We report transient radiation-induced effects on solid core microstructured optical fibers (MOFs). The kinetics and levels of radiation-induced attenuation (RIA) in the visible and near-infrared part of the spectrum (600 nm-2000 nm) were characterized. It is found that the two tested MOFs, fabricated by the stack-and-draw technique, present a good radiation tolerance. Both have similar geometry but one has been made with pure-silica tubes and the other one with Fluorine-doped silica tubes. We compared their pulsed X-ray radiation sensitivities to those of different classes of conventional optical fibers with pure-silica-cores or cores doped with Phosphorus or Germanium. The pulsed radiation sensitivity of MOFs seems to be mainly governed by the glass composition whereas their particular structure does not contribute significantly. Similarly for doped silica fibers, the measured spectral dependence of RIA for the MOFs cannot be correctly reproduced with the various absorption bands associated with the Si-related defects identified in the literature. However, our analysis confirms the preponderant role of self-trapped holes with their visible and infrared absorption bands in the transient behaviors of pure-silica of F-doped fibers. The results of this study showed that pure-silica or fluorine-doped MOFs, which offers specific advantages compared to conventional fibers, are promising for use in harsh environments due to their radiation tolerance.
© 2011 OSA
Promising fiber-based applications (sensors, control-command, diagnostics...) are considered for possible integration in the harsh environments associated with space, nuclear power plants or high energy physics facilities. Even if optical fibers present strong advantages for use in these harsh environments such as their electromagnetic immunity, multiplexing capability, radiations are known to degrade their performances. Previous studies showed that three main degradation mechanisms occurring during irradiation of silica-based glasses limit their possible uses. Initially, radiations induce absorbing point defects in the silica-based matrix constituting the fiber core and cladding. These defects lead to an increase in the waveguide linear attenuation, also called radiation-induced attenuation (RIA). Secondly, radiations may induce parasitic light inside the fiber through Cerenkov or radioluminescence emissions. Thirdly, a compaction phenomenon can occur, especially at high dose levels (> 1 MGy), that can modify the refractive-index of the glass and thereby the guiding properties of fibers. The parameters having significant effects on the amplitude of the RIA are well known. A very influent one concerns the nature of the dopants used to modulate the refractive index of the fiber core and cladding [1,2]. Other parameters affect the fiber radiation sensitivity by influencing the concentrations, growth and evolution kinetics of defects: the irradiation characteristics (dose, dose rate), the fabrication process , the fiber profile of use [4–6], or the temperature .
The development of microstructured optical fibers (MOFs) is one of the most recent innovative progresses in the field of optical waveguides. These fibers present wavelength-scale structures (presence of tiny air holes in their cladding for example, see Fig. 1 ) with high refractive index contrast providing them unusual properties . Two types of MOFs are usually distinguished : photonic band gap (PBG) MOFs in which the light remains confined in a low index core thanks to a PBG cladding and high index core fibers in which light is guided via modified total internal reflection (MTIR). We previously discussed the transient radiation response of hollow core PBG fibers in  whereas their steady state radiation responses have been presented by other authors in [10,11]. From these studies, it appears that if hollow core PBG fibers are very promising for integration in steady state radiative environments where they show a very low RIA compared to other fiber types, the results are more complex for transient - or pulsed -irradiation. We previously reported RIA levels quite comparable to classical SMF28 fiber from Corning at 1550 nm for a hollow core PBG fiber after 1 MeV X-ray pulse  whereas Fraunhofer Institute researchers presented promising results on a 19 cells hollow core PBG fiber from the same manufacturer under pulsed electron irradiation . From these studies, it seemed that the structure of the PBGs can affect their radiation responses. In this paper, we focused our work on the other class of MOFs, namely MTIR MOFs.
MTIR MOFs present several advantages that can be used to design new laser or plasma diagnostics. Depending on their geometry, such fibers can be endlessly single-mode, present low bending loss, possibility of dispersion tailoring, large mode field diameters… Another possible advantage, under irradiation, consists in their homogeneity in terms of glass properties. Such fibers can be made with a unique glass composition and this particularity may appear very useful to distinguish between the relative influences of the various dopants or impurities on the fiber responses . Only few studies have been devoted to the study of the radiation response of this class of waveguides [12,13]. In , a prototype MOF was tested and its high radiation sensitivity was attributed to the silica glass tubes used to design the fiber (Heraeus F300). A more complete comparative study  between MOFs and conventional fibers made with the same silica glass reveal that MOFs present radiation responses comparable to their standard counterparts.
In this paper, we focused our work on the behaviors of solid core MTIR MOFS after transient irradiation to evaluate the vulnerability of this fiber type to these specific harsh environments. We characterized, for the first time to our knowledge, the pulsed (35 ns) X-ray radiation-induced attenuation of silica-based MOFs in the visible and near-infrared range (600 nm – 2000 nm). Such environment is representative for example, of the radiations associated with the future megajoule class laser facilities devoted to the study of the fusion by inertial confinement (LMJ, NIF) [14,15]. We also investigated the nature and properties of the radiation-induced defects that govern the fiber radiation responses.
2. Experimental procedure
Tested optical fibers
Two MOFs have been fabricated by the PhLAM Laboratory in Lille (France) using the stack-and-draw technique. Both fibers present the same geometry with a pitch of 3.85 µm and a hole diameter of 1.7 µm. The two fibers mainly differ by the nature of the glass tubes and rods used for their fabrication. MOF1 is entirely made of pure-silica glass (F300 from Heraeus) whereas the core and the whole microstructured cladding of the second one, MOF2, was made of fluorine-doped silica glass with a doping level of around 0.23 wt.% (F320 from Heraeus). The geometry of these two fibers is illustrated in the inset of Fig. 1 that gives a microscopic view of MOF1 as well as the spectral dependence of its linear attenuation before irradiation. Similar microstructure, loss and spectral dependence levels were observed for the MOF2 sample. Both fibers were endlessly single mode. For our experimental conditions (transient exposure, infrared range, for times > 1 ms after irradiation), our previous studies showed that pure-silica-core and Fluorine-doped (F-doped) silica optical fibers present the lowest semi-permanent RIA levels. These results explain our choice for MOF1 and MOF2 fibers composition. To estimate their radiation tolerance, we compared the radiation responses of these two solid-core MOFs to the responses of three conventional optical fibers that have been made by the Modified Chemical Vapor Deposition (MCVD) process. The cores of these fibers are respectively made of pure-silica or silica doped with Germanium (Ge); Phosphorus (P). P-doped optical fibers are known to exhibit the highest losses for this irradiation conditions whereas Ge-doped optical fibers present interesting behaviors that can be suitable for most of the targeted applications. Pure-silica cores fibers present low RIA for this temporal range (> 1 ms) after irradiation; however it is also known that they suffer from very high transient RIA (< 1 µs) just after the pulse. The comparison between the responses of MOFs and MCVD fibers will permit evaluating the vulnerability of the microstructured samples.
Pulsed X(1 MeV)-ray irradiation tests have been performed using the X-ray generator ASTERIX from CEA, Gramat . The incident photons have energy of about 1 MeV; the dose rate exceeds 1 MGy/s and typical deposited dose per 35 ns pulse remains below one kGy. All experiments have been made at room temperature.
We measured the spectral dependence of the RIA in the visible and near-infrared ranges [600-2000 nm]. Only the tested sample is exposed to the X-rays whereas the other part of our equipment is kept in a shielded room located at around 20 m far away from the irradiation source. Fibers transmission measurements were conducted by using a supercontinuum white light source (Koheras) which is directly injected into a single-mode fiber pigtail of 30 m long that is connected to the sample under test. The investigated samples consist in fiber coils of ~10 cm diameter with lengths varying from 46 m (MOF1) to 100 m (MOF2). The transmitted signal is injected in another 30 m long pigtail connected to a 50/50 fiber coupler. One output is analyzed with a NIR256 spectrometer [800-2000 nm] from Ocean Optics while the other output is connected to a HR4000 spectrometer [600-1100 nm] from the same manufacturer. With this setup, we were able to measure the changes in fiber transmission, over a large spectral domain, during and after the X-ray pulse with a few milliseconds time resolution.
3. Experimental results
These figures illustrate the spectral dependence (600-2000 nm) of the pulsed X-ray RIA for the two investigated MOFs. As for conventional optical fibers, MOFs suffer from a strong increase in their transmission losses over the whole range of wavelengths. For both fibers, RIA is higher at shorter wavelengths and exceeds 100 dB/km for various application conditions. Most of the radiation-induced defects seem to absorb in the ultraviolet and visible parts of the spectrum explaining the higher radiation sensitivity observed for our fibers in this spectral region. A specificity of the transient exposures remains the contribution of room-temperature unstable defects to the induced losses that is clearly discernable for shorter times after the end of the irradiation pulse. In this paper, we focused our study on quite long times after irradiation (>1-10 ms after irradiation). From our previous studies, we can state that this corresponds to the time range where the last unstable defects disappear.
The difference in fiber lengths for MOF1 and MOF2 samples explains the losses that have been more precisely measured for the 100 m-long MOF2 sample. The decay kinetics of the RIA for the two fibers can be analyzed in the 100 ms – 100 s range illustrating the bleaching of the radiation-induced point defects unstable at room temperature in this time range. Another important point is that the RIA does not depend linearly on the deposited dose per pulse in the two fibers, in opposition to what has been observed for Ge- or P-doped samples but in agreement with our results on pure-silica or F-doped silica samples. However, as we were unable to use different samples for the different shots, the exact dose dependence of RIA for these fibers was not unambiguously characterized.
We first consider the vulnerability of MOFs to transient irradiations. To estimate the relative radiation sensitivity of the tested MOFs, we compared their RIA spectral dependence to those of MCVD fibers. Figure 3 illustrates the RIA spectra measured 100 ms after an irradiation pulse in these different fibers. All spectra have been normalized by the deposited dose to correct the dose fluctuation between the different pulses. This normalization is correct for the P- and Ge-doped fibers; it remains also pertinent in Fig. 3 as the dose difference between the shots used for MOF1, MOF2 and pure-silica core fiber remains below 15%.
From this figure, it appears that the two MOFs present similar responses and the lower RIA levels for these conditions. At such dose levels (around 250 Gy), RIA remains below 0.1 dB km−1 Gy−1 at 1310 nm and 1550 nm. Induced optical losses are minima in the 1500 - 1800 nm range. Their response is similar to the one measured for the pure-silica-core MCVD fiber. Compared to the two doped fibers, RIA in the MOFs is at least one order of magnitude lower.
Another important point consists in indentifying the point defects that are responsible for these transient induced losses. For P-doped optical fibers, they have been shown to exhibit high permanent RIA levels after pulsed X-rays or under γ-rays. It is well established that P1 defects (P-related E’ centers) are responsible for the optical absorption (OA) band around 1600 nm in this kind of fibers . However, other defects with OA bands peaking also in the IR should contribute to the RIA as the different P-related defects associated with absorption bands in the visible (POHCs, P2, P4...)  are unable to reproduce the measured spectral dependence of RIA . The Ge-doped fibers exhibit high transient RIA but recover rapidly their induced losses after irradiation as compared to P-doped fibers. Like for P-doped fibers, the well-known Ge-related defects (GeNBOHC, GeODC, GeX, GeE’,..) with OA bands located in the UV and visible are unable to reproduce the IR pulsed X-radiation response . The addition of “new” OA bands peaking in the IR remains necessary.
Last class of fibers consists in “radiation-hardened” pure-silica-core fibers. These fibers can present very high transient RIA (for times < 1 ms). However, the defects at the origin of RIA are unstable at room temperature, leading to a quick decrease of RIA for times > 1 ms after a X-ray pulse or during γ-ray irradiation. We previously reported that Self-Trapped Holes (STHs) explained the transient visible RIA in fibers with low concentration of hydroxyls groups . These defects are associated with OA bands at 1.87 eV (660 nm) and 1.63 eV (760 nm) that may also impact the fiber response in the IR part of the spectrum . Two forms of STHS exist: STH1 which consists in a hole trapped on a single bridging oxygen and STH2 which consists in a hole delocalized over two equivalent bridging oxygens of the same SiO4 tetrahedron . Chernov et al. also assumed that an OA band at ~0.69 eV (1800 nm) is related to these room-temperature unstable defects .
We considered the ability of the various well-known point defects (SiE’, ODC, STHs, NBOHC, POR) to explain the measured RIA spectra at different times after pulse. Our analysis shows that the absorption bands peaking at energies greater than 3 eV cannot contribute to the induced losses at tested wavelengths. In Fig. 4 , we illustrate the RIA spectrum measured 1 s after a 110 Gy pulse as well as the absorption bands of Si-related defects peaking in the 0.6 – 3 eV (400 – 2000nm) energy range and the best fit obtained by combining these different absorption bands.
The best fit of our experimental data is obtained by a combination of the three OA bands centered at 0.7 eV (Full Width at Half Maximum = 0.3 eV); 1.88 eV (FWHM = 0.6 eV) and 2.6 eV (FWHM = 1.12 eV) all associated with STHs [18–23]. The other OA bands at 1.97 eV (FWHM = 0.2 eV, NBOHC) and 1.63 eV (FWHM, 0.46 eV, STHs) seem to not contribute to the RIA spectrum for these fibers.
From these results, it appears that STHs, that are known to be very unstable at room temperature, have a preponderant influence of the F-doped and pure-silica MOFs. The generation efficiency of these defects is affected by the glass history parameters like impurities contents  and fictive temperature . More work should be done to adjust such parameters in order to reduce the MOF fiber radiation sensitivity.
We characterize the pulsed X-ray radiation-induced effects in two solid core MOFs made with two different glasses: pure- and F-doped silica. Such harsh environment is representative of those encountered in facilities devoted to the study of fusion by inertial confinement. Our results demonstrate that the main radiation-induced phenomenon in both near-infrared and infrared domains is the radiation-induced attenuation (RIA). The kinetics and amplitudes of RIA are quite similar for the two tested fibers that possess the same microstructure but have been made with different compositions: pure-silica and fluorine-doped silica. The comparison of these fibers behaviour to pulsed X-rays with those of MCVD fibers illustrates that MOFs have a quite comparable vulnerability than pure-silica optical fiber. We provide no evidence for a degradation process linked to the air holes structure, as for hollow core PBG fibers. Point defects responsible for the fiber degradation still need to be identified as the Si-related defects identified in the literature failed to reproduce the measured RIA spectra. However, our study reveals the preponderant role of room-temperature unstable self-trapped holes (STHs) in the transient responses of the tested fibers. As the generation efficiency of these defects is affected by the glass properties, like fictive temperature, it seems possible to decrease their contribution to fiber degradation and consequently to build more radiation-tolerant fibers.
As a consequence, MOFs with their specific optical features (endlessly single-mode, low bending loss, dispersion tailoring, and large mode field diameter) are good candidates for integration as part of diagnostics in nuclear environments like those associated with megajoule class lasers. Additional work is under progress to enhance the resistance of these fibers to such a harsh environment and to identify the whole set of radiation-induced point defects involved in their radiation responses.
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