Monitoring of gamma-irradiated Yb-doped optical fibers through pump induced refractive index changes effect

Abstract

We discuss a radioactivity sensing based on monitoring of color centers formation in Yb-doped fiber under gamma irradiation. New method exploits the dynamic effect of refractive index changes (RIC) induced by laser pumping into the fiber absorption band. In our experiment four identical samples of the single-mode aluminum silicate Yb-doped optical fiber have been γ-irradiated with different doses from a ⁶⁰Co source. All fibers passed the test in the intereferometric setup for the purpose of the pump induced RIC effect. During the test the phase shifts induced in the fiber by 1-mssquare pump pulses at 980 nm were recorded with a probe signal at eleven different wavelengths ranging from ~1.46 to ~1.61 μm. The phase traces have been normalized to their maximum values and averaged over 100 traces for each probe wavelength and also over all probe wavelengths. The averaged phase traces highlight the differences in their growing and decaying parts in respect to the case of non-irradiated fibers. These differences are found to be in correlation with the fiber irradiation dose. For non-irradiated fibers decay parts are perfectly fitted by one exponential function with the relaxation time constant equal to the Yb-ion excited state life-time ~750 μs, to be the same for all fiber samples. However, for irradiated fibers the similar fitting gives a triple exponential decay with time constants estimated as ~750, ~500 and 40μs. For higher irradiation dose the difference with one exponential fitting is more pronounced. Having in mind that the obtained difference in phase shift dynamics could be associated with excitation of some color centers induced in the fiber matrix by gamma irradiation, we represent the normalized phase shifts as a superposition of two contributions. The first contribution is due to excitation of Yb-ion, the same for all fiber samples. The second is due to excitation of color centers. The amplitude of the second part highlights a degree of fiber degradation under gamma radiation and is directly proportional to the concentration of the excited color centers that, in its turn, linearly grows with the irradiation dose. Therefore, new method is regarded as a good candidate for potential applications in fiber dosimeters.