Tm-doped fiber laser or amplifier can be applied in varied adverse environments. In this work, we demonstrate the pump bleaching of Tm-doped silica fiber with 793nm pump source under gamma-ray irradiation in the range 50Gy-675Gy. The recovery time, the fiber slope efficiency and the fiber cladding absorption spectra after irradiation and bleaching have been measured. It is found that the recovery time and radiation induce absorption are positively associated with doses, however, the fiber slope efficiency of irradiated TDF and bleached TDF are both negatively correlated with doses. Based on the simulation of the fiber core temperature, the probable mechanism of pump bleaching is also discussed.
© 2015 Optical Society of America
The systems based on Thulium doped fiber(TDF) like lasers or amplifiers operating at 2μm are an attractive choice for space exploration and laser communication [1,2 ], and considerable efforts have been devoted to the applications. For example, thulium doped fiber laser(TDFL) can be a suitable laser source used for airborne and space-borne atmosphere measurements in atmospheric lidar sensing applications [3,4 ]. It can also work as a water vapor lidar for remote sensing of water under arid conditions, such as those found on Mars . And the thulium doped fiber amplifier(TDFA) is capable of creating a route to significantly enhanced amplification bandwidths that can be extensively applied in potential future telecommunication and space communication network operating around 2μm . For these purposes, TDF will be exposed and have to survive to the harsh environment. It is clear that rare earth-doped fiber is the most radiation sensitive part of the fiber-based systems, exhibiting high radiation-induced attenuation(RIA),in space-based and adverse radiation environments(like X-rays, gamma-rays or protons) [7–10 ].In this case, the radiation resistance of TDF largely determines the properties of TDFL and TDFA. With TDF-based device being widely applied for military and space exploration, it is urgent to investigate the radiation effects of TDF in adverse operating environments.
In recent decades, amount of efforts have been devoted to the radiation effects on the properties of pure silica fiber and X-doped fiber(Yb, Er, Al, Ge, P…) and there have been many excellent achievements in this field [7–18 ]. It has been found that the X-doped fiber is very sensitive to radiation compared to passive fiber(pure silica). Furthermore, photobleaching  or thermal bleaching  can lead to the recovery of the RIA of the doped fiber. Ge-doped silica core fiber under a 37Gy dose of ionizing radiation at a dose rate of 120Gy/min can be bleached by 850nm light at room temperature . Er-doped fiber exposed to gamma-ray irradiation from 0.1to 10kGy for 18h can be bleached by 980nm light . Silica core fiber under a 30Gy dose of gamma-ray radiation at 15Gy/h can be bleached by 623.8nm laser . The transmission of Er-doped fiber irradiated with 180MeV protons can recover to 70% of the original after annealing at 450 K . Researchers have also developed radiation hardening techniques such as the introduction of Ce ions into the active glass matrix that can improve the radiation resistance [15,16 ],that the coexistence of Ce3+ and Ce4+ in the glass matrix possibly provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses [15,16,19,20 ]. It has also been shown that the preloading with hydrogen  can drastically improve the radiation tolerance of fiber. Up to now, numerous efforts have been put on the radiation efforts and resistance of Yb, Er and Yb/Er doped fiber, however, few researchers focus on characterization of Tm-doped fiber in an active configuration under gamma-ray irradiation
In our previous work, we have investigated the radiation effects on Tm-doped fiber under 500Gy gamma-ray irradiation and found the pump bleaching of TDF: the properties of irradiated TDF, such as slope efficiency and fiber cladding absorption of the visible (VIS) and near infrared (NIR) regions, can recover towards to the pristine TDF obviously with 793nm pump source . As TDFL or TDFA application environments are varied, the doses TDFs receive are also different. Therefore, it is highly essential to investigate the radiation effects on the pump bleaching under multi-dose radiation. In this work, TDFs were gamma-irradiated to the doses of 50Gy, 185Gy, 325Gy, 500Gy and 675Gy, respectively. The radiation doses effects on the pump bleaching of TDFs were investigated. It was found that the recovery time of the irradiated TDF samples increased with doses. And the decrease and increment of the slope efficiency due to irradiation and pump-bleaching, respectively, were also positively associated with doses. The fiber cladding absorption in the range 600nm-1300nm was measured. And the result shows that the irradiated TDF had the highly stronger broad band absorption in VIS and NIR region than the pristine TDF and the RIA increased with doses. It also got recovery towards the pristine TDF after 793 LD pumping for hours. Moreover, the RIA of the irradiated TDF and bleached TDF at around 780nm were both accord with the Power Law . The core temperate of TDF was calculated and the process that the irradiated TDFs were bleached by 793nm pump source was also discussed.
2. Experimental setup
The TDFs were prepared with the traditional modified chemical vapor deposition and the vapor-solution-doping method .The introduction of Al2O3 and Ge in the fiber is to reduce the cluster effect of high-concentration active ions and increase the refractive index of the core. Fibers have a 125μm octagonal-shaped pump core and 10μm laser core, shown in Fig. 1 inset. The 60Co gamma source with radioactivity of 9.99*1015 Bg and energy of 1.33MeV was submerged in water on moveable platform, which was raised into the test cell during the test. Spools of test fibers and Ag2Cr2O7 dosimeters mounted onto the plate were vertically mounted onto a plate facing the source . The total average doses received by the fibers were calculated from the Ag2Cr2O7 dosimeters readings to be 50Gy, 185Gy, 325Gy, 500Gy and 675Gy at an average dose rate of 250Gy/h, respectively. The laser properties of 4m fibers were measure by all-fiber laser system composed of 793nm LD, fiber Bragg grating (FBG) at 2μm, and TDF, shown in Fig. 1. An optical filter was used to filter the residual pump light. The fiber cladding absorption spectra were performed by PK2500. To avoid noticeable radiation-induced absorption relaxation, the experiments were conducted within 24h after finishing the irradiation and at room temperature.
3. Experimental results and discussion
In the experiment, the maximum launched pump power was 23.6 W and the 2μm laser power plus residual pump power of the pristine TDF was 13.1 W, of which 2μm laser power was 12.2 W. After the gamma-ray irradiation of 50Gy, 185Gy, 325Gy, 500Gy and 675Gy, the laser power and residual pump power became 11.8 W, 8.26 W, 6.82 W, 5.6 W and 3.53 W, respectively, of which the 2μm laser power were 11.1 W, 7.90 W, 6.58 W, 5.11W and 3.25 W. Furthermore, if the launched pump power was kept unchanged at the maximal value, the power gradually increased and finally kept constant values of 12.3 W, 10.8 W, 10.2W, 8.95W and 8.35 W, of which the 2μm laser power were 11.4 W, 10.2 W, 9.52 W, 8.45 W and 7.85 W, respectively. It is obvious that all irradiated TDFs were bleached. The relationship between laser power plus residual pump power and time is shown in Fig. 2 . It is well known that the power of 793 nm pump light degraded along the fiber axial direction in TDF. Therefore, the input end could get greater bleaching effect than the output end. To obtain the maximum bleaching, we would change the pump end to another one when the output power of one fiber end tended towards stable. During the time that the launched power increased from zero to 23.6W, the output power was not monitored. As a consequence, the jump points in each cure were generated shown in Fig. 2. It follows from the figure that the time taken for recovery increased with the doses. The longest one was about 1700mins in the case of 675Gy. It was also found that the maximal output power of irradiated TDF and bleached TDF both had a negative correlation with doses.
Figures 3 and 4 show the 2μm output power versus launched power and the doses dependency of the slope efficiency of the irradiated TDFs and bleached TDFs, respectively. It is concluded that the higher dose gamma-ray irradiation resulted in the lower slope efficiency. After the pump bleaching, the efficiencies of bleached TDFs were also accord with the above conclusion. And it follows from the Fig. 4 that the increment of the slope efficiency due to pump bleaching was also positively correlated with the irradiation doses. In the case of low dose irradiation, the irradiated TDF can be almost completely bleached by the 793nm pump source.
The fiber absorption spectra of the pristine TDF, irradiated TDFs, and bleached TDFs were also measured, shown in Fig. 5 . Figure 5 indicates that the gamma-ray irradiation led toa very strong broadband absorption from 600 to ~800nm. It is well known that radiation-induced photodarkening in glass can cause a strong broad absorption band centered in the UV(ultraviolet) and VIS region, and the tail extends into the NIR region . The results of our experiment shown in Fig. 6 are highly consistent with it. It also follows that higher dose could result in the higher additional absorption coefficient. After pump bleaching for hours, the absorption spectra of irradiated TDFs recovered towards to the pristine TDF’s, and the decrease were roughly positively correlated with dose. Figure 6 shows the RIA of the irradiated TDFs and bleached TDFs at 780nm. The dose dependence of RIA in irradiatedTDFs is described sufficiently well by the power law :
It is well known that photobleaching  and thermal bleaching  can lead to the recovery of the RIA in the doped fiber. In our previous work, it was found that gamma-ray irradiation can lead to additional absorption at 793nm , and researchers has also found Ge-doped silica core fiber under ionizing radiation can be bleached by 850nm light at room temperature , Er-doped fiber exposed to gamma-ray irradiation can be bleached by 980nm light  and silica core fiber under gamma-ray radiation can be bleached by 623.8nm laser . Therefore, it is inferred that there is a huge possibility that the color centers in the irradiated fiber could be bleached by 793nm protons directly. It means that there exists photo-bleaching effect in irradiated TDFs when pumped by 793nm diode laser(LD). As is well known, an amount of heat is generated in active fiber when TDFL is at work. To investigate whether there exists thermal bleaching in irradiated TDF when pumped by 793nm LD, the core temperature of fiber must be explored. The heat distribution in TDF is largely consistent with pump power distribution. The distribution of pump power along the fiber can be written:
In TDFL, since the fiber length is much larger than fiber cross-section, the capability of heat dissipation from the fiber end facet is much lower than from the fiber side, therefore the stable temperature distribution is given by the following thermal conductive equation in symmetric cylindrical coordinates:
By considering Eqs. (2)-(4) and according to the boundary conditions, continuity conditions and the Newton’s law of cooling, we conclude that
The parameter values of the pristine TDF case are shown in Table 1 .
The curves of T0 distribution in the pristine TDF along fiber axial direction are shown in Fig. 7 . It is suggested that the input end has the highest core temperature of 375.3K and the core center temperature has been degraded along the fiber axial direction. In , theEr-doped fiber was irradiated with 180MeV protons and the recovery of up to 70% of the original transmission was reached after annealing at 450 K for nearly one day. Therefore, it is inferred that there exist thermal bleaching in the irradiated TDF when the TDFL is active, however, the effect is not supposed to be strong. In conclusion, the pump bleaching is the result of a combination of photo bleaching and thermal bleaching, shown in Fig. 8 . When 793nm pump light is launched into the irradiated TDF, some 793nm photons are absorbed by the color centers directly and then the defects are bleached. The other 793 pump light is absorbed by Tm3+ and 2μm laser are emitted. Therefore, there exist heat generation from the quantum defect, leading to thermal bleaching of the color centers. In our work, as the core temperature of the fiber calculated is not so high that the defects cannot be thermal-bleached significantly, it is inferred that the photo-bleaching may be the more prominent factor, resulting in the recovery of the irradiated TDF. Here, we conclude the bleaching effect in X-doped silica fiber(Er, Tm…) for comparison shown in Table 2 . Next work, we will investigate the quantitative effect of the photo-bleaching and thermal bleaching.
We particularly reported on the pump bleaching of TDF under multi-dose gamma-ray irradiation for the first time. The higher dose results in the more recovery time, the higher RIA and the lower fiber slope efficiency. The 793nm pump source can bleached the irradiated TDF significantly and the extent of the recovery is also positively correlated with dose. From the experiments and calculation, it follows that the photo-bleaching may be more effective to retrieve the fiber performance than the thermal bleaching. In conclude, the pump bleaching with 793nm pump source, as an active recovery effect, can make TDF obtain wide applications in adverse operation circumstances.
This work was financially supported by the National High-Technology Research and Development Program of China (Grant No. 2013AA031501), the National Science Foundation of China (Grant NO. 61378070).
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