiation-induced darkening has not been reached, especially, in the phosphate glasses fibers which are widely being utilized in single frequency fiber lasers and relatively less investigated. The phosphate fibers, even with higher doping concentrations of Yb³⁺, show greatly reduced PD loss compared to silica fiber when pumped at 980 nm [17]. A color center absorption band with a peak near 467 nm in Er³⁺/Yb³⁺-codoped and undoped phosphate fibers after 193 nm irradiation has been observed. In addition, both photo- and thermal-bleaching techniques have been proposed to remove the PD in phosphate glass fibers [18]. But, it is not advisable for space applications because the additional devices used for generating photons or heat increase the complexity of the system,the weight of a space instrument and the cost of power. Therefore, it remains crucial to understand the mechanisms of radiation-induced absorption and to improve the radiation resistance of the multicomponent phosphate glass fibers in harsh environment.

In this study, we focus on the reduction of radiation-induced defects of a multicomponent phosphate glass fiber. The multicomponent phosphate glasses used for higher optical gain single mode fiber were chosen as subjects for the investigation. Considering that Nb and Sb don’t affect the optical properties of phosphate glass fiber, we analyzed the effects of Nb⁵⁺ and Sb³⁺ on the radiation-induced defects of phosphate glass. We also identify the optical absorption centers by means of band separation of the optical spectra and reveal interactions between Nb/Sb ions and paramagnetic defect centers.

2. Experimental

The phosphate glass samples used in the present study were synthesized based on the Er³⁺/Yb³⁺ co-doped single mode phosphate glass fiber composition used for single frequency fiber lasers [2]. The basic host glass was of the nominal composition P₂O₅ 60%, K₂O 17%, BaO 14% and Al₂O₃ 9% (mol %). In order to investigate the respective role of Nb and Sb in the darkening of such fibers under gamma irradiation, the set of samples has been designed with a variable amount of Nb and Sb, which were added in the form of Nb₂O₅ and Sb₂O₃ to the basic host glass. The compositions of the studied glasses are given in Table 1 .

Table 1. Chemical composition of the studied glasses

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All samples were prepared by conventional melt quenching technique using powders of high purity reagents (99.999%) under atmosphere. The weighed batches were thoroughly mixed in an agate mortar and the homogeneous mixture was transferred into a platinum crucible and melted at 1100 °C in an electric furnace for 1 h. Then the melt was air quenched by pouring it onto a preheated stainless steel molds. After pouring, the glass samples were annealed at 365 °C for 1 h to remove thermal strain and allowed to cool slowly inside the furnace down to room temperature.

For investigations of the radiation effects, the polished samples with the dimensions 2 × 10 × 10 mm were exposed to gamma rays from a ⁶⁰Co γ-source at room temperature up to total accumulated doses of 5, 10 and 20 kGy, respectively, at a dose rate of 2 kGy/h. To avoid noticeable radiation-induced absorption relaxation during the measurements, the spectra of the glasses were measured within one day after finishing the irradiation.

Ultraviolet and visible optical absorption spectra of highly polished samples of equal thickness (2 mm ± 0.05 mm) were recorded in the range 200 – 1000 nm before and after γ-ray irradiation using a double beam spectrophotometer (Perkin Elmer Lambda 900 UV/Vis/NIR spectrophotometer) at room temperature.

Paramagnetic centers in glasses before and after gamma irradiation were detected at 100 K by electron paramagnetic resonance performed on a Bruker A300 spectrometer operating at X-band frequencies (υ~9.44 GHz) with 100 kHz magnetic field modulation, 1 G amplitude modulation and 0.2 mW microwave power. The magnetic field was scanned from 480 to 6480 Gauss. For each EPR testing, the same quartz tube and the same total mass of the sample were used.

3. Results

The optical absorption spectra of the phosphate glasses having different composition (Table 1) before gamma irradiation at room temperature are shown in Fig. 1 . The spectra were recorded from 200 nm to 1000 nm. Because no absorption bands were observed in the visible region, only the ultraviolet region is represented. It also shows that Nb or Sb don’t cause new absorption bands in the multicomponent phosphate glasses.

 

Fig. 1 UV absorption spectra of the phosphate glasses before gamma irradiation: (a) host and Nb₂O₅ doped glasses; (b) host and Sb₂O₃ doped glasses.

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Figure 1(a) displays the UV absorption spectrum of the basic host glass and four samples containing increasing Nb₂O₅ contents. It is obvious that with increasing the Nb₂O₅ content, the end of the UV absorption is continuously shifted from 320 nm to 330 nm. Figure 1(b) illustrates the UV absorption spectrum of the basic host glass and four samples containing increasing Sb₂O₃ contents. The important feature was a red shift of the absorption band edge from 275 nm to 281 nm, which corresponds with the increasing concentration of Sb₂O₃. The UV cut off shift to longer wavelengths with increasing dopant concentration is caused by the charge transfer (CT) transitions of the dopants. In our experiments, it is the d → d transitions of Nb⁵⁺ and s → p transitions of Sb³⁺, respectively [19, 20 ].

To study the radiation damage as a function of the dose, we chose the irradiation doses at 5kGy, 10kGy and 20kGy, respectively. Figure 2 shows that the gamma irradiation results in a broad intense absorption band from 330 nm to 700 nm in the host glass. The band comes from the formation of color centers [21]. The color centers will be identified later by EPR measurements. It is noted that the intensity of the band, in the investigated dose range, continuously grows with the irradiation dose.

 

Fig. 2 UV–visible absorption spectra of host glass before and after gamma irradiation of 5 kGy, 10 kGy and 20 kGy.

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To have a qualitative comparison of the gamma ray irradiation effects on glasses with different dopants, the optical absorption spectra of the host and doped samples after the same gamma ray dose irradiation are shown in Fig. 3 . Figure 3(a) reveals the absorption spectra of (0.5, 1, 1.5 and 2%) Nb₂O₅-doped glasses after 20kGy gamma irradiation. It is obvious that the presence of Nb₂O₅ causes a shielding towards the effect of gamma irradiation and the shielding or retardation effect becomes moresignificant with the increase of Nb₂O₅ content. Figure 3(b) displays the absorption spectra for (0.25, 0.5, 0.75 and 1%) Sb₂O₃-doped glasses after 20 kGy gamma irradiation, and a significant mitigation effect has been observed. Even when the concentration of Sb₂O₃ is less than the Nb₂O₅, the former mitigation effect is better than the latter.

 

Fig. 3 UV–visible absorption spectra of (a) 0.5, 1, 1.5 and 2% Nb₂O₅; (b) 0.25, 0.5, 0.75 and 1% Sb₂O₃ doped glasses after 20 kGy gamma irradiation.

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In order to identify the color centers generated by γ-irradiation and responsible of the induced absorption, we conducted continuous-wave EPR measurements. The samples used for this experiment were ground into powder and placed in a quartz tube. The masses of different doped samples for EPR testing are the same. It is appropriate to emphasize here that the there was no detectable EPR signal prior to irradiation, and the EPR signal appeared only after γ-ray irradiation.

It can be seen from Fig. 4 that the samples doped with different dopants exhibit different EPR spectra. Figure 4(a) shows the EPR spectrum of the irradiated host glass with two main EPR signals. We identify the two EPR signals as A and B. The central part of the EPR spectra near 3400 Gauss is signal A, and the two weaker signals in the wings of EPR spectra are signal B. Figure 4(b) reveals the EPR spectrum of the 1% Nb₂O₅ doped sample and only signal A is observed. In other words, the presence of niobium prevents the emergence of signal B, i.e., inhibits the formation of other centers. The reasons will be discussed in detail later. Figure 4(c) illustrates the EPR spectrum of the 0.25% Sb₂O₃ doped glass. It can be observed that both signals A and B are somewhat weaker compared with that in Fig. 4(a). These EPR signals attenuated with the increases of Sb₂O₃ doped concentration. When Sb₂O₃ is up to 1%, no EPR signals can be seen shown in Fig. 4(d), which means that the presence of antimony also has serious implications on the formation of the centers. So we may conclude that different EPR signals correspond to the different radiation-induced absorptions and the dopants play different roles in radiation-induced decay process of the multicomponent phosphate glass fiber.

 

Fig. 4 EPR spectra of the samples after 20 kGy gamma irradiation: (a) host glass. (b) 1% Nb₂O₅. (c) 0.25% Sb₂O₃. (d) 1% Sb₂O₃.

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4. Discussion

The structure of phosphate glasses can be described as a network of [PO₄] tetrahedra that are classified using Qⁱ terminology [22], where i represents the number of bridging oxygen atoms per tetrahedra. Based on the composition of the oxygen to phosphorus ratio, phosphate glasses are classified as ultra-phosphates ([O]:[P] < 3), meta-phosphates ([O]:[P] = 3), and polyphosphates ([O]:[P] > 3.0). It is accepted that the structure of ultra-phosphates, meta-phosphates, and polyphosphates are dominated by Q³ & Q², Q², and Q² & Q¹ units, respectively. For our host glass composition, with a nominal [O]/[P] ratio of 2.98, long meta-phosphate chains dominated by Q² tetrahedra are expected.

Several color center species are believed to result from exposure to high-energy electromagnetic radiation (UV, X-rays, gamma rays). Most of the current understanding of P-related defects derives from EPR experiments on irradiated phosphorus containing glasses. EPR unambiguously identifies four main P-related paramagnetic point defects, referred to PO₂, PO₃, PO₄, and phosphorus oxygen hole center (POHC) [23]. The POHC is characterized by an unpaired electron shared by two nonbridging oxygen atoms bonded to the same phosphorus. Because of the small effect of nuclear magnetic moment and large spin-spin interaction, the POHC splitting is small and the resonances are narrow. Other defects, such as PO₂, PO₃ and PO₄ complexes which consist of unpaired electron localized on the central P atom are generally called phosphorus-oxygen electron centers (POECs) [24]. In contrast to POHC, the POEC signal is broad with a much larger hyperfine splitting due to the large effect of nuclear magnetic moment and small spin-spin interaction. In our particular case, signal A is usually associated with POHC. Meanwhile, the weaker signal B is commonly related to POECs.

To single out the different radiation induced attenuation (RIA) components, the acquired absorption spectra of the host glass after 20 kGy gamma irradiation has been fitted by Gaussian curves, as shown in Fig. 5 . The absorption band can be fitted by three Gaussian curves, centered at 520 nm, 404 nm, and 267 nm and with FWHM of 151 nm, 111 nm, and 194 nm respectively. The coefficient of determination R² is 0.999664, and the fitting standard error is 0.012. R² is very close to 1, which indicates that the agreement between the experimental data (gray solid line) and the fitted data (red solid line) is good. The comparison of present data with the results by previous workers [25, 26 ] allows us to identify the 520 nm and 404 nm absorption bands as belonging to POHC, while the 267 nm band may be tentatively associated with POECs, respectively.

 

Fig. 5 Decomposition with Gaussian absorption bands of the RIA of host glass after 20 kGy gamma irradiation. Dashed lines represent the Gaussian components.

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From Fig. 4, it is noted that the influences of dopants on γ-ray induced RD of phosphate glass is different. Only the POHC related EPR signal is observed in the Nb-doped samples. The presence of Sb significantly reduces the EPR signal intensity.

The above results can be explained by using a model that accounts for the chemical reactions occurring in the glass under γ-ray irradiation. The absorbed γ-ray ejects a primary projectile electron, from an inner shell, that ionizes the medium and generates electrons and holes, which can be captured at pre-existing traps (precursors). Due to the photo generation process, a hole capturing reaction typically takes place in the vicinity of an electron capturing reaction. There are at least two primary reactions which consume γ-ray generated electrons and holes in phosphate glasses [27]

We assume that Nb⁵⁺ ions act as electron-trapping centers in γ-ray irradiated Nb₂O₅ doped samples. Some of the electrons are captured by Nb⁵⁺ ions (which then become Nb⁴⁺) while the corresponding holes must be captured by some hole-trapping centers. So, we can write a chemical reaction of the form

Equations (2) and (3) clearly show that Nb⁵⁺ ions and some of PO precursors compete for electrons in the vicinity of reaction defined by Eq. (1), which captures holes. Besides, the d1-ion Nb⁴⁺ could form Nb⁴⁺-O-Nb⁴⁺ binuclear or Nb⁴⁺-O^- complex, and then became Nb³⁺-O-Nb⁵⁺ [28–30 ]. These processes will also trap holes, and further capture electronics. Therefore, it is reasonable to believe that POECs formation in the glass would be completely suppressed and POHC formation were partially inhibited by increasing the Nb⁵⁺ doping concentration. This will explain why POECs signal is not observed and RIA decreased in Nb₂O₅ doped glass after γ-ray irradiated (Fig. 3(a) and 4(b) ).

In contrast, for the Sb₂O₃ doped glasses, Sb³⁺ serve as hole-trapping centers. The holes created by high energy γ-rays are captured by Sb³⁺ ions which then form Sb⁴⁺. Applying the same rationale to the effect of γ-ray irradiation, we can write another chemical reaction

Similarly, the pair of reactions defined by Eqs. (1) and (4) obviously compete for holes. As a result, Eq. (4) can effectively suppress the POHC creation. In addition, like Nb4 + , the Sb4 + ions are unstable and will be converted to Sb⁵⁺. The Sb⁵⁺ ions could absorb electrons that may be captured by the nearest neighboring POECs precursors according to Eq. (2) to inhibit POEC formation. Owing to the Sb³⁺ ⇄ Sb⁵⁺ interconversion, antimony ions function as the centers to annihilate both holes and electrons generated by γ-ray irradiation in the vicinity of antimony ions. This restrains POHC and POEC creation in the multicomponent phosphate glasses. When the concentration of Sb₂O₃ reaches 1 mol %, the reactions described in Eqs. (1) and (2) almost do not occur, as shown in Fig. 4(d).

5. Conclusions

The presence of Nb⁵⁺ or Sb³⁺ dopants significantly reduced the radiation-induced defects of multicomponent phosphate glasses. The RIA decreases with increasing Nb⁵⁺ or Sb³⁺ doping concentration. Only the POHC related EPR signals are observed in the Nb⁵⁺-doped samples, while both POHC and POEC defect centers related EPR signals decrease with increasing Sb doping concentration. Owing to the Sb³⁺ ⇄ Sb⁵⁺ interconversion, antimony ions function as the centers to annihilate both holes and electrons generated by γ-ray irradiation in the vicinity of antimony ions. When the concentration of Sb₂O₃ reaches 1 mol %, the POHC and POEC formation is completely inhibited in the multicomponent phosphate glasses. Such studies may be helpful in understanding the basic physical mechanism of RIA and finding out the method of suppressing RIA in multicomponent phosphate glass for the application of single frequency fiber lasers in space.