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Barium gallo-germanate (BGG) glass is an important glass matrix material used for mid-infrared transmission and mid-infrared fiber laser. In this study, we investigated the γ-ray irradiation induced darkening effect of BGG glass. Optical transmittance spectra, electron paramagnetic resonance (EPR) and thermoluminescence (TL) spectra were employed to investigate the γ-ray irradiation induced defects. Two kinds of Ge-related defects in the irradiated BGG glass, named Ge-related non-bridging oxygen hole center (Ge-NBOHC) and Ge-related electron centers (GEC), were verified. In addition, the absorption bands of the two defects have been separated and the peak absorptivity of Ge-NBOHC and GEC defects is at 375 nm and 315 nm, respectively.
© 2016 Optical Society of America
1. Introduction
Fiber laser operating at eye-safe 2.0 μm spectral region has attracted much attention due to its wide applications in coherent laser radar system, light detection, remote sensing, laser surgery and pump source for 3-5 μm infrared (IR) laser [1–4 ]. In recent years, multicomponent germanate glasses have been studied as promising glass matrix materials in mid-IR optical applications and high energy fiber laser systems [5–7 ]. Particularly, one glass system, barium gallo-germanate (BGG) glass, has been practically applied in 2.0 μm fiber lasers due to its excellent combinations of superior IR transparency, good mechanical characteristics, comparatively low photon energy, and high rare-earth solubility [8–10 ]. Comparing to silica or tellurite based active optical fiber, rear earth doped BGG glass optical fiber could provide higher optical gain with shorter fiber length [5]· Owing to their low weight and small volume, the compact mid-IR BGG glass fiber lasers may be used in harsh radiation environments especially in the space environment [11,12 ]. However, for these fiber lasers operating in radiation environment, the radiation-induce darkening (RD) often occurs. This has been recognized as a serious issue for the utility of these laser devices in space because the RD gives rise not only to optical excess loss from ultraviolet to visible even down to near infrared region [13,14 ], but also causes continuous decrease in the output power, efficiency, lifetime and reliability of these fiber lasers system [15–19 ]. Though the Tm3+-doped BGG glass optical fiber laser operates at 2.0 μm spectral region, the pumping wavelength (808 nm or 790 nm) of the laser, may reach RD range [20]. Up to now, the most research about RD phenomenon mainly focused on rare earth or germanium doped silica glass fibers and phosphate glasses fibers [21–33 ], very few reports pay attention to the RD effect of germanate glass fibers [26,29 ], especially no one refers to the RD of BGG glasses or fibers.
In this study, we focused on the γ-ray irradiation induced darkening in the BGG glass. In order to investigate the origin of RD in the BGG glass, the transmission spectrum, EPR spectrum and thermoluminescence spectrum of the BGG glass were measured. The relationship between radiation-induced optical attenuations and color center defects for BGG glass has been clarified.
2. Sample preparation and experiment detail
The BGG host glass were synthesized based on a homemade Tm3+ doped BGG single mode fiber used for 2.0 μm laser as described in [5]. The host glass composition was 15BaO-19Ga2O3-66GeO2 (in mol percentage). The BGG glass was prepared by the conventional melting-quenching technique. The raw materials were high purity reagents (99.99% minimum). Well-mixed raw materials (100 g) were placed in a covered alumina crucible and melted at 1350°C for 60 min in air atmosphere. Then, the melts were quickly poured on preheated stainless steel plates about 300 °C and annealed at 620°C for 2 h to remove thermal strain and allowed to cool slowly in the furnace down to room temperature. Lastly the annealed glasses were cut and polished to a size of 20 × 20 × 2 mm3.
For investigations of the radiation effects, the samples were exposed to Gamma ray using a 60Co source with an average dose rate of 41 Gy/min and total accumulated dose of 50 KGy. To avoid noticeable radiation-induced absorption degradation during the measurements, the spectra of the glasses were measured within one day after γ-ray irradiation.
Optical transmission spectra of these prepared samples before and after γ-ray irradiation were measured by a Perkin-Elmer Lambda 900/UV/Vis/NIR spectrophotometer in the range from 200 to 1200 nm at the room temperature.
Thermoluminescence measurements were performed on a RIS TL/OSL-12 system and the samples were heated from 300 K to 773 K with the heating rate of 2 K/s. TL glow curves were detected by a photomultiplier tube (300–600 nm spectral range).
EPR measurements were 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 1000 to 6000 Gauss. The measurements were carried out with dynamically changing temperature from 100 K to 400 K. For each EPR testing, the same quartz tube and the same total mass of the sample were used.
3. Results and discussion
Figure 1 shows the transmission spectra of BGG glasses before and after a γ-ray irradiation of 50 kGy total dose. The spectra were recorded from 200 to 1200 nm. Because no infrared absorption loss bands were observed in both the irradiated and the unirradiated BGG glass samples, only the region from ultraviolet to visible is represented in Fig. 1. Obviously, the transmittance of the unirridiated sample significantly decreases from 370 to 260 nm, indicating the ultraviolet absorption edge of the BGG glass. For the γ-ray irradiation BGG glass sample, a large optical excess loss band in the wavelength range from 260 to 800 nm can be observed. On the basis of previous reports about germanate glass, the optical excess loss is attributed to forming color center defects induced by γ-ray radiation [23].
Fig. 1 Transmission spectrum for BGG glasses before and after gamma-ray irradiation with the total exposure dose of 50 KGy (sample thickness d = 2 mm).
In order to understand the types of color center defects, continuous-wave EPR measurements were performed and the principle of EPR is presented elsewhere (see, e.g., [33]). The samples used for the experiments were ground into powder and placed in a quartz tube. The test conditions and the mass of each sample for EPR testing were the same. Figure 2 shows the EPR spectra of these irradiated and unirradiated BGG glasses at 100 K. It can be seen that a weak EPR signal or even almost no signal was detected in the unirradiated sample, while two remarkable EPR signals were observed from the irradiated sample, revealing that two kinds of paramagnetic centers (PC) may be induced in the BGG glass by γ-ray irradiation. In the EPR spectrum, the two kinds of PC can be described by a spin Hamiltonian of rhombic symmetry with g-values which are characterized for hole centers (g1>ge = 2.0023) and electron centers (g2<ge = 2.0023) where ge is the free-electron g-value [26].
Ge-related defect centers have been reported early in the Ge-doped silica glasses fibers. Some germanium related paramagnetic defects, containing Ge-NBOHC, GEC, E'(Ge), Ge lone pare center (GLPC)+ and GeO3+, are found [23–27,29 ]. The g values of these defects are listed in Table 1 . Because the g-value of the same defect in different local environments may be slightly shifted [23] and based on the g-values in Table 1, the g-values in Fig. 2 are more close to these of Ge-NBOHC and GEC than others in Table 1. Therefore we can deduce that the stronger signal with 2.0067 g1-value in Fig. 2 could be identified as Ge-NBOHC and the weaker one with 1.9934 g2-value assigned as GEC in the irradiated BGG glass.
Table 1. Empirical g-value of Different Germanium Related Paramagnetic Defects
To investigate the stability of these defects, EPR spectrum at different temperature were measured. Figure 3 shows the EPR spectra of the irradiated sample from 100 K to 400 K. As can be seen from Fig. 3, the intensity of the signal that represents the hole defects is more inclined to disappear compared with electron defects with the increment of temperature from 100 K to 400 K. Therefore it can be inferred that the electron defect is more stable than the hole defect.
To validate types of these Ge-related PC defects further, TL spectrum measurement was adopted. Figure 4 describes the principle of thermoluminescence. In Fig. 4, n and m represent the electron captured by the electron defect trap and the hole captured by the hole defect trap, respectively, while n1 and m1 represent a free hole and a free electron which uncaptured by these defect traps, respectively. When a irradiated sample is heated up to an adequate temperature, there are mainly two kinds of radiation-producing recombination between electrons and holes which escaped from these defect traps: (a) when enough thermal energy is available, the captured electron n can escape into the conduction band and returns to the free hole n1 with emitting a photon in Fig. 4(a); (b) thermal energy causes the trapped hole m to be released into the valence band and then the hole m can recombine with the free electron m1 with emitting a photon [32–35 ] in Fig. 4(b). In this process, activation energy (ΔE – that is energy depth) is required to excite the electron captured in electron defect trap to conduction band or the hole trapped in hole defect trap to valence band, as showed in Fig. 4. Each PC defect has a special ΔE. Moreover, the larger the ΔE, the higher the peak temperature in TL spectrum and the better the stability of the defect. So it is possible that the types and stability of PC defects can be deduced from the number of peaks and the peak temperature in TL spectrum [36].
Figure 5 displays the TL spectrum of the γ-ray irradiation BGG glass heated from 300 K to 773 K. It is obvious that the experimental TL curve of the irradiated BGG glass ascended with increasing the heating temperature beyond 400 K. Two TL peaks centered at 530 K and 720 K can be observed clearly in Fig. 5. Therefore, we can deduce that γ-ray irradiation BGG glass induced two kinds of defects in glass sample. This is identical with the previous EPR measurement result from Fig. 2. To characterize average energy depth of the two kinds of defects further, the TL spectrum of the irradiated BGG glass had been fitted by TL Glow Curve Analyzer [37]. In this method proposed by Chen [38], the general order kinetics equation is used, as following:
where ΔE is the energy depth of defect trap, I(T) is the luminescence intensity at temperature T, T is the heated temperature, S' is the escape frequency factor, n 0 is the initial concentration of trapped electrons prior to heating, b is the kinetic order (1≤b≤2), β is the heating rate, and k is the Boltzmann constant. The two fitted peaks centered at 530 K and 720 K were obtained. The fitting coefficient of determination R2 is 0.99742, very close to 1, indicating that the agreement between the experimental data and the fitted data is good. The obtained results and the kinetics parameters are summarized in Table 2 . The energy depths of the two kinds of defects corresponding to peaks temperature at 530 K and 720 K are 1.71 ev and 2.19 ev, respectively. It is clear that the larger the energy depth of defect, the higher the stability of the defect. Combining the EPR measurement results at different temperature above, it can been inferred that the TL peak centered at 530 K can be assigned to the annihilation of Ge-NBOHC defect, while the other peak centered at 720 K is attributed to the annihilation of GEC. The annihilation process of Ge-NBOHC means that the defects recombine with some free electrons uncaptured by electron traps, while the annihilation of GEC means the recombination between the GEC defects and some free holes to maintain charge balance in the BGG glass [29].Fig. 5 TL spectrum of the irradiated BGG glass which is heated from 300 K to 770 K with the heating rate of 2 K/S. Dashed lines represent the fitted components.
Table 2. Kinetics Parameters of Glow Peaks in TL Glow Curve
Another intriguing question is where the absorption bands of the two kinds of defect locate. In Fig. 1, only one broad absorption band from 260 to 800 nm can be observed because of the overlap of the two defect absorption bands. In order to ascertain the location of RD absorption band of each defect in the BGG glass, isochronal thermal treatment based on the peak temperatures of TL spectrum for the irradiated glasses was adopted. Firstly, the irradiated BGG glasses were incubated for 3 min at 530 K and 720 K respectively, then transmission spectra of these samples were measured. The absorptivity of the samples and the according symbols is as follows:
A530K—The absorptivity of the sample with a heat treatment at 530 K after γ-irradiation
A720K—The absorptivity of the sample with a heat treatment at 720 K after γ-irradiation
ART—The absorptivity of the sample without a heat treatment after γ-irradiation
Aunirradiated sample—The absorptivity of the unirradiated sample
On the basis of TL spectrum and previous reports on these germanium related defects stability [39], the 720 K thermal treatment can eliminate all two color center defects induced by γ-ray radiation, while 530 K thermal treatment can only annihilate Ge-NBOHC defect, i.e., after a heat treatment at 530 K, only GEC defect exists in the γ-irradiated BGG glass. Therefore, it can reasonably be speculated that A720K subtraction from A530K, A530K - A720K, should express the absorptive characteristic of GEC defect. In a similar way, from the difference between ART and A530K (ART - A530K), the separate absorption band of Ge-NBOHC defect can be obtained. Figure 6 shows the absorption bands of Ge-NBOHC and GEC defects induced by γ-ray irradiation in the BGG glass. It is clearly observed that the absorption band of GEC defect (red line, A530K - A720K) centers around 315 nm, and the absorption peak of Ge-NBOHC defect (black line, ART - A530K) is at 375 nm. In Fig. 6, the blue line, A720K - Aunirradiated sample, is almost horizontal, indicating that the sample with a heat treatment at 720 K after γ-irradiation has same absorptivity with the unirradiated sample. This confirms that the 720 K thermal treatment can eliminate all color center defects induced by γ-ray irradiation.
Fig. 6 The absorption band of Ge-NBOHC and GEC defects induced by γ-ray irradiation in the BGG glass.
4. Conclusion
In conclusion, the γ-ray irradiation BGG glass can induce defects which result in darkening effect of the BGG glass. From the experimental results of optical transmittance spectra, EPR and TL measurements for these irradiated and unirradiated BGG glasses, two kinds of Ge-related defects induced by γ-ray irradiation, named Ge-NBOHC and GEC, were verified. The absorption bands of two defects have been separated and the peak absorptivity of Ge-NBOHC and GEC defects is at 375 nm and 315 nm, respectively. This investigation may be helpful in understanding the RD mechanism of the BGG glass and finding the methods to suppress the RD for using the BGG glass optical fiber in the space environment.
Acknowledgments
This research was supported by the High-level Personnel Special Support Program of Guangdong Province (2014TX01C087), the Science and Technology Project of Guangdong (2015B090926010, 2013B090500028 and 2014B050505007), the China State 863 Hi-tech Program (2013AA031502), Fundamental Research Funds for the Central Universities (2015ZP019), National Natural Science Foundation of China (NSFC) (11375278).
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