Experimental study on γ ray radiation effect of silica optical fiber near infrared band

Objective With the continuous development of optical fiber production technology, optical fiber sensing technology has been widely used in radiation environments such as outer space and nuclear power plants. However, under the action of radiation sources such as X-ray, γ-ray, electron and neutron, optical fiber will produce radiation induced loss especially in the intense pulse radiation field, the transient induced loss of optical fiber can reach thousands of dB, which has been widely concerned. To carry out the experimental study on the mechanical effect of radiation beam acting on materials, a set of optical fiber displacement interferometry system was established to obtain the dynamic mechanical response parameters of various materials under irradiation. In order to improve the radiation resistance of the interferometry system, it is necessary to carry out the examination experiments of various types of radiation-resistant quartz optical fibers and select the optical fibers with excellent radiation resistance.

Methods Based on the established optical fiber displacement interferometry system, it is necessary to carry out irradiation experiments. Under the conditions of working wavelength of 1 550 nm and injected optical power of 40 mW, the online induced loss values of various radiation-resistant single-mode fibers under steady and transient irradiation are obtained, and there is no relevant data at home and abroad at present. The radiation sources selected in the experiment include ⁶⁰Co radiation source and " Qiangguang-Ⅰ" γ pulse radiation source, respectively with radiation dose rates of 0.5 Gy·s⁻¹ and 5.0×10⁹ Gy·s⁻¹, where the pulse width of the pulse radiation source is about 20 ns. The fiber samples selected in the experiment include Ge-doped radiation-resistant single-mode quartz fiber, F-doped radiation-resistant single-mode quartz fiber and G.652 ordinary single-mode fiber. All optical fiber samples were arranged in the radiation window, and the radiation induced loss of optical fiber in steady and transient irradiation experiments was obtained at the same time to obtain its radiation resistance.

Results and Discussions Under the steady irradiation of ⁶⁰Co with a dose rate of 0.5 Gy·s⁻¹ and a total dose of about 2.5 kGy, the radiation-induced loss of the samples of Ge-doped and F-doped radiation-resistant fiber is less than 17 dB·km⁻¹, and that of the corresponding 10 m optical fiber is less than 0.2 dB. With the increase of irradiation time, the radiation-induced loss is saturated. However, the radiation-induced loss of conventional G.652 single-mode fiber increases with the increase of irradiation time, and the induced loss is more than 40 dB·km⁻¹. Under the action of intense γ pulse with a peak dose rate of 5×10⁹ Gy·s⁻¹ and a pulse width of about 20 ns, two different types of F-doped optical fibers have the best radiation resistance, respectively with the peak radiation-induced loss of 471.5 dB·km⁻¹ and 608.6 dB·km⁻¹, and the recovery time of communication is about μs. The peak radiation-induced loss of conventional G.652 single-mode fiber is 2 378.2 dB·km⁻¹. Ge-doped fiber has the worst radiation resistance, and the peak radiation-induced loss is 2 691.3 dB·km⁻¹, which keeps the induced loss saturated in μs time scale. Ge-doped fiber can be used in steady-state radiation environment, but not in transient radiation environment. The reason for this phenomenon is that Ge-doped fiber is easier to produce defects under irradiation, but the recovery speed of defects in this kind of fiber is relatively fast. Under transient strong radiation, the defects produced by Ge-doped fiber are more obvious, and the phenomenon of "color center" appears for a long time.

Conclusions The experimental results of steady-state and transient irradiation can be summarized as follows: 1) The designed optical fiber irradiation experiment successfully obtained the radiation-induced loss of each optical fiber sample under steady-state and transient irradiation conditions. Especially, the introduction of reference light path not only eliminates the influence of the fluctuation of laser and photoelectric converter on the measurement, but also improves the reliability of the measurement data and plays an important role. 2) The real-time induced loss curves of Ge-doped, F-doped and G.652 single-mode fibers under the same radiation source are successfully obtained at the working wavelength of 1 550 nm, and the response differences of various types of radiation-resistant fibers are compared and analyzed. 3) Compared with others, Ge-doped radiation-resistant single-mode fiber is more sensitive to irradiation dose rate and can be used in steady-state irradiation. F-doped radiation-resistant single-mode fiber has good radiation resistance, which can be used in steady and transient intense irradiation conditions.