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In this paper we present the results on the effect of γ-irradiation on the optical properties of bismuth-doped germanosilicate fibers. It has been revealed that the absorption and emission bands of the bismuth-related active centers are mildly affected by γ-radiation with a dose ranging from 1 to 8 kGy. Experimental data on the radiation-induced absorption (RIA) dynamics of Bi-doped fibers under irradiation and post-irradiation recovery have been obtained. The RIA spectra of the Bi-doped germanosilicate fibers have been measured. The effect of Bi concentration on the RIA level is discussed.
© 2016 Optical Society of America
1. Introduction
Bismuth-doped germanosilicate (with high content of GeO2) fibers possess wideband luminescence and can provide optical amplification in the 1700-nm band [1]. One of the advantages of the Bi-doped germanosilicate fibers in comparison with other optically active media, in particular with Er-doped fibers, is a broad amplification spectrum of more than 50 nm. Recently, CW and mode-locked laser generation in the 1620-1775-nm region has been obtained on this type of fibers [1, 2]. At present, the maximum efficiency of these lasers is ~30% [3]. Among the potential applications of Bi-doped fibers are space amplifiers for intra-satellite communication [4] and superfluorescent light sources for gyroscopes [5, 6]. For such applications the fibers should be radiation-resistant. That is why the study of radiation resistance of the bismuth-doped fibers is of great interest. Moreover, this research has an important fundamental aspect, because new data on the properties of the Bi-related active centers can be obtained with the help of ionizing radiation.
The effect of γ-radiation on the optical properties of Bi-doped aluminosilicate fibers has been addressed in a few papers [7–9]. It has been shown that, depending on the irradiation dose, the level of radiation induced absorption (RIA) as well as the IR-luminescence intensity in the 1100-1400-nm range can be strongly affected. The authors of those papers supposed that the formation of defects in the host glass and bismuth reduction can have an effect on the level of RIA. However, the contribution of each of the factors has not been determined. Radiation-induced effects in Bi-doped fibers with other hosts have not been studied.
Therefore, the aim of this paper was to investigate the effect of γ-irradiation on the optical properties of bismuth-doped germanosilicate (with high GeO2 content) fibers.
2. Experimental
The bismuth-doped fiber preforms were fabricated by the MCVD technique. In the first stage, a porous layer of unsintered material (“white soot”) of required composition was deposited on a fused SiO2 tube (Heraeus). High-purity silicon and germanium chlorides and O2 were used as the raw materials in this stage. In the second stage, the porous layer was sintered into transparent glass in the flow of the products of the high-temperature decomposition reaction of a bismuth precursor. The temperature of the chemical reaction zone was monitored by an IR-pyrometer. Thereafter, the substrate tube with the deposited and sintered layers was collapsed into a solid transparent preform. The refractive index profile in the preforms obtained was measured using a P-102 Preform Analyzer (“York-Technology”, USA). A typical refractive index profile of a bismuth-doped fiber preform containing about 50 mol.% of GeO2 is shown in Fig. 1. Thereafter, a jacketing tube was collapsed upon the preform to obtain the required core/cladding diameter ratio. A single-mode fiber with a core diameter of 2 μm, a cladding diameter of 125 μm, and a cut-off wavelength of 1.1 μm was drawn from the preform.
We studied the optical properties of fibers with various bismuth and GeO2 content (Table 1). 50GeO2-50SiO2 fiber containing no bismuth in the core was also tested for comparison. The total Bi concentration given in Table 1 was determined by means of the electrothermal atomic absorption spectrometry and inductively coupled plasma atomic emission spectroscopy. A high content of GeO2 was made as a required component for the formation of Bi-related active centers (BACs) associated with Ge emitting in the 1700-nm region. Also, germanium oxide was used to provide the appropriate refractive index of core relative to the pure silica cladding in order to ensure light guiding in the fiber. It is worth noting that the amount of BACs can significantly differ from the total Bi content because of the influence of a number of factors, such as the temperature and atmosphere of the glass synthesis, glass composition etc [10]. The BAC concentration in the fibers investigated was estimated as the magnitude of the optical absorption at the wavelength of ~1650 nm. This parameter is also indicated in Table 1.
Table 1. Characteristics of fibers tested
The tested fibers were γ-radiated with a 60Co-source to different doses in the range 1-8 kGy. For this, the fibers were wound in coils, 5 cm in diameter and placed in the 60Co-source room in a calibrated position ensuring uniform irradiation of all the fiber length.
The small-signal absorption spectra, the luminescence spectra and level of unsaturable optical loss at 1550 nm were measured in the pristine and irradiated fibers. Typical lengths of fibers in the experiments were 5 – 10 m. Small-signal absorption spectra were measured using the cut-back technique with a halogen lamp as a probe light source. RIA was determined as the difference of the pre- and post-irradiation loss spectra. In order to investigate the dynamics of RIA, optical transmission spectra were measured at regular time intervals. The level of unsaturable loss was measured at a wavelength of 1550 nm using a semiconductor laser diode instead of the halogen lamp. The output power of the laser diode was up to 30 mW. Radiation of this laser diode was also used as the excitation light in the luminescence measurements. The luminescence spectra were recorded using an optical spectrum analyzer.
3. Results and discussion
Figures 2(a) and 2(b) demonstrate the absorption spectra in the 1200-1700-nm range of the pristine fibers (e. g. Fiber #2 and Fiber #3). As can be seen that in both cases there are two bands peaking at 1400 and 1650 nm, which are due to BACs associated with SiO2 and GeO2, respectively (see [11] for details). It should be noted that only small amount (estimated as ~1%) of total concentration of Bi contributes to the formation of the BACs. Thereby, the other centers associated with Bi exist in glass matrix and their amount could be much higher than that of the BACs. Therefore, apart from the observed absorption bands of the BACs, the change of background loss level caused by the presence of the other forms of bismuth may be observed. The background loss level in this type of fibers is determined by the measurement of unsaturable loss. It is known that the background loss in this type of fibers increases monotonically with the decrease of the wavelength [10]. Although the exact nature of the background loss is still unclear, there is a hypothesis that possible sources of these losses can be some reduced forms of bismuth (metallic, dimeric etc.) [10, 12]. Thereby, γ-radiation could affect the absorption of BACs as well as other forms responsible for the background loss. That is why we measured total absorption and background losses before and after irradiation.
Fig. 2 Absorption spectra of Fiber #2 (a) and Fiber #3 (b) before and after γ-irradiation with a dose of 1 kGy. Dependence of loss at 1550 nm for Fiber #2 versus pump power is shown in inset (a). The unsaturated loss level of Fiber #3 is indicated by points (b).
The absorption spectra of active fibers irradiated with a dose of 1 kGy are also presented in Figs. 2(a) and 2(b). It is seen, that after γ-irradiation the shape of the spectra of the investigated fibers changed insignificantly, whereas the loss levels of the irradiated samples became higher. The same results were obtained for the other Bi-doped fibers shown in Table 1. To determine the change in the background loss, the levels of the unsaturable loss in the irradiated fibers were measured at 1550 nm. Typical dependencies of absorption on the pump power for Fiber #2 are illustrated in inset of Fig. 2(a). It turned out that after the irradiation the unsaturable loss level of all the fibers increased by the same value as the small-signal loss. Therefore, the observed growth of the loss is caused by an induced background loss.
The results on the absorption changes are in agreement with experimental data on luminescence. Typical luminescence spectra of this type of fibers (e.g. for Fiber #2) before and after irradiation under excitation of 1550 nm are shown in Fig. 3. It is seen that the negligible (within the limits of the measurements accuracy) change of the intensity of IR luminescence took place for all the fibers. Analyzing the results of the absorption and luminescence measurements, one can see that the contribution of the BACs to RIA is almost not present. Probably, it is due to the low concentration of BACs in comparison with the other forms of Bi.
Fig. 3 Luminescence spectra (Fiber #2) of pristine and irradiated fibers upon excitation at 1550 nm. Irradiation dose is 1 kGy.
We carried out the same measurements for all the fibers listed in Table 1. Figure 4 illustrates RIA at 1550 nm as a function of GeO2 content. The RIA growth with the increase of the germania and total bismuth concentrations was revealed. It can be seen in Fig. 4 that the germanosilicate fiber containing no bismuth (Fiber #5) has the RIA level 3-8 times lower than the analogous fiber doped with bismuth. Thus, the contribution of bismuth to the RIA level is higher than that of germanium. On the basis of the experimental data, we suggest that RIA is mainly caused by the Bi-related radiation-induced color centers. It should be noted that the formation of Er-related radiation-induced color centers contributing to RIA has already been observed in germanosilicate fibers [13].
Fig. 4 RIA at 1550 nm versus GeO2 concentration in fibers doped with bismuth. All fibers were γ-irradiated to a dose of 1 kGy. The digits indicate the fiber numbers in Table 1.
We also investigated the effect of radiation dose on RIA. We are of interest to study a fiber with high gain efficiency. That is why for this purpose, we tested Fiber #2 because it has gain efficiency higher than the others. It was irradiated by γ-radiation to different doses ranging from 1 to 8 kGy. The measurements of the absorption spectra of the fibers before and after irradiation allowed us to determine the RIA spectra in the range where the absorption bands of BACs are absent (Fig. 5). It is seen that RIA slowly decreases with the increase of wavelength. As expected, the RIA level became higher in the whole region of measurements, from 1000 to 1300 nm, with the increase of irradiation dose.
Fig. 5 RIA spectra of Fiber #2 irradiated to doses of 1-8 kGy. RIA spectrum of Fiber #5 after γ-irradiation with a dose of 1 kGy is also shown for comparison. Dotted lines show interpolation of RIA in the spectral range where there are cut-off wavelengths of tested fiber.
A typical temporal dependence of RIA in Fiber #2 during irradiation (0-1000 s) and subsequent recovery (1000-2700 s) is depicted in Fig. 6. The RIA level monotonically grows with γ-irradiation. It is seen that during the irradiation to a dose of 1 kGy the saturation of RIA was absent. The post-irradiation recovery process depended on wavelength. In particular, the RIA recovered by 20% at the 1500-nm wavelength, while the recovery at 1600 nm was almost absent.
Fig. 6 Kinetics of RIA at 1500 and 1600 nm for Fiber #2 during irradiation (up to 1000 s) and its post-irradiation relaxation (after 1000 s). Dose of irradiation for 1000 s is approximately 1 kGy.
Thus, the results obtained show that bismuth has a considerable contribution to the RIA level: the increase of the bismuth content leads to a growth of RIA. Because efficient Bi-doped fiber lasers and amplifiers have been developed using fibers doped with a low Bi concentration (similar to Fiber #2) (Table 1), it is expected that RIA in these fibers upon irradiation to a dose of 1 kGy will not exceed a level of about 0.1-0.2 dB/m. As a result, no considerable decrease of the efficiency of devices based on such fibers will occur. In the following investigations, the effect of γ-irradiation on the characteristics of bismuth-doped devices (lasers, amplifiers, and superfluorescent sources) will be studied.
4. Conclusion
In summary, the first results of the study of the effect of γ-radiation to a dose of 1 – 8 kGy on the optical properties of bismuth-doped germanosilicate fibers have been presented. The considerable changes in the luminescence intensity at 1700-nm belonging to BACs has not been observed in the fibers irradiated by γ-radiation. The RIA spectra of this type of fibers have been obtained for different irradiation doses. It has been revealed that addition of bismuth has a considerable contribution to the level of RIA in the investigated fibers (an increase of the bismuth concentration leads to an increase of RIA). At the same time, the contribution of the BACs was insignificant. It is probably caused by small amount of the BACs with respect to the total Bi concentration. The experimental data on the dynamics of RIA in the Bi-doped fibers during the irradiation process up to a dose of 1 kGy and post-irradiated recovery have been obtained.
Funding
Russian Science Foundation (RSCF) (Grant No. 16-12-10230).
Acknowledgments
The authors are grateful to A. L. Tomashuk for valuable advice.
References and links
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