1. Introduction

With the development of space exploring and communications, Er-doped fibers can be used in space for optical intersatellite links and other applications, such as amplifiers for highly-distributed data networks and broadband super luminescent sources for fiber gyroscopes [1–3]. The fibers would be withstand a total dose of ~500 – 2000 Gy of protons and electrons in the spacecraft lifetime which is a relatively large dose over 5-10 years in space [4]. However, when exposed to gamma-ray irradiation, rare-earth doped fibers are more sensitive to irradiation than rare-earth free optical fibers [5–8]. As the carriers of the rare earth doped fibers can be excited into trap states and subsequently form defects or color centers in the glass matrix [9], which lead to the radiation-induced absorption (RIA) and radiation-induced luminescence (RIL) [6]. There are many factors affect the radiation induced loss on doped fibers, such as the type of ionizing radiation, total absorbed dose, dose rate, rare-earth dopant concentrations and the fabrication parameters [7,10–13]. There is no doubt that the low radiation resistance of Er-doped fibers calls into questions to their space applications.

Research on the development of RIA of the glasses has been actively continued in the past few years [14–17]. The results show that the RIA arise from color centers generation due to the trapping of holes and electrons at pre-existing or radiation induced defect sites. The decrease in optical transmittance of the glasses with respect to ionizing-radiation dose and dose rate has been described by a power law [9,17]. It is well known that the radiation resistance of the passive and scintillating glasses is improved with Ce co-doping [18,19]. By the reason that Ce³⁺ and Ce⁴⁺ coexist in the glass, Ce³⁺ ions can capture the radiation-induced holes and then inhibit the formation of all other kinds of centers due to trapped holes while Ce⁴⁺ inhibit the formation of all other kinds of centers result from trapped electrons and inhibit recombination of electrons with trapped holes through capture the electrons [18,20]. Some investigations have been performed in the Er/Ce co-doped fibers or glasses and the results show that the telluride fibers have a low noise figure around 1.55 μm band when pumped at 980 nm [21], and the fluorescence characteristics at 1.55 μm of the fluoride host glasses can effectively be improved when pumped at 980 nm [22]. However, few studies have been carried out on the radiation resistance of the Er/Ce co-doped silicate glasses. As we know, Ce³⁺ has a 4f¹ ground electronic configuration with two free ion states separated by 2000 cm⁻¹ and as to Ce⁴⁺ is a 4f⁰ system [23]. The property of the Ce ions has very slight effect on the radiative transition of other rear earth ions that could not increase the non-radiative quenching and energy transfer. Furthermore, Ce ions could easily introduced in silicate glass with aluminum and other rare earth ions.

The aim of this work was to investigate the radiation resistance of the Er/Ce co-doped silicate glasses under 5 kGy gamma-ray irradiation. We demonstrate that the addition of Ce into glass matrix can improve the radiation resistance of Er-doped silica-based glass for the first time. The influence of Ce co-doping on optical properties of Er-doped glasses has also been investigated. The results imply that Er/Ce co-doped silicate glasses can be used for active materials in harsh radiation environments.

2. Experiments

In this experiment, the glasses with composition in mol% was 65% SiO₂, 10% Al₂O₃ and 25% CaO (SAC) were prepared by a conventional melting method under atmosphere. The doped samples were prepared with 0.5 mol% Er₂O₃ and different CeO₂ concentrations (0, 0.2, 0.5, 0.7, 1.5, and 2 mol%). The glasses were prepared by mixing appropriate quantities of analytical grade in a sintered alumina crucible and melting the mixture for 2h at 1580°C in a normal atmosphere. The melts were poured on the pre-heated steel mold and then annealed for 2h at 550°C to obtain thermal and structural stability.

Then the sample glasses were cut and polished into with dimensions of 15 × 15 × 2 mm³ for subsequent measurements. The absorption spectra before and after irradiation in the ultra-violet and visible wavelength ranges were performed by using a Perkin Elmer Lambda 35 UV-Vis spectrophotometer. The fluorescence spectra in the near infrared region and the up-conversion spectra in the visible range were measured on a ZOLIX SBP300 spectrophotometer under an 976 nm LD excitation. Photoluminescence decay measurements were carried out by exciting the samples with a modulated 980 nm LD, and monitoring 1535 nm emission with an InGaAs photodetector in TRIAX550 spectrofluorometer. The fluorescence data was processed by a Tektronix oscilloscope to determine the lifetime. In order to investigate the radiation resistance of the silicate glasses, the samples were exposed to gamma rays from a ⁶⁰Co γ-source under normal atmosphere up to total doses of 5 kGy at a dose rate of 250 Gy/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. All of measurements were carried out at room temperature.

3. Results and discussions

The initial and 5 kGy irradiated glass samples are shown in Fig. 1 . It is known that Ce ions exhibit both trivalent and tetravalent oxidation state (Ce³⁺ and Ce⁴⁺) in oxide glasses under normal conditions of melting. Ce³⁺ in oxide glasses has very little, if any, absorption in the visible region, therefore, the color of Ce-containing glasses origins from Ce⁴⁺ due to the tail of strong charge transfer absorption band in the near ultraviolet region [23]. From Fig. 1 we know that the color of the single Er-doped glass changes from light pink to brown owning to irradiation while no obvious color change is observed in the Er/Ce co-doped glass. This can be confirmed from Fig. 2 , the change in the optical absorption coefficient spectra of corresponding sample glasses upon irradiation.

 

Fig. 1 The photography of the Er-0.5 mol% Ce co-doped glasses (a, pristine; b, irradiated), and the single Er-doped glasses (c, pristine; d, irradiated).

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Fig. 2 UV–visible absorption spectra of host, single Er-doped, and Er/Ce co-doped glasses (a) before irradiation (b) after irradiation. Inset: absorption spectra at 980 nm.

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The distinctive feature of the Fig. 2 was a red shift of the UV cut-off positions, which corresponded with the increasing concentration of CeO₂. From Figs. 2(a) and 2(b), we can see that the absorption coefficient of the host material and single Er-doped glass in the visible range have a more substantial increase after irradiation while the change of the Er/Ce co-doped glasses is small. The absorption peaks have little excursion after irradiation which indicates that the microscopic environment of the Er³⁺ ions is not much affected by the gamma radiation [1]. In Er/Ce co-doped glasses, a strong broad absorption band is observed in the UV region and that extends to the visible region (~400 nm). This broad absorption band is assumed to be a superposition of both charge transfer transitions of Ce⁴⁺, i.e., Ce⁴⁺ + O^2-→Ce³⁺ + O^-, where O is an oxygen ligand, and 4f¹→4f5d¹ transitions of Ce³⁺. Since the f – f related transition of Ce³⁺ is located in the far-infrared range (~5 μm) where the silicate glass is opaque. The ratio of the two valence states being dependent on the composition of the glass, the temperature of melting [23] so that the amount of Ce ions in each valence state is very difficult to estimate. As shown in the inset of Fig. 2 the absorption peak around 980 nm has little shift after irradiation, and the absorption coefficient of the single Er-doped sample witnesses an increase about 0.75 after irradiation while the absorption coefficient does not change in Er/Ce co-doped glasses. The strong absorption in the UV region (<300 nm) is considered to be the absorption edge of the host material.

The RIA and up-conversion are important characteristic parameters for evaluating the performance of radiation resistance materials. The RIA can be obtained by digitally subtracting the absorption coefficient spectrum of the un-irradiated glass from that measured after each exposure. The RIA spectra of the host material and the co-doped glasses with the concentration of CeO₂ was 0, 0.2, 0.5 mol% are shown in Fig. 3(a) ; Fig. 3(b) shows the up-conversion spectra with the CeO₂ concentration was 0, 1.0 mol% in the visible range. As can be seen, the free CeO₂ silicate glass form a strong broad absorption band around 400 nm and extends to the near infrared region while there is slight change in the Er/Ce co-doped glasses. From the spectra of the up-conversion in Fig. 3(b) we know that the intensity of Er doped glasses obviously minished after irradiation while the intensity of Er/Ce co-doped glasses with no change. It is suggested that co-doped CeO₂ could improve the radiation resistance of the sample via inhibiting the formation of color centers in silicate glasses under gamma radiation. Therefore compared with the glass free of CeO₂, the induced absorption bands of the Er/Ce co-doped glass extending from the UV into the visible insight range are strongly suppressed. The Ce³⁺ and Ce⁴⁺ could coexist in the silicate glass, and Ce ions could effectively trap ionization charge induced by radiation due to its ability to change the charge state between Ce⁴⁺ and Ce³⁺: Ce⁴⁺ + e^-→Ce³⁺ or Ce³⁺ + h⁺→Ce⁴⁺, thus diminishing the potential for gamma-induced color-center formation. In all the glasses and solutions, Ce⁴⁺ produces a very strong absorption around 250 nm [23]. The 4f-5d transitions of Ce³⁺ make a strong absorption between 320 ~360 nm. The absorption coefficient of the Er/Ce co-doped glasses have a tendency of increase around 400 nm, we speculate that is related to the valence change of the Ce ions during the radiation.

 

Fig. 3 (a) The radiation induced absorption (RIA) spectra and (b) up-conversion spectra of the typical glass samples.

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We also measured the emission properties of the sample glasses before and after irradiation, as shown in Fig. 4 . Figure 4(a) shows the fluorescence spectra of typical non- and 5 kGy irradiated samples under 980 nm excitation, and the peak wavelength near 1535 nm. All spectra have been divided by the peak wavelength intensity of un-irradiated Er-doped glass for comparison. Figure 4(b) shows the intensity of Er/Ce co-doped glasses as a function of CeO₂ concentrations at a wavelength of 1535 nm. As can be seen, the FWHM (full width at half maximum) and the peak wavelength have slightly change while the luminescence intensity of the glasses decreased obviously with increasing CeO₂ concentration and only half when the CeO₂ concentration up to 1.5 mol%. The intensity of the Er/Ce co-doped glass has barely changed after irradiation, as compared with single Er-doped glass. This also means that the radiation resistance of the Er-doped glass can be greatly improved with CeO₂ co-doping.

 

Fig. 4 (a) Fluorescence spectra of single Er-doped, and Er/Ce co-doped glasses, pristine (solid curve) and 5 kGy irradiated (dashed curve); (b) luminescence (1535 nm) intensity of Er-doped glasses as a function of Ce concentration.

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The decay time of the photoluminescence (PL) intensity at 1535 nm versus CeO2 concentration (Ce = 0,0.2,1.5 mol%) before irradiation and that of Ce co-doped sample (0.2 mol%) in non- and 5 kGy-irradiated glass are shown in Fig. 5 . All the curves of the samples were approximated by the exponential decay, and in Fig. 5(a) the lifetime of the samples were 6.5, 6.4, and 3.4 ms, respectively. That is to say low concentration of CeO₂ has little effect on the fluorescence lifetime of the upper level in the Er³⁺, while the fluorescence lifetime of Er³⁺ decreased significantly in the higher CeO₂ concentration samples. This is opposite with previous studies. Many investigations have shown that Ce³⁺ co-doping effectively improve the 1.55 µm fluorescence characteristics of Er³⁺-doped fluoride glasses or tellurite glasses with 980 nm excitation [22,24]. The resonant energy transfer between Er³⁺ in the ⁴I_11/2 state and Ce³⁺ in ⁴F_7/2 state results in the improvement in the ⁴I_11/2→⁴I_13/2 transition of Er³⁺, thus reduce excited state absorption (ESA) without shorting the lifetime of the ⁴I_13/2 state, which could improve the fluorescence quantum yield and the characteristics of gain significantly. We think that the contrary may be result from the difference of the host material and may be more likely to the reason that most of Ce ions is in the form of Ce⁴⁺ in the samples. There is a overlap between the charge transfer absorption band of the Ce⁴⁺ and the absorption band of the Er³⁺ upper level, part of the pump power can be transferred to Ce⁴⁺ via the ESA of the Er³⁺, which immediately reduced the intensity of the glasses. The lifetime of the samples in Fig. 5(b) were 6.4 and 6.3 ms, respectively. Co-doped with Ce could improve the radiation resistance of the silicate glasses when compared with the lifetime of the single Er-doped is 4.1 ms (not given in the figure) after irradiation.

 

Fig. 5 Decay curve of Er ions transition (a) doped with different Ce concentrations (b) doped Ce in non- and 5 kGy-irradiated glass.

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4. Conclusion

We have demonstrated that Ce ions co-doping by some amount could effectively improve the radiation resistance of the Er-doped silicate glasses. The radiation induced loss of Er/Ce co-doped glass diminished significantly as compared with single Er-doped glass. However, taking into account the influence of Ce ions on the luminescent characteristics of Er³⁺ around 1.55 µm, an optimal concentration of Ce should be exist. We can reduce the negative impact of doped Ce to acceptable levels by controlling the ratio of Ce³⁺ and Ce⁴⁺ via adjusting the redox atmosphere.