† Corresponding author. E-mail:
Project supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, China.
The effects of color centers’ absorption on fibers and interferometric fiber optical gyroscopes (IFOGs) are studied in the paper. The irradiation induced attenuation (RIA) spectra of three types of polarization-maintaining fibers (PMFs), i.e., P-doped, Ge-doped, and pure silica, irradiated at 100 Gy and 1000 Gy are measured in a wavelength range from 1100 nm to 1600 nm and decomposed according to the Gaussian model. The relationship of the color centers absorption intensity with radiation dose is investigated based on a power model. Furthermore, the effects of all color centers’ absorption on RIA and mean wavelength shifts (MWS) at 1300 nm and 1550 nm are discussed respectively. Finally, the random walk coefficient (RWC) degradation induced from RIA and the scale factor error induced by MWS of the IFOG are simulated and tested at a wavelength of 1300 nm. This research will contribute to the applications of the fibers in radiation environments.
The interferometric fiber optical gyroscope (IFOG) is one of the most important achievements in the optical fiber sensing field, which can be used to measure vector rotation angular velocity relative to the inertial space and has been widely applied to spacecraft systems recently.[1,2] The sensing coil wound by optical fibers is the essential component of IFOGs. However, the presence of highly energetic radiation in space induces additional optical attenuation greatly. The radiation-induced attenuation (RIA) is primarily caused by the trapping of radiolytic electrons and holes at defect sites in the fiber.[3,4] Furthermore, it affects the performance of the IFOGs in space environment.[5] The smaller optical power received by the photodetector resulting from the RIA can cause the degradation of random walk coefficient (RWC). In Ref. [6], authors reported that the filtering effect arising from the wavelength-dependent RIA in the fiber coil can cause a change in the scale factor of an IFOG, but no experimental results exist. In Ref. [7], the change of wavelength centroid with time for a fiber coil during proton irradiation was tested, which showed a steady shift in wavelength. Moreover, the effect of the RIA spectrum dependence on the mean wavelength shift (MWS) during the light transmission can lead to the scale factor error.[8] It has been found that the RIA spectrum can be fitted by several color centers’ absorption bands based on a Gaussian model.[9–11] A series of color centers have been studied in detail through special spectroscopic techniques.[12,13] Giacomazzi et al. presented a first-principles investigation of Ge paramagnetic centers in Ge-doped vitreous silica based on calculations of the electron paramagnetic resonance (EPR) parameters.[14] However, there are also some near-infrared (NIR) defects that are not so well-known, except for P-doped fibers, in which an NIR γ-induced absorption has already been attributed to P1 defects.[12] The RIA of fibers and its induced degradation on IFOGs have been investigated.[15] However, the correlation between the color centers’ absorption and its influence on IFOGs are rarely studied. In addition, the effects of the radiation dose on the RIA and MWS of all color centers have not been studied, which will be helpful in assessing the performance of the space-borne IFOGs.
In this paper, to investigate the experimental spectrum results ranging from 1100 nm to 1600 nm in phosphorus (P)-doped, germanium (Ge)-doped, and pure-silica-core (PSC) PMFs at two radiation dose levels of 100 Gy and 1000 Gy, the Gaussian model is adopted based on the color centers’ absorption. In addition, the influences of all color centers on RIA and MWS of the experimental fibers are discussed at the wavelengths of 1300 nm and 1550 nm respectively. Moreover, the relationship of the color centers’ absorption intensity with radiation dose is investigated based on a power law model. Finally, the degradation of the RWC and the scale factor error of the IFOG induced by color centers’ absorption are estimated and tested. The work will help to evaluate the performances of optical fibers and optical sensors in other radiation environments, such as military facilities and nuclear power plants.
Three prototype PMFs with different dopants were chosen and irradiated by a 60Co gamma radiation source at room temperature. Table
![]() | Table 1. Main characteristics of the fibers. . |
The setup for measuring the RIA spectrum is shown in Fig.
Since the types of color centers are directly related to the dopants in fibers, the radiation effects will depend on the composition of the fibers, such as core and cladding dopants, impurity levels, and stoichiometry.[16,17] However, these intrinsic parameters of the fibers are generally unavailable for the researchers who apply them to radiation study as these parameters are mostly considered to be confidential by the fiber manufacturers. Therefore, all common dopants are employed in this paper.
Figure
![]() | Table 2. Main parameters values of color centers. . |
Substituting the values listed in Table
The RIA spectra result from three main contributions: the UV absorption tail, visible absorption bands, and NIR-contribution.[13] At the observation wavelengths, NIR absorption bands play the most important roles. The fitting results indicate that the RIA spectra of P-doped fibers can be decomposed into a sum of Gaussian functions corresponding to P1, Ge-NBOHC and Ge(X) color centers. The RIA spectra of Ge-doped fibers depend strongly on Ge-impacted Self-Trapped-Hole (STH) defects, Ge-NBOHC, and Ge(X) color centers. The RIA spectra of PSC fibers are decomposed by STH defects and the Si–NBOHC color centers. The fitting parameter values of the color centers are listed in Table
![]() | Table 3.(a) Parameter values of P-doped fiber. . |
![]() | Table 3.(b) Parameter values of Ge-doped fiber. . |
![]() | Table 3.(c) Parameter values of PSC fiber. . |
Each kind of color center has specific absorption characteristics, and it can absorb light in a specific wavelength range. Thus the wavelength dependence of the color centers’ absorption may change the transmission properties of PMFs. The scale factor of IFOGs is associated with the mean wavelength of light signals, so it is necessary to study the characteristics of the MWS.
The mean wavelength λ can be calculated by the centroid of the spectrum weighted, which can be approximated by a linear weighting factor as follows:
![]() | Table 4. RIA and MWS computed at total dose of 100 Gy. . |
![]() | Table 5. RIA and MWS computed at total dose of 1000 Gy. . |
As illustrated in Tables
The RIA can be explained with the power-law model
The relationship between the color centers’ absorption intensity and the radiation dose is studied quantitatively. It can be concluded that the color centers’ absorption intensity and radiation dose conform to the power law relationship. The fitting curves are shown in Fig.
![]() | Table 6. Coefficients of fitting functions of ai(D). . |
The scale factor error εk of IFOGs caused by the MWS can be computed from the following formula:[23]
To verify the RIA of fibers and the effects of all color centers on RWC and scale factor error of IFOGs, an experimental IFOG with sensing coils of F4, F5, and F6 is tested. The parameters of the IFOG and its physical quantities are shown in Table
![]() | Table 7. Typical design parameters of experimental IFOG. . |
Table
![]() | Table 8. Results of the effects of RIA spectra on the experimental IFOG at 1300 nm. . |
In this paper, the RIA spectra in P-doped, Ge-doped, and PSC fibers with the radiation dose of 100 Gy and 1000 Gy ranging from 1100 nm to 1600 nm are measured. According to the Gaussian model, the RIA spectra of P-doped fibers are decomposed into P1, Ge-NBOHC, and Ge(X) color centers. The RIA spectra of Ge-doped fibers depend strongly on Ge-STH, Ge-NBOHC, and Ge(X) color centers, and the RIA spectra of PSC fibers are decomposed into STH and Si-NBOHC color centers. The relationship between color centers’ absorption intensity and radiation dose is studied quantitatively and conforms to a power-law model, which is helpful in decomposing the spectrum into Gaussian functions with dynamic radiation dose. The effects of all color centers on the RIA and MWS in PMFs are investigated respectively. For the fibers and the tested IFOG, the P1 color centers account for a significant proportion in RWC variations and the scale factor variations in P-doped fibers, and almost all the total RWC and the scale factor variations are attributed to the STH band for Ge-doped and PSC fibers. Moreover, the RWC and the scale factor error of the IFOGs at 1300 nm are simulated based on color centers’ absorption and also tested. The calculated and tested results of the RWC as well as the scale factor error in the IFOG are accordant with each other. Further studies should be conducted to investigate the cross effect of radiation and temperature on the RIA spectrum in space.
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