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Abstract
We present the deuterium-loading induced radical recovery of 2µm laser fiber and absorption efficiency in thulium (Tm)-doped silica fibers under gamma irradiation in the range of 185Gy-1000Gy. The optical-optical efficiency and fiber cladding absorption spectra of the Tm-doped silica fibers with multi-dose irradiation and deuterium treatment have been measured. It was found that the reduction of the optical-optical efficiency and the additional absorption, both induced by irradiation, were positively associated with the dose. Under the deuterium loading, the efficiencies of the multi-irradiated TDFs could be restored to above 91.9% of the pristine, and the radiation induced absorptions were almost completely vanished. The results indicate that deuterium-loading has an extensive prospect for the radiation hardness of Tm-doped silica fiber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the operating wavelength of ~2μm covering or avoiding the absorption bands, thulium doped silica fiber lasers (TDFL) or amplifiers (TDFA) [1–11] are of great interest for laser surgery [8], atmospheric communication [9], polymer machining [10], and certain applications of radiation environments, such as military defense [7] and remote sensing [11]. TDFL is the suitable laser source of the atmospheric lidar and water vapor lidar for airborne atmosphere measurements and space exploration [11]. Meanwhile, TDFA is capable of creating a route to significantly enhance amplification bandwidths that can be extensively applied in potential future telecommunication and space communication network operating around 2μm [9]. In the above application cases, TDF will be exposed to the radiation. It is well known that the rare earth doped silica fiber is drastically sensitive to radiation, as high-energy ions can induce the point defects of color centers responsible for the significant additional absorption and degradation of laser performance [12–14]. Therefore, it is urgent to investigate the radiation hardening techniques for the rare earth doped fiber for the application in severe conditions.
In the last decades, several effective radiation resistance techniques of X-doped fiber (Yb, Er, Al, Ge, P…) [12–31] have been developed. The introduction of Ce ions into the active fiber could reduce the radiation-induced optical losses in both pure silica fiber and X-doped fiber, that the coexistence of Ce3+ and Ce4+ in the glass matrix possibly provides means for trapping both hole- and electron-related color centers [12–16]. It was also found that radiation-induced attenuation (RIA) of the fiber could be restrained by thermal-bleaching [22] and photobleaching [23–26]. The transmission of Er-doped fiber irradiated with 180MeV protons can recover to 70% of the original after annealing at 450 K [22]. Ge doped silica core fiber under ionizing radiation was able to be bleached by 850nm laser at room temperature [23]. Gamma-ray irradiated Er, Er/Yb or F doped fiber and pure silica core fiber could be bleached by laser at 980nm [17,24,25] and 623.8nm [26], respectively. The gamma-irradiated Tm-doped silica fiber could be restored by pump bleaching [33,34] composing of 793nm photobleaching and thermal-bleaching of heat induced by quantum defect. Furthermore, the fiber radiation tolerance could also be improved by hydrogen loading and deuterium loading [35–37]. The radiation resistance to gamma-radiation of UV silica fiber [35] and Er/Yb-doped fiber [36] can be improved by H2 loading and preloading. However, as the -OH induced by H2 in the glass matrix exhibit a strong absorption effect in the 2µm band [38], H2-loading is no longer suitable for the radiation hardness of the TDF operating at 2µm. In our previous work [36], we have observed the bleaching effects of deuterium loading on the TDF under 300Gy gamma-ray irradiation. It was found that the irradiated TDF exhibit a profound recovery with deuterium treatment: the optical-optical slope efficiency of the irradiated TDF could recover from 25.3% to 58.4%, highly close to the non-irradiated TDF (60.7%). Further, the additional absorption from 600nm to 900nm induced by radiation was almost completely vanished, implying that the absorption spectra of the bleached TDF and the non-irradiated TDF nearly coincided. As the TDFLs or TDFAs will face the harsh radiation environment of various doses during their life cycle, it is highly essential to investigate the multi-dose radiation effects on the passive bleaching of TDF with deuterium-loading.
In this work, TDFs were gamma-irradiated with the doses of 185Gy, 325Gy, 600Gy, 675Gy and 1000Gy, respectively. The passive bleaching effects of deuterium on the multi-irradiated TDFs were investigated. It was found that the radiation induced degradation of the optical-optical efficiency was positively associated with doses, as well as the growth of the efficiency of the irradiated TDFs due to deuterium loading. And the optical-optical efficiencies of the bleached TDFs were all above 52.4%, which is 91.9 percent of the optical-optical efficiency of the non-irradiated TDF (57%). The fiber cladding absorptions in the range 600nm-1000nm were measured. The result shown that the irradiated TDFs had the highly stronger broad band absorption than the non-irradiated TDF in the visible (VIS) and near-infrared (NIR) regions. And the RIAs were also positively related to doses, however, all of they were almost completely suppressed under deuterium loading, even for the 1000Gy-irradietd fiber. The possible mechanisms of the radiation induced attenuation of optical performance and the bleaching effect of deuterium loading were also discussed, respectively.
2. Experimental setup
The thulium-doped silica fibers were fabricated by traditional modified chemical vapor deposition (MCVD) with the vapor-solution co-doping method. The introduction of Al in TDF is to reduce the cluster effect of high-concentration Tm3+ ions and regulate the numerical aperture (NA) between the core and the inner cladding. The refractive index profiles (RIPs) of the non-irradiated TDF preform sample and the 1000Gy-irradiated TDF preform sample are shown in Fig. 1(a)
Fig. 1 (a) Index profiles of TDF preform samples of the non-irradiated and the 1000Gy-irradiated; (b) the doped-ion concentration distributions of Tm2O3 and Al2O3.
The 60Co gamma source with radioactivity of 9.99 × 1015Bg and energy of 1.33 MeV was submerged in water on a moveable platform, which was raised into the test cell during the test. Spools of test fibers were vertically mounted onto a plate facing the irradiation source, and Ag2Cr2O7 dosimeters were mounted onto the plate in proximity to the fiber windings and ends, shown in Fig. 2
. Then the fiber samples were treated with D2-N2(5:95) mixtures for 48 h at 3bars and room temperature. The N2 was used to increase the hybrid gas pressure to accelerate the D2 to interact with the irradiated TDF. And in our previous work [36], N2 has no bleaching effect on the irradiated TDF, and has no influence on the optical performance of TDF.The laser properties of 4.5m fibers were measured by the all-fiber laser system composed of 793nm laser diode (LD), combiner, fiber Bragg grating (FBG) with the reflectivity of 99.9% at 2μm, TDF, cladding power stripper (CPS), and filter, shown in Fig. 3(a)
Fig. 3 (a) Experimental setup to measure the laser property of TDF; (b) experimental setup for fiber absorption measurement.
3. Results and discussions
In the experiments of lase property, the maximum launched pump power at 793nm was 25.3W and the 2μm laser output power of the non-irradiated TDF was 14.48W, shown as line 1 in Fig. 4
. After being irradiated with 185Gy-1000Gy, the laser output power were 9.94W,8.08W, 4.21W,3.8W and 1.43W, respectively, corresponding to lin2-line6 in Fig. 4. Under the deuterium loading, the laser output power recovered to 14.31W, 14.05W, 13.65W, 13.4W and 12.3W, respectively, corresponding to line7-line11 in Fig. 4. It can be seen that the there is still a linear relationship between the launched pump power and laser output power in both irradiated TDF and bleached TDF, like the pristine TDF. From the line2-line6, it is concluded that the higher dose of gamma-ray irradiation led to the lower output power and the 2μm laser output power of the irradiated TDF with deuterium loading were all close to the pristine. The Fig. 5 shows the doses dependency of the optical-optical efficiency of the irradiated TDF and bleached TDF. The optical-optical efficiencies of the non-irradiated TDF and the irradiated TDF with the dose of 185Gy-1000Gy were 57%, 39.2%, 31.84%, 16.59%, 14.97% and 5.63%. From the line1, it also follows that the degradation of the optical-optical efficiency of the irradiated TDF was positively correlated with the irradiation dose. Under the deuterium loading, the optical-optical efficiency of the bleached TDF were 56.38%, 55.36%, 53.78%.52.8% and 52.4%, which are all above 91.9 percent of that of the non-irradiated TDF. From the comparison of line1 and line2 in Fig. 5, it is found that the there was a positive relationship between the increment of the optical-optical efficiency due to the deuterium loading and the irradiation dose. In the case of the TDF with 1000-Gy irradiation, the fiber under deuterium loading had a factor of 9 improvement in the optical-optical efficiency, implying the deuterium loading is an awfully effective method for radiation hardness of TDF.The fiber absorption spectra of the non-irradiated TDF, gamma-irradiated TDF and the deuterium-loaded TDF were measured shown in Fig. 6
and Fig. 7Fig. 7 Comparison of the absorption coefficients of the non-irradiated TDF, irradiated TDF, and bleached TDF.
From the Fig. 1(a), it can be seen that the gamma-ray irradiation has no significant influence on the refractive index profile of the TDF. It is meant that the degradation of the optical-optical efficiency induced by irradiation will not be attributed to the waveguide change of the TDF. Thus, it is easy to deduce that the generation of color centers induced by irradiation in the TDF is the main reason for the efficiency attenuation. The explanation of the gamma-ray irradiation suppressing the 2μm laser emission is shown in Fig. 8
Fig. 8 The explanation of the color centers to suppress the 2μm lase emission: Process 1: Additional absorption of 793nm induced by the color centers. Process 2: Non-radiative transition of 3H4 to 3H6 and the energy is absorbed by the color centers.
As the irradiated TDFs exhibit the strong additional absorption at around 630 nm, it is derived that the gamma-ray radiation was supposed to lead to the generation of the color centers of non-bridging oxygen hole centers (NBOHC), the absorption peak of which is at 630 nm. As the TDFs contain Al3+ ions shown as Fig. 1(b) and the additional absorption tails of certain silica defects may extend to 600 nm region, considerable color centers of Aluminum oxygen hole centers (AlOHC) and peroxy radical (POR) may also be induced by radiation, both of which own the absorption peak at 539 nm [32]. When the irradiated TDFs were under the deuterium loading, the deuterium ions penetrated into the SiO2 matrix would react with the dangling oxygen bond of the color centers to generate the more stable bond of ─O─D, which exhibits no significant additional absorption in the VIS and NIR [38]. Meanwhile, the free electrons would bind with the hole-related defects. Thus, the related color centers were eliminated by the deuterium, shown in Fig. 9
.4. Conclusion
We proposed the radical bleaching effect of the deuterium loading on the multi-dose irradiated TDF for the first time. The higher irradiation dose result in the lower optical-optical efficiency and the stronger additional absorption in the VIS and NIR region. The deuterium-loading could sharply restore the optical-optical efficiency of the irradiated TDF. For the lower dose, the efficiency of the bleached TDF (56.38%) is almost the same as the non-irradiated TDF (57%). Even if under the 1000Gy, the efficiency could also be recovered from 5.63% to above 52.4%. Meanwhile, it also can almost completely suppress the addition absorption induced by irradiation in the range of 600nm-1000nm. The results indicate that deuterium-loading will be a highly effective method for radiation hardness of TDF and mid-infrared fiber.
Funding
China Postdoctoral Science Foundation (2018M632857); National Natural Science Foundation of China (61735007).
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