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We have investigated gamma-ray radiation response at 1550 nm of fluorine-doped radiation hard single-mode optical fiber. Radiation-induced attenuation (RIA) of the optical fiber was measured under intermittent gamma-ray irradiations with dose rate of ~10 kGy/h. No radiation hardening effect on the RIA by the gamma-ray pre-dose was found when the exposed fiber was bleached for long periods of time (27~47 days) at room-temperature. Photo-bleaching scheme upon 980 nm LD pumping has proven to be an effective deterrent to the RIA, particularly by suppressing the incipient RIA due to room-temperature unstable self-trapped hole defects (STHs). Large temperature dependence of the RIA of the optical fiber together with the photo-bleaching effect are worthy of note for reinforcing its radiation hard characteristics.
© 2016 Optical Society of America
1. Introduction
Radiation-induced attenuation (RIA) in silica-based optical fiber caused by high energy ionizing radiation seriously degrades the capabilities of most fiber-based devices such as fiber lasers, high-speed optical fiber communication systems, nonlinear optical fiber devices, and fiber sensors, etc [1–4]. Radiation-hardening against such the light attenuation is therefore definitely needed for proper implementation of the fiber-based devices in radiation-harsh environments. Previous works to reduce the RIA have been addressed by i) minimizing defect precursors or impurities, ii) adding another types of dopants (F, Ce), iii) pre-treatments (pre-irradiation, H2 loading), and iv) photo- and thermal-bleaching methods [5–9]. Among them, modification of glass composition of optical fiber can significantly alter the radiation response. For example, a pure-silica glass core fiber showed strong radiation resistance compared to the germano-silicate glass core fiber due to the lack of extrinsic Ge-related defects. Presence of metal ions or phosphorus in silica glass network has been found to significantly increase the RIA, whereas OH or fluorine (F) in the glass (Si-OH and Si-F groups) are well-known to effectively reduce the RIA [10,11]. The silica optical fiber doped with F has shown very strong radiation resistant characteristics particularly under the steady-state gamma ray (γ-ray) irradiation condition [11,12]. However, one should be aware of that the RIA response of the F-doped fiber can significantly vary with the fiber manufacturing process due to different processing parameters and various concentrations of impurities such as Cl, O, OH, which are likely to be formed in the fiber glass during the process [13,14].
The radiation resistance of silica glass optical fiber is well-known to be enhanced by incorporating F in the fiber core due to its role of inhibiting the defects generation during γ-ray irradiation. The effect of F on the RIA is prominent in the visible and UV by reducing NBOHC (Non-bridging oxygen hole center) and E' center. In near infrared (NIR) region, the RIA has been attributed to STH (Self-trapped hole) defect with a secondary UV-tail contribution [15,16]. Thus the high radiation resistance in the NIR region of the F-doped fiber is thought to be mainly due to suppressed generation of the STH defect by radiation, as shown in our present work. Fluorine is usually heavily doped in the fiber cladding than the fiber core for light guiding via total internal reflection, since the refractive index (RI) of the silica glass decreases with F addition. Although the strong radiation resistance has been manifested in the highly F-doped optical fibers, it was also noted that high fluorine doping can degrade the radiation resistance [17,18]. Thus optimum concentration of fluorine regarding radiation resistant characteristics of the F-doped optical fiber core and cladding has been determined upon consideration of both the RIA response and refractive index (RI) profile of the fiber. Recently developed F-doped radiation hard optical fiber (F-doped RHF) with optimized F-doped profile showed very low RIA sensitivity and satisfied fundamental optical requirements (cut-off wavelength, attenuation coefficient, dispersion, bending loss, etc.) as a waveguide for optical telecommunication [12], making it suitable for reliable fiber-based communication and sensor applications in radiation-harsh environments. Despite such high feasibility of the future applications based on the F-doped RHF, however, radiation response of the optical fiber under diverse radiation conditions (dose rate, accumulated dose, types of radiation, continuous/intermittent/transient irradiations), different environments (temperature, strain, humidity), and measurement conditions (source wavelength, power) have not been fully investigated yet. Thus, we have investigated radiation dose dependence of the RIA at 1550 nm of the F-doped RHF under intermittent γ-ray irradiation. By comparing the measured RIAs for each irradiation with different conditions such as accumulated dose, bleaching time, temperature, and photo-bleaching, the radiation-hardening behavior against the RIA of the optical fiber has been discussed.
2. Experiments
2.1 Fluorine-doped radiation hard single-mode optical fiber
A commercially available F-doped radiation hard single-mode optical fiber (F-doped RHF, YOFC) manufactured using the plasma activated chemical vapor deposition (PCVD) process was used in the experiment. Figure 1 shows the refractive index (RI) and stress profile of the optical fiber measured by fiber index profiler (Interfiber Analysis, IFA-100).
Fig. 1 (a) Refractive index profile and (b) axial stress of the F-doped radiation hard optical fiber.
The concentrations of F in the fiber core and cladding were estimated from the RI profile to be ~2.4 mol % (Δn (nF-SiO2 - nSiO2) = −0.005) and ~4.1 mol % (Δn = −0.010), respectively [19]. Compared to the standard Ge-doped SMF, no significant difference in internal stress between the core and the cladding of the F-doped RHF was found due to the nonlinear and small deviation of the axial stress by the refractive index change (or F concentration) in the low-concentration F-doped silica glass [20]. Core diameter, attenuation, and cut-off wavelength of the F-doped RHF were about 9 μm, 0.26 dB/km (@1550 nm), and 1130 nm, respectively.
2.2 Gamma-ray irradiation and radiation-induced attenuation (RIA) measurement
A 15 mm thin spool wounded with 200 m length of the F-doped RHF was placed parallel to a 60Co radiation source (MSD Nordion, pencil type/C-188 sealed), so that the fiber can receive almost uniform dose longitudinally along the fiber. The optical fiber was irradiated intermittently by gamma-ray from the 60Co source, and the RIA of the optical fiber was measured during and after the irradiation. The irradiation was carried out 14 times separately with different bleaching times at room temperature. Radiation dose rate and irradiation time of each irradiation were fixed to be ~10 kGy/h and 1 hour, respectively. The experimental setup to measure the RIA of the optical fiber is shown in Fig. 2. The ASE light source (OPTOWARE-B200) and optical spectrum analyzer (OSA, YOKOGAWA-AQ6370C) were used for measuring optical transmission changes upon the gamma ray irradiation. Note that the launched source light was only maintained when the RIA measurement was being performed. A 980 nm laser diode (LD) module was inserted in the experimental setup to investigate photo-bleaching effect on the RIA of the optical fiber. The LD at 980 nm was chosen for the photo-bleaching experiment due to its sufficient light energy which was able to partially bleach radiation losses due to the radiation-induced defects [7]. Moreover, the LD with high power output was highly accessible, and we easily applied the LD using a typical WDM coupler (980/1550 nm) for the purpose without any interference to optical communication bands. The fiber adapters and patch cords used were protected by the radiation shielding Pb plates to avoid the exposure of the gamma-ray which may cause unexpected optical losses. The change in optical transmission of the optical fiber at 1550 nm with respect to the radiation dose was measured discretely upon continuous illumination of the source light and the RIA was calculated by using the Eq. (1).
where P1 is optical transmission power before the gamma-ray irradiation and P2 is the power during or after the irradiation, and l is the exposed optical fiber length. The irradiation history and details for the RIA measurement are listed in Table 1.Fig. 2 Experimental setup for the gamma-ray irradiation and the RIA measurement of the optical fiber. VOA: Variable Optical Attenuator.
Table 1. Gamma-ray irradiation history and detailed experimental conditions for the RIA measurement of the F-doped radiation hard optical fiber
3. Results and discussion
Figure 3(a) shows the measured RIA of the F-doped RHF with respect to time under the intermittent irradiation (Irr. #1, #2, and #3). The time interval between the end of the previous irradiation and the execution of the subsequent irradiation ( = bleaching time) was 1 hour for this measurement. Temperature inside the irradiation room during the measurement was maintained to be 18 ± 2 °C. RIA drastically increased just after the beginning of the irradiation and it became gradually increased with the increase of the exposed radiation dose (Dose [Gy] = dose rate [Gy/min] x irradiation time [min]). We plotted the RIAs for each of the irradiations with individual time scale (0 ~60 min: during irradiation, 60 ~120 min: after the end of the irradiation) to compare with each other as shown in Fig. 3(b). The RIAs after the Irr. #1, #2, and #3 (@ 60 min) were 3.2 dB/km, 2.8 dB/km, and 2.3 dB/km, respectively, indicating that the RIA sensitivity decreased when the subsequent irradiation was made after the previous irradiations. When the irradiation was terminated, the RIA partially recovered with time due to rehabilitating process of the radiation-induced defects by both spontaneous thermal and optical bleaching. Similarly to the RIA growth curve, the RIA rapidly decreased within a minute from the termination of the irradiation, and it became gradually decreased with time. RIA recovery rates at 60 minutes after the end of the Irr. #1, #2, and #3 were 52%, 70%, and 90%, respectively. This is because the different residual ( = initial) RIA levels corresponding to conversion level of precursors into defects would seriously involve the subsequent RIA response. Figure 3(c) shows the RIA per 10 kGy dose for each irradiation (Irr. #1, #2, and #3) with their residual RIA. The RIA decreased with the increase of the residual RIA, and it is thought to be due to the lack of the precursors after the pre-dose.
Fig. 3 (a) RIA at 1550 nm of the F-doped radiation hard optical fiber during and after the gamma-ray irradiation #1, #2, and #3. (b) A comparison of the RIAs of the optical fiber for each of the irradiation (0 ~60 min) and the consequent RIA recoveries with time after the end of the irradiations (60 ~120 min). (c) RIAs per 10 kGy dose for each of the irradiation with their residual ( = initial) RIA level.
Irradiation #4 and #5 were carried out gradationally after 44 days of the previous Irr. #3. In the same manner, Irr. #6, #7 and Irr. #8, #9 were also carried out after 27 days and 30 days of the Irr. #4, #5 and Irr. #6, #7, respectively. Temperature inside the irradiation room during the measurement was maintained to be 13 ± 2 °C. Figure 4(a) shows the RIAs of the optical fiber for the Irr. #4-#9, and Fig. 4(b) compares them individually. Almost identical but modestly increased RIA growth curves have been found in the progression of the Irr. #4, #6, and #8. Whereas the RIAs for the Irr. #5, #7, and #9 decreased compared to those by the previous Irr. #4, #6, and #8, respectively, and it is thought to be mainly due to influence of the residual RIA and remaining rehabilitating process that can lead the RIA recovery. Figure 5(a) shows the RIAs per 10 kGy dose of the optical fiber after each of the Irr. #4-#9 with respect to accumulated dose. The RIA slightly increased compared to those by the previous irradiations after the long-term bleaching times of 27 ~47 days (Irr. #4 -> Irr. #6 -> Irr. #8). On the contrary, when the bleaching time was only 1 hour, the RIA significantly decreased compared to to those by the previous irradiations (Irr. #4 -> Irr. #5, Irr. #6 -> Irr. #7, and Irr. #8 -> Irr. #9). It seems that after the long-term bleaching periods of time (27 ~47 days) at room temperature, the temporal rehabilitating process would be negligibly small and the possible residual RIAs were close to those before the pre-dose, thus they negligibly affected the subsequent RIA growths. Figure 5(b) shows the RIA recovery rates after 60 minutes of each of the Irr. #4-#9. The RIA recovery rates increased with the repeated irradiations when the bleaching time was 1 hour, but decreased when the bleaching time was long enough about 27 ~47 days.
Fig. 4 (a) RIA at 1550 nm of the F-doped radiation hard optical fiber during and after the gamma-ray irradiation #4-#9. (b) A comparison of the RIAs of the optical fiber for each of the irradiation (0 ~60 min) and the consequent RIA recoveries with time after the end of the irradiations (60 ~120 min).
Fig. 5 (a) RIAs per 10 kGy dose at 1550 nm of the F-doped radiation hard optical fiber at the end of the gamma-ray irradiation #4-#9. (b) RIA recovery rates for each of the irradiation after 60 min of the irradiations.
Irradiation #10 was carried out after 47 days of the previous Irr. #9, and subsequent Irr. #11 was made after 1 hour of the end of the Irr. #10. Figure 6 shows the RIA of the optical fiber during and after the Irr. #10, #11. RIA response for the Irr. #10 showed similar tendency as before but overall RIA level slightly decreased, and it is thought to be due to temperature increase. More details about the temperature dependence of the RIA response of the optical fiber will be followed in the discussion part.
Fig. 6 RIA at 1550 nm of the F-doped radiation hard optical fiber during and after the gamma-ray irradiation #10 and #11. The LD light power at 980 nm was ~40 mW.
Since application of sufficient photon energy of light is known to introduce photo-bleaching effect through recombination of photo-ionizing trapped-charge defects in glasses formed under ionizing radiation, we have tried to investigate the photo-bleaching effect, if any, on RIA behavior of the optical fiber [21,22]. In this regard, we have applied a 980 nm light (~40 mW) from laser diode (LD) into the fiber core in the intervals during and after the Irr. #11. As shown in Fig. 6, significant reduction of the RIA of the fiber was found to occur immediately when the LD turned on. Note that the photo-bleaching effect can also occur by the used 1550 nm light source itself with power of 1.5 ± 0.5 mW. However, the effect commonly affected for all the RIA measurements, so the distinguishing photo-bleaching effect appeared during the Irr. #11 was entirely by the 980 nm light energy. The RIA was 2.1 dB/km at 25 minutes after the execution of the Irr. #11, and it decreased to 0.6 dB/km after 1 minute of the launching the laser light. Then the RIA gradually increased with respect to dose. When the LD was turned off (45 min), the RIA again drastically increased. Approximately 70% the RIA reduction was obtained by applying the 980 nm LD. The photo-bleaching effect upon the LD pumping was found to occur instantly and efficiently reduced the RIA during the irradiation, and it also appeared even after the end of the irradiation as shown in Fig. 6. The result shows that the rehabilitating process of the defects generated by the irradiation can be accelerated with the 980 nm LD light. The photo-bleaching effect became diminished as the unstable defects would be restored themselves with time, and consequently the RIA recovery was reaching to saturation limit. The resultant RIA at 60 minutes after the end of the Irr. #11 was only 0.4 dB/km.
RIA of the optical fiber by Irr. #12 was measured after 29 days of the Irr. #11 and it is shown in Fig. 7. Note that in this measurement, 980 nm LD (~40 mW) laser was applied consistently during and after the irradiation. As we have confirmed from the results after the Irr. #11, RIA dose sensitivity for the Irr. #12 has shown to be significantly reduced due to the photo-bleaching effect. Particularly, no drastic RIA increase in early stage (< 1 min) of the irradiation was found to appear unlike the previous irradiations. RIA at 1 min of the Irr. #11 was 0.7 dB/km, which was about 3 times smaller than that of by the previous irradiations. Then the RIA increased gradually with respect to radiation dose by the end of the irradiation (60 min) reaching to 2.0 dB/km. No instant recovery of the RIA after termination of the irradiation was found to appear, whereas it gradually recovered with time. The RIA was reduced to 1.2 dB/km after 60 minutes of the termination representing ~60% of RIA recovery rate. The extent of RIA reduction by the photo-bleaching seems to be determined by amount of unstable defects involving photo-ionizing trapped-charge defects, represented by the residual RIA, since the photo-bleaching effect was more powerful during the irradiation than after the irradiation as shown in Fig. 6. This has been accounted for the relatively weaker photo-bleaching effect at the Irr. #12 than the Irr. #11 which was performed after 1 hour of the previous Irr. #10. The larger RIA reduction by the photo-bleaching during the Irr. #11 was thought to be due to the influence of remaining the unstable defects generated by the Irr. #10. As shown in the Irr. #12, the photo-bleaching effect by the 980 nm light was particularly effective in the suppression of the incipient RIA that may arise from the unstable defects at room temperature such as the STHs [16,23]. Since the STHs are localized at the top of the valance band, they are likely to be de-trapped with electrons excited from the valance band by thermal and/or optical stimulation resulting in the faster decay of the STHs [21]. As a consequence, there was no rapid increase and decrease in the RIA at the beginning of the irradiation as well as at the termination, compared to those by the previous irradiations performed without applying the light. From the results it seems clear that the photo-bleaching using the 980 nm light can be an effective deterrent for the RIA at 1550 nm of the F-doped RHF under the steady-state radiation condition. It is also highly probable that the photo-bleaching effect would bring about great reduction of RIA under pulsed or transient radiation conditions since the light power has proven to be effective for suppression of the incipient RIA as shown in Fig. 7.
Fig. 7 RIA at 1550 nm of the F-doped radiation hard optical fiber during and after the gamma-ray irradiation #12. The LD light power at 980 nm was ~40 mW.
In the previous investigations on radiation-induced defects of silica glass, it has been clarified that fluorine doping can effectively reduce the defects formation in the glass, thereby reinforcing the radiation resistance. The major role of fluorine doping for radiation hardening of the silica glass is reduction of defect precursor sites such as strained Si-O bonds which is likely to form the pair of NBOHC (Non-bridging oxygen hole center, ≡O−Si˙) and E' center (≡Si˙) by radiolysis [18,24]. The NBOHC and E' center are considered to be mainly responsible for the RIA in UV-visible spectral range of the optical fibers, since their absorption bands are located around 185 nm, 260 nm, 620 nm for NHOHC, and 215 nm for E' center [1,15]. Those defects, however, do not seem to crucially affect the RIA at near infrared (NIR) region. Instead, absorption from self-trapped hole defects (STHs) have been proposed as a main contributor to the RIA at NIR region (> 1300 nm) with a secondary UV-visible absorption tail [15]. Figure 8(a) shows the optical absorption spectra of the gamma-ray irradiated optical fibers (F-doped RHF, Ge-doped SMF28e + , and pure-silica glass core optical fiber (PSCF), Accumulated dose: ~100 kGy). Strong absorption below ~900 nm of the SMF 28e + was attributed to combination of Ge-related defects rather than Si-related defects [25]. Absorption coefficients over the spectral range of the PSCF were significantly lower than that of the SMF28e + due to the lack of the Ge-related defects. Distinct absorption bands from the NBOHCs of the PSCF and the F-doped RHF appeared around 620 nm [26,27]. No significant absorption below 500 nm has been found in the F-doped RHF, which indicates suppression of generation of the UV located defects such as E’ center in the fiber core. The inset of Fig. 8 shows the absorption spectra of the optical fibers in the wavelength region from 1200 nm to 1650 nm. The optical absorption coefficients of the optical fibers increased with the increase of wavelength becoming dominantly affected by broad absorption band centered in NIR (> 1650 nm) that has been assigned to the STHs [15,16]. Figure 8(b) compares the RIA responses at 1550 nm during and after the gamma-ray irradiation of the optical fibers. As we discussed earlier, such high radiation resistance characteristics ( = Low RIA) at 1550 nm of the F-doped RHF is thought to be mainly due to lower generation of the STHs.
Fig. 8 (a) Absorption spectra of the gamma-ray irradiated F-doped radiation hard optical fiber, SMF28e + , and pure-silica glass core optical fiber (PSCF) (Accumulated dose: ~100 kGy). (b) Comparison of RIA responses at 1550 nm of the optical fibers.
To evaluate the radiation hardening effect by the pre-dose or photo-bleaching, as well as temperature dependence of the RIA response of the F-doped RHF upon the intermittent irradiation, we plotted the RIAs of the optical fiber at the end of each irradiation with corresponding temperatures for the RIA measurements as shown in Fig. 9.
Fig. 9 RIAs at 1550 nm of the F-doped radiation hard optical fiber at the end of each intermittent irradiation with respect to accumulated dose, and corresponding temperatures for the RIA measurements.
Note that the RIAs for the Irr. #2, #3, #5, #7, #9, and #11 were not included in this plot because of the short bleaching time (1 hour), the values were partly influenced by remaining rehabilitating process as a consequence of the previous irradiations that leads the RIA reduction. The RIAs for the Irr. #4, #6, and #8 that were measured at 13 ± 2 °C were almost similar to each other, indicating that no radiation hardening effect by the pre-dose was found when the exposed fiber was bleached for the long periods of time (27 ~47 days). The plot in Fig. 9 also shows large temperature dependence of the RIA response of the F-doped RHF as well as the relatively negligible influence by the pre-dose. The RIA has only modestly increased when the temperature has maintained (Accumulated dose: 40~80 kGy), but it changed significantly with temperature change during the measurement. Temperature dependence of the RIA (ΔRIA/ΔTemp.) of the optical fiber under the measurement condition was estimated to be about −0.076 dB/km/°C without consideration of other minor factors affecting the RIA. Note that the sharp drop in the RIA for the Irr. #12 (accumulated dose: 120 kGy) was attributed to the photo-bleaching effect by the 980 nm light.
4. Conclusion
We have investigated the radiation response at 1550 nm of the F-doped RHF under the intermittent gamma-ray irradiations (Irr. #1~#14). While when the bleaching time after the irradiation was short (1 hour) at room-temperature, the subsequent RIA level was reduced to its corresponding residual RIA, the RIA after the long-term bleaching times about 27 ~47 days was not changed without showing radiation hardening effect upon the pre-dose. It seems that the possible residual RIA after the bleaching time was close to that before giving the dose and the temporal rehabilitating process was also negligibly small and thus they resulted in the similar RIA level by the subsequent irradiations. On the other hand, the RIA at 1550 nm of the F-doped RHF was significantly reduced by pumping with the 980 nm LD (~40 mW) through the photo-bleaching effect (Irr. #11 and #12), and it was particularly effective for the incipient RIA which was mainly attributed to the room-temperature unstable STHs. The optical fiber has also shown the large temperature dependence of the RIA, as it varied significantly with changing temperature (Irr. #1 -> Irr. #4, Irr. #8 -> Irr. #10). The temperature dependence of the RIA (ΔRIA/ΔTemp.) of the optical fiber was estimated to be about −0.076 dB/km/°C.
Acknowledgments
This work was partially supported by the Advanced Technology Radiation Laboratory of the Korea Atomic Energy Research Institute, the New Growth Engine Industry Project of the Ministry of Trade, Industry and Energy, the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2013R1A1A2063250), the Brain Korea-21 Plus Information Technology Project through a grant provided by the Gwangju Institute of Science and Technology, South Korea.
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