Effect of Ce doping on radiation resistance of erbium-doped fiber for space laser communication

  Objective   Space laser communication has the advantages of fast transmission speed, large bandwidth and good confidentiality, and is one of the key development directions of future interplanetary communication. Laser communication requires fast enough transmission rate and high enough transmission power, and erbium-doped fiber amplifier with erbium-doped fiber as the core device is widely used as a signal amplifier in the transmitter and receiver of space laser communication. However, erbium-doped fibers are inevitably affected by the irradiation of space particles in space, which can cause a large number of color-centered defects inside the erbium-doped fiber, resulting in a dramatic decrease in the gain capability and slope efficiency of the device, and then affect the smooth implementation of space laser communication missions. Cerium (Ce) doping is considered as an option to suppress the irradiation loss in optical fibers. Ce can be easily doped into SiO₂ glass together with Al, and Ce can suppress the formation of color-centered defects in optical fibers by trapping carriers. Further understanding of the radiation-induced absorption mechanism of Ce doped erbium-doped fibers and enhancing the gain performance of fibers in irradiated environments is essential for the development of space laser communications.

  Methods   Three kinds of erbium-doped optical fibers, namely, high Ce doped(HCe), low Ce doped(LCe) and non-Ce doped(NCe) fibers were prepared by chelate vapor deposition. The fibers were irradiated at a cumulative dose of 100 krad and a dose rate of 6.17 rad/s using a ⁶⁰Co irradiation source at room temperature. The effect of Ce doping on the performance of the erbium-doped fibers under 100 krad gamma irradiation was investigated. The changes of the fiber color center defects were analyzed by absorption coefficient, loss, and Electron Paramagnetic Resonance (EPR) spectra before and after irradiation of the fiber. By testing the fluorescence lifetime and gain coefficient of the fiber, verification of Ce doping enhances the irradiation resistance of erbium-doped fibers.

  Results and Discussions   The fiber loss and absorption spectra were tested and found that the loss values of all three fibers decreased gradually with the increase of wavelength after irradiation, and the loss changes in the range of 900-1600 nm showed the characteristics of short wavelength and high loss, and it was speculated that there might be higher absorption peaks before 900 nm. Through the EPR test, The paramagnetic defects are mainly Al-OHC, Ge(1), Ge(2) and other Ge/Si related defects, and the EPR test verified that the irradiation loss in the operating band of the fiber is mainly due to Al-OHC, and Ce³⁺/Ce⁴⁺ can effectively reduce the number of Al-OHC and Ge(1)/Ge(2) related defects number. Thus making the absorption of radiation-induced color-centered defects suppressed. The fluorescence lifetime and gain performance tests showed that the fluorescence lifetime was reduced by 1.099 ms for the NCe and 0.578 ms for the HCe, and the gain of the HCe was 4.15 dB higher than that of the NCe after irradiation. This is due to the fact that Ce doping reduces the AL-OHC defects, decreases the irradiation loss in the working band of the fiber, makes the pump light of the fiber more absorbed by rare-earth ions rather than by color-center defects, and improves the irradiation resistance of the erbium-doped fiber.

  Conclusions   Ce doping can reduce the number of carriers during fiber irradiation and thus suppress the formation of color-centered defects during fiber irradiation. Three types of erbium-doped fibers containing different ratios of Ce ions were selected to study the radiation damage from both macroscopic gain performance and microstructural changes. The loss spectra and absorption spectra before and after irradiation were tested, and it was assumed that the main cause of irradiation loss was the trailing of the color-centered absorption peak before 900 nm in the infrared band. Through the EPR test, it was found that the irradiation loss of fibers with high Ce content is smaller and less color-centered defects appear. The analysis is due to the opposite change induced by the valence state of Ce^3+/4+ which tends to keep the balance of the ratio of Ce³⁺ and Ce⁴⁺ ions in the glass, The fluorescence lifetime test before and after fiber irradiation shows that the samples with less change in fluorescence lifetime have stronger irradiation resistance, and Ce doping can suppress the shortening of fluorescence lifetime of erbium-doped fibers, which verifies that Ce doping can effectively improve the irradiation resistance of erbium-doped fibers. The gain performance of the fiber before and after irradiation shows that Ce doping can effectively reduce the number of color center defects in the fiber due to irradiation, which can improve the gain performance of the fiber after irradiation. The results of this study can be used as a reference for the subsequent spatially irradiation-resistant reinforcement technology and space applications of erbium-doped fibers.