The Rayleigh scattering loss in low water peak single-mode optical fibers under varying Gamma rays irradiation has been investigated. We observed that the Rayleigh scattering coefficient (CR) of the fiber is almost linearly increased with the increase of Gamma irradiation in the low-dose range (< 500 Gy). Based on the electron spin resonance (ESR) spectra analysis, we confirmed that the Rayleigh scattering mainly results from the irradiation-induced defect centers associated with electron transfer or charge density redistribution around Ge and O atoms. This work provides a new interpretation of the optical loss and reveals a new mechanism on irradiation influence on Rayleigh scattering.
© 2011 OSA
With the development of new fiber fabrication techniques, low water peak single-mode (LWPSM) fiber, as a result of eliminating interior hydroxyl, is currently utilized not only in long-distance optical transmission system but also in prospective application [1, 2] to harsh environments, such as atomic power plants and military applications, etc. In general, there are many optical loss factors in optical fibers such as intrinsic absorptions, impurity material absorption, scatterings and so on [3–5]. As literatures [5–10] reported, the infrared absorption is unchanged regardless of the specific fabrication process, but the Rayleigh scattering does vary with fabrication conditions such as drawing tension, furnace temperature and drawing speed, etc. The Rayleigh scattering is an inherent factor that critically determines the minimum material transmission loss. The scattering loss due to imperfections in waveguides can normally be reduced by appropriately selecting the drawing conditions or the precise process control. For example, the Rayleigh scattering can be reduced by heat treatment [6–8].
Previously a number of literatures reported the irradiation influence on optical fiber material [11–14]. Nevertheless, the irradiation influence on Rayleigh scattering of the optical fibers, particularly, the LWPSM optical fibers, has not been studied. In this paper, we reported our investigation of irradiation influence on Rayleigh scattering in LWPSM fibers. We found that the loss increase of the optical fibers irradiated with low-dose Gamma rays mainly results from Rayleigh scattering loss (RSL). The trend of loss increase is almost consistent with that of Rayleigh scattering coefficient increase. We also measured and analyzed ESR spectra characteristics of the fiber material and light scattering properties of the optical fibers under different doses Gamma rays irradiation. Based on these results, we proposed a new mechanism that relates the irradiation influence on Rayleigh scattering to radiation-induced electron transfer.
2. Experimental section
In our experiments, the samples of low water peak single-mode optical fiber (ITU-T G.652D, 500 m in length, Jiangsu Fasten Photonics Co., Ltd.) were irradiated with cumulative doses at about 20 Gy, 50 Gy, 100 Gy, 150 Gy, 200 Gy, 300 Gy, 1.0 kGy, 5.0 kGy at room temperature, respectively. The radiation is from a Cobalt-60 source with a dose rate of 1.4 Gy/s. The irradiations were carried out at the Irradiation Center at the Medical College of Soochow University, China.
Loss spectra, both before and after irradiation, were measured by the well-known cutback technique using a broadband optical spectrum analysis (OSA) (YOKOGAWA AQ6315A) in the 1100-1700 nm range, and with the resolution is 0.2 nm. Each fiber sample is up to 500 m in length. The germanium concentration in the fiber core is less than 10.0 mol %. The diameter of the GeO2-doped core is about 8 μm.
The relative-index profile (RIP) of the optical fiber samplers was measured with an Exfo, Inc., Model NR-9200 fiber analyzer. The resolution is 10−4 and the uncertainty is 10−3. The fiber shows a graded shape of the refractive index difference (RID) between the cladding layer and the core region.
The ESR measurements were performed with Varian E112 spectrometer (Shanghai Institute of Applied Physics, Chinese Academy of Sciences) operating at 9.53 GHz (X band) and employing a modulation field of frequency fm = 100 kHz. The center magnetic field strength is 3410 Gauss. The sweep range is 50 Gauss. The response time constant is 0.25 s. The microwave power is 50 mW. All ESR spectra are obtained at room temperature. The ESR fiber samples are prepared by removing their coating material and cutting into 80-90 pieces of 40 mm in length with a weight of about 40 mg. The intensities of the observed signals are normalized by the standard pitch signal. The unpaired electron in the molecule structure will split under the action of the direct current magnetic field H, while a frequency ν of electromagnetic waves is added in the vertical direction of the direct current magnetic field. When the relationship satisfies the relation of , the electron in the upper and lower energy level will have stimulated transitions. The absorption signal generated in the process is treated by some electronic system and is recorded by the spectrum-meter. Here is Planck's constant, is the microwave frequency, β is the Bohr magneton, is the magnetic field strength and g value is for the Landé g-factor.
3. Experimental results and discussion
In general, the Rayleigh scattering coefficient can be expressed byEq. (2), where is the refractive index, the photoelastic coefficient, the Boltzmann constant, the fictive temperature and the isothermal compressibility at [15, 16]. The Eq. (2) indicates that is directly proportional to . This has been confirmed by the fact that the Rayleigh scattering exhibits a linear relation with in the pure silica core fibers [8, 17, 18]. Now we discuss the effect induced by the Gamma rays irradiation. In our experiments, the optical fiber samples are only treated by using Gamma rays irradiation and all of the parameters: , , , and are constant. So the irradiation influence on Rayleigh scattering in the LWPSM fiber results from the change of fiber material properties, e.g. density or RID. And we propose the density change mainly comes from the charge density redistribution. The attenuation spectra of the fiber samples irradiated by Gamma rays with dose of 0 Gy, 20 Gy, 100 Gy, 150 Gy, 200 Gy, 300 Gy, 1.0 kGy and 5.0 kGy are shown in Fig. 1 . Obviously the radiation-induced attenuation is increased with the increase of cumulative irradiation doses. However the attenuation of the LWPSM fibers in long wavelengths is not increased as much as in the short wavelength, for low cumulative irradiation doses up to ~300 Gy. But the increase in long wavelength becomes remarkable for high cumulative irradiation doses at 1.0 kGy and 5.0 kGy. The attenuation values at three wavelengths: 1310 nm, 1380 nm, 1550 nm, respectively, are shown in Table 1 . The RIDs of the LWPSM fiber un- and irradiated with 1.0 kGy, 5.0 kGy, 10.0 kGy Gamma rays are measured using a NR-9200 fiber analyzer. The measured results are shown in Fig. 2 . The RID is about 0.0053 and the refractive index at core and cladding layer regions are 1.4572 and 1.4625, respectively. The radius of the core region is about 4 μm. The change of RID is not obvious when the fiber samples are irradiated with ~5.0 kGy Gamma rays. Even if the irradiation doses are increased to 10.0 kGy, the RID changes little, except that the ripple of RID appears in the core region, as seen in Fig. 2. We haveinvestigated the formation of different defect centers in the irradiated optical fiber samples through ESR measurements. In the pristine fiber, very weak or almost no ESR spectrum signal could be observed. Therefore, we consider no defect centers are pre-existing before our optical fiber samples are irradiated. After the fiber samples are irradiated, the ESR spectra could be obviously observed. As shown in Fig. 3 . All g-values observed in the previous study and our experiments are also listed in Table 2 . According to the literatures [19–22], the ESR spectrum signals observed in the region g < 2.000 could be assigned to germanium electron trapped center (GEC) associated with fourfold-coordinated germanium ion. The downward peaks of g2 = 1.9991 (gGe(1) = 1.9994) and g3 = 1.9952 (gGe(3) = 1.9945) agree with these of reported Ge(1) and Ge(3) within a little g-value excursion range. We suppose the Ge(1) and Ge(3) could be formed, depending on the number of the second nearest-neighboring germanium atom. The upward peak in the region g > 2.0000 basically agrees with that of the electron self-trapped hole (STH) centers of bridging oxygen bond (g(STH) = 2.0030). We attribute this upward peak (g1 = 2.0028) to STH defect center. Figure 3 shows the intensity of g1-value spectrum signal increases and the intensity of g2- and g3-value spectrum signal decreases with the increase of irradiation dose. Further, when the fiber samples are irradiated up to 10.0 kGy, the conversion of defect centers is generated. The conversion mechanism has been expounded in our previous paper .
Figure 4 shows the attenuation spectrum of the standard LWPSM fiber versus , which is made normalization processing. The slope coefficient CR = 0.946 (, where is the wavelength, and CR is the Rayleigh scattering coefficient). The RIP of the optical fiber is also shown in the inset of Fig. 4, and the RID (%) is 0.526. The other Rayleigh scatteringcoefficients, no normalization processing, are shown in the Fig. 5 . The CR and RID of optical fiber samples irradiated with various doses are also shown in Table 1. The increasing trend of Rayleigh scattering coefficient can clearly be seen in the Fig. 6 . Both optical loss and CR are increased with the increase of irradiation dose, which can be seen in Table 1. We consider, under low-dose irradiation (< 500.0 Gy), there is a close relation between the optical attenuation and the CR, and the increase of the loss may mainly comes from the increase of Rayleigh scattering loss.
Under low-dose irradiation, the attenuation of the optical fiber is relatively low in wavelength range from 1250 nm to 1700 nm, which can be seen in Fig. 1. In these low-dose irradiation cases, we consider that, on the basis of ESR spectra signal analysis, the density change comes mainly from the charge transfer. The defect centers of GEC and STH are attributed to germanium-related rather than silicon-related defect structure. With the increase of the irradiation cumulative doses, the intensity of the ESR spectrum signal of these defect centers is enhanced, leading to the increased loss of the LWPSM optical fiber (see in Fig. 1 and Fig. 3). As shown in Fig. 7 , the electron-trapped centers associated with fourfold coordinated germanium ion and the STH centers of bridging oxygen between Ge atom and Siatom are generated under low-dose irradiation. The radiation induced electron is transferred from the bridging oxygen to the fourfold coordinated germanium atom. From our previous research , Germanium electron center and STH center, without bond cleavage, are formed in optical fiber under low-dose irradiation. These are only due to charge transfer, that is, redistribution of local charge between the Ge atom and the oxygen atom. As the charge density on the Ge atom increases more and more with the increase of irradiation dose, the Rayleigh scattering coefficient is increased with the charge density increase on the Ge atom. However, when the irradiation doses are increased further and more significantly, the molecular structure bond will be broken up and the new defect centers are formed. Hence Rayleigh scattering will not be dominant factor contributing to fiber attenuation under high-dose irradiation (1.0 kGy~). We consider, under low-dose irradiation, there being no other absorption losses, there must be a close relationship between the optical attenuation and the Rayleigh scattering loss, and the increase of the optical attenuation may be attributed to the Rayleigh scattering effect.
We have investigated the influence of Gamma rays irradiation on Rayleigh scattering of the low water peak single-mode optical fibers. We found that, under the low-dose irradiation (< 500 Gy), the CR is increased with the increase of irradiation dose. By experimental measurements of ESR and optical attenuation under various radiation doses, our work provides an interpretation of the optical loss and reveals a mechanism on irradiation influence on Rayleigh scattering. The irradiation influence on optical fiber results from the formation and enhancement of defect centers, such as GEC and STH center. These defect centers represent the radiation-induced electron transfer and the charge density redistribution around Ge and O atoms, which leads to the increase of Rayleigh scattering. Hence the increase of the irradiation dose results in the increase of Rayleigh scattering, ultimately the increase of optical attenuation or transmission loss in these low water peak single-mode optical fibers.
In next stage, we will investigate the relationship between CR and defect concentration, explore the model of various defect centers in detail, and further clarify the density change from a fiber material microstructure aspect by the computer simulation with density functional theory calculation, in a way extending our previous researches , using GAUSSIAN software package.
This work is supported by Shanghai Leading Academic Discipline Project and Science Committee (Grant Nos. S30108, 08DZ2231100, and 08DZ2271700), National Natural Science Foundation of China (Grant Nos. 60937003, 61077068) and Shanghai Natural Science Foundation (Grant No. 10ZR1411900).
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