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 1 lists their main characteristics. The operating wavelength and cut-off wavelength of all fibers are 1300 nm and about 1100 nm. Fibers F1, F2, and F3 (F4, F5, and F6) were employed to compare for the discrepancy among different dopants of fibers, and the fibers F1 and F4 (F2 and F5 or F3 and F6) were used to analyze the effect of radiation dose on the RIA spectrum.

Table 1.

Table 1.

Table 1. Main characteristics of the fibers. . Fiber Dopant Length/km Dose/Gy F1/F4 P∼1 mol% 0.3 100/1000 F2/F5 Ge∼15 mol% 0.3 100/1000 F3/F6 PSC∼none 0.3 100/1000

Table 1. Main characteristics of the fibers. .

The setup for measuring the RIA spectrum is shown in Fig. 1. A super continuum white light source with a spectrum width from 400 nm to 1700 nm was used, and its output signal was split into nine paths by light splitting components. Three were used as references to compensate for the variation of light intensity, and the others were used as interrogation signals. The RIA spectra of six experimental fibers were measured with an optical spectrum analyzer (OSA). The light source, the light splitters and the OSA were located outside the radiation room to avoid the influence of irradiation. The spectrum measurements were conducted in a wavelength range from 1100 nm to 1600 nm. Then the experimental fibers were annealed at room temperature for a few weeks until most of the unstable color centers had been decomposed and transformed after irradiation. The RIA spectrum can be obtained by using the following formula:

Fig. 1.

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	Fig. 1. Schematic diagram of the experimental setup.

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 2 shows the RIA spectra obtained from the experiments. The RIA spectra are smoothed appropriately. The RIA spectrum decomposition is to be addressed as the superposition of several corresponding color centers. Therefore, the RIA spectra are fitted by Gaussian functions^[18] as widely assumed for defects in fibers. The fitting formula is expressed as

Fig. 2.

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	Fig. 2. Smoothed RIA spectra of six fibers.

Table 2.

Table 2.

Table 2. Main parameters values of color centers. . Center name Ei/eV ωi/eV PSC-STH 0.7 0.3 Ge-STH 0.72 0.3±0.1 P1 0.765 0.22 Ge-NBOHC 1.97 0.3–0.6 Si-NBOHC 2.0 0.18 Ge(X) 2.61 0.82

Table 2. Main parameters values of color centers. .

Substituting the values listed in Table 2 into Eq. (2), the fitting curves of RIA spectra can be obtained. The values of parameters may have subtle changes when fitting the data with higher precision. By the superposition of Gaussian functions, the RIA spectra can be decomposed. Figures 3(a)–3(f) illustrate the experimental smoothed spectra and decomposed results of six fibers. The corresponding color centers are also given there. The unit of horizontal axis in Fig. 3 is converted from nanometer (nm) to electron volt (eV).

Fig. 3.

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	Fig. 3. RIA spectra and their fitting curves: (a) F1, (b) F4, (c) F2, (d) F5, (e) F3, (f) F6.

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 3. Thus the numerical fitting functions of fibers are obtained by inserting these values into Eq. (2).

Table 3.(a)

Table 3.(a)

Table 3.(a) Parameter values of P-doped fiber. . Color center Dose/Gy ai Ei/eV ωi/eV P1 100 56.44 0.74 0.199 1000 243.1 0.74 0.199 Ge-NBOHC 100 162.8 1.962 0.5022 1000 1447 2 0.456 Ge(X) 100 0.0005 2.618 0.756 1000 0.001 2.621 0.763

Table 3.(a) Parameter values of P-doped fiber. .

Table 3.(b)

Table 3.(b)

Table 3.(b) Parameter values of Ge-doped fiber. . Color center Dose/Gy ai Ei/eV ωi/eV Ge-STH 100 15.53 0.7392 0.2003 1000 43.76 0.7176 0.236 Ge-NBOHC 100 15450 1.994 0.3 1000 19380 2 0.3 Ge(X) 100 0.0076 2.64 0.73 1000 0.06944 2.64 0.7317

Table 3.(b) Parameter values of Ge-doped fiber. .

Table 3.(c)

Table 3.(c)

Table 3.(c) Parameter values of PSC fiber. . Color center Dose/Gy ai Ei/eV ωi/eV PSC-STH 100 5.622 0.692 0.4 1000 21.59 0.65 0.3617 Si-NBOHC 100 4.59 2 0.18 1000 18.39 2 0.18

Table 3.(c) Parameter values of PSC fiber. .

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.