Performance Degeneration And Mechanism Of Silica Polarization-maintaining Optical Fibers Under Irradiation

The fiber optical gyroscope has wide applications in space technology asaccurate positioning and navigation device. Its normal operation will be influencedby the space radiation environment. The major reason for that is due to theperformance degradation of its optic fiber under space radiation. The variation ofirradiation-induced loss was thus systematically investigated under irradiation of-ray different dose rates, electrons and protons with different energies for twodomestic silica polarization-maintaining optical fibers of "Panda" type and"Capsule" type. The mechanism for formation and evolution of irradiation-induceddefects was also studied to reveal the nature of the increase in irradiation-inducedloss of silica polarization-maintaining optical fiber. Mathematical model was thenestablished for describing the irradiation-induced loss of silica fiber. The resultsprovide data support and theoretical reference in order to improve the spaceenvironment applicability and the on-orbit operation reliability of the domesticpolarization-maintaining optical fibers.The test results show that both the irradiation-induced loss of twopolarization-maintaining silica optical fibers is increased under certain energy range.Only the charged particles that reach optical fiber core show effect on the opticaltransmission of fiber. The minimum energy threshold is100keV for electrons and2.5MeV for protons. The irradiation-induced loss of the polarization maintainingsilica optical fibers shows a natural exponential increase with irradiation dose (orfluence), which rises up sharply at beginning, then slowly and finally reaches tosaturate state. The irradiation-induced losses are depending on many factors such asthe type, energy, and dose rate (or flux) of irradiation, the light conditions, and thetype of optical fiber etc. The irradiation-induce optical loss is greater under higherdose rates or without light than that under lower dose rates or with light. Under thesame irradiation conditions, the "Panda" type silica polarization-maintainingexhibits better radiation resistance in contrast with the "Capsule" type silicapolarization-maintaining, which shows significantly lower irradiation-induced lossthan that of "Capsule" type silica polarization-maintaining.The main reason for the optical loss at1310nm is due to the absorption by theSi-OH defect which forms induced by irradiation in the fiber core. Based on the testdata and by aid of Electron Paramagnetic Resonance (EPR), X-ray PhotoelectronSpectroscopy (XPS), Broadband Optical Spectrum Analysis (BOSA) and FourierTransform Infrared Spectrometer (FTIR), the formation mechanism of the irradiation-induced defects was analyzed. It shows that the formation of Si-OHdefect is related with two processes in the optical fiber core. Firstly, the incidentcharged particles break the Si-O-Si bonds of silica, forming the non-bridgingoxygen defect of Si-O-. Secondly, there is irradiation-induced degradation reactionoccurring in PMMA of clad, producing H and C=C. The hydrogen atom will moveinto fiber core through diffusion mechanism. The hydrogen reacts with Si-O-whenmeets it and forms Si-OH structure. The proton irradiation provides hydrogen thatthe above reaction needs, which promotes formation of Si-OH to a certain extent.Since a certain amount of F doped in the silica fiber core and clad of the "Panda"type optical fiber, they react with Si-OH and forms stable HF and SiO2, decreasingconcentration of Si-OH defects. In this way, the irradiation-induced loss of the"Panda" type optical fiber is far lower than that of the "Capsule" type.The variation of irradiation-induced loss of silica optical fiber depends on theaccumulation of Si-OH defects in the fiber core, of which is related to thedistribution of the absorbed dose in the fiber. The calculation by the SRIM and theCASINO software shows that, the energy loss of the penetrating charged particlesvaries with the incident energy resulting in difference of absorbed dose distribution,of which influences the formation and accumulation of the Si-OH defects in theoptical fiber core.Based on the formation mechanism of irradiation-induced defect for silicaoptical fiber, two mathematical models on irradiation-induced loss were establishedby mathematical modeling the connection between the defect accumulation inoptical fiber core and the irradiation-induced loss of optical fiber. One model allowsfor-ray radiation and gives the irradiation-induced loss as a natural exponentialfunction of irradiation dose, which takes irradiation dose rate into consideration. Itreflects the effect of dose rate on the increasing rate and the saturated valve of theirradiation-induced loss of silica optical fiber. The other model provides a naturalexponential relationship between the irradiation-induced loss and irradiation fluencewithout flux term. It describes that the irradiation-induced loss of the silica dependson total fluence and energy under charged particles irradiation. The comparisonbetween calculation by the models and the test results shows that, two models welldescribe the variation of the irradiation-induced loss of silica optical fiber underdifferent irradiation conditions.