y-Radiation-Induced Absorption in Pure-Silica-Core Fibers in the Visible Spectral Region: the Effect of H2-Loahg

A. L. Tomashuk', E. M.Dianov', K. M. Golant ', and A. 0 .Rybaltovskii2 'Fiber Optics Research Center at the General Physics Institute of the Russian Academy of Sciences (FORC), 38 Vavilov St.;

117942 Moscow, Russia

'Nuclear Physics Institute of Moscow State University, 119899 Moscow, Russia

Abstract y-radiation-induced absorption in the visible spectral

region is studied in H2-loaded and untreated optical fibers with high-OH low-Cl KU core and low-OH low-Cl KS-4V core. H2-loading is found to suppress the low-dosage transient absorption band centered at 670 nm in KS-4V fibers. After irradiation to 1.7 MGy(Si), H2-loaded fibers show losses many times lower than the untreated fibers. H2-loaded and irradiated fibers are found to retain very high radiation resistance under subsequent irradiations even in the absence of molecular hydrogen. It is concluded that H2-loading followed by pre-irradiation can be used as an efficient radiation hardening techque for pure-silica optical fibers.

I. Introduction Among the promising applications of fiber optics in the

nuclear industry, fibroscopes for visual inspection of inaccessible parts of nuclear installations and fibers for the analysis of plasma emission spectrum in the fusion reactor are high on the agenda. Preforms for such fibers and fibroscopes are commonly produced by plasma outside deposition (POD) process [l] which consists in depositing an F-doped silica layer (light-reflecting cladding) on a substrate silica rod (core). Radiation resistance of the final fiber is governed primarily by the properties of the specific synthetic silica from whch the rod was produced. Recent comparison experiments on fibers with different silicas in the core [2, 31 have shown that the key factor of radiation-induced absorption are chlorine and hydroxyl impurities in silica. Most of the existing fabrication technologies provide either high-chlorine low-hydroxyl (e.g. Suprasil F-300) or high-hydroxyl low- chlorine (KU and Suprasil F-100) silicas.

In [2] it was shown that the best suited core material for radiation-resistant fibers intended for use in the visible region at megagray doses should contain as little chlorine and hydroxyl as possible. Hydroxyl and peroxy linkage arising in 'wet' silicas during the cladding deposition process are the precursors of radiation-induced non-bridging oxygen hole center (NBOHC) with absorption bands at 630 and 260 nm, whereas chlorine-associated radiation color centers produce a band tail extending from the UV to near-IR region. It was also shown in [2]and [4] that the best suited commercially available silica for fibers to be used at a megagray level of

radiation is KS-4V silica of Iruvisil (St. -Petenburg), which features extremely small concentrations of both C1 and OH.

An interesting peculiarity of fibers made from KS-4V silica is a transient absorption band centered at about 670 nm which arises at low doses (- lo2 Gy) and then vanishes with dose. The 670 nm band can be annealed by pre-irradiation and does not show up, if we irradiate the fiber again [ 5 ] . Based on radiation response of bulk samples treated under a constant electric field, Amosov et al. [6] deduced this band to be due to an alkali impurity. Recently Griscom has investigated it in two types of low-Cl low-OH fibers 171 to propose that this transient absorption is inherent in silicas free of both C1 and OH and might be due to holes trapped at strained Si-0 bonds. As soon as the 670 nm band vanishes, in the dose range 1-10 MGy, the induced absorption in KS-4V fibers in the visible region turns out at saturation at a level of 2-2.5 dT3/m. At doses of about 10 MGy ths absorption level is much lower than in fibers with other commercial silicas, although some laboratory fiber samples with an F-doped silica core feature high radiation resistance as well [2]. Apparently, the above level of absorption at megagray doses is unlikely to be reduced in the nearest future by improving the silica technology. However, radiation hardening techniques can be applied directly to as-drawn fibers.

H2-loading can harden fibres against radiation (e.g. see [ 81 and references therein). Molecular hydrogen dissolved in the fiber glass serves as a source of atomic hydrogen which heals radiation-disrupted bonds thereby removing color centers. Previous works on H2-loading mainly dealt with fibers intended for use in the telecommunication spectral windows (0.85, 1.30, and 1.55 pm), at doses below 1 MGy, and at low temperatures. In particular, it was shown that H2- loading can be efficiently used to harden fibers for space applications [SI, [9]. Nagasawa et al. [lo] demonstrated suppression of the radiation-induced 2 eV absorption band by hydrogen loadmg. Lyons and Looney [ 111 observed a lower induced absorption at 850 nm in a high-OH fiber that was H2- loaded and pre-irradiated. It was concluded, however, that the pre-irradiation dose was too small (500 Gy) to reveal the potentialities of the radiation hardening effect in full measure.

The aim of this work was to investigate the effect of H2- loading on y-radiation-induced absorption at high doses in well-studied fibers which are considered as candidates for

0-7803-4071-X/98/$10.00 0 1998 IEEE. 476

applications in the nuclear industry in the visible spectral region.

11. Experimental Using KS-4V and KU silica rods we fabricated two

preforms by the POD-process which were then drawn into polymer-coated fibres. The fibres had the same geometry (core and cladding diameters of 100 and 120 pm, respectively) and the same numerical aperture NA=0.16. As to the principal impurities, the KS-4V fiber core contained 0.25 ppm hydroxyl and 20 ppm chlorine, and the KU fiber core contained 800 ppm hydroxyl and 80 ppm chlorine.

Pieces of the two fibers were kept in H2 atmosphere at a pressure of 24 atm. and at room temperature for 11 days. Thereafter one hydrogen-loaded piece and one as-drawn piece of the KS-4V fiber were subjected to heat treatment at 200°C for 200 min. in air atmosphere. On finishing the heat treatment, the fibers were slowly cooled down to room temperature. This procedure was expected to result in removal of molecular hydrogen from the fiber and its partial conversion into OH-groups. Thus we had four pieces of the KS-4V fiber which underwent different treatments before the irradiation: 1) as-drawn, 2) as-drawn and heat-treated, 3 ) H2- loaded, and 4) H2-loaded and heat-treated.

The two pieces (as-drawn and H2-loaded) of the KU fiber and the four pieces of the KS-4V fiber were then subjected to three successive runs of y-irradiation with a cobalt source: run #1 to a dose of 297 Gy(Si), run #2 to 1.7 MGy(Si), and run #3 to 1.7 MGy(Si). Thus, the total dose absorbed after the three runs was 3.4 MGy(Si). The dose rate was 6.6 Gyh for runs #1, #2 and 6.5 Gyh for run #3. The irradiation temperature was slightly elevated (- 40 "C), because the fibers were irradiated in air and were in contact with the steel wall of the irradiation unit. Runs #1 and #2 were performed within several days after finishing the H2-loading procedure. Run #3 was performed three months after run #2. Before run #2, whenever possible, the fibers were kept in a refrigerator (-6 "C) to prevent molecular hydrogen from leaving the fibers. Between runs #2 and #3 the fibers were kept at room temperature.

Optical loss was measured by the standard cut-back technique (e.g. see [4]) before the treatments, after H2- loading, after heat treatment, and within several hours after completion of each irradiation run.

111. Results and Discussion Let us first consider evolution of the loss spectra in the

KS-4V fiber as the result of H,-loading and heat treatment (Fig. 1). As one might expect, H2-loading gave rise to the 1.24 pm characteristic absorption band of molecular hydrogen [12] and to a slight increase in the OH-group absorption at 1.38 pm. The 0.63 pm absorption band of NBOHC, which is typical of as-drawn polymer-coated KS-4V

0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8

wavelength, pm Figure 1: Total loss in RS4V fiber pieces which underwent different treatments. fibers [4], disappeared in the process. The heat treatment resulted in a further growth of the OH-group absorption and complete removal of molecular hydrogen from the fiber. Thus, the H2-loaded and heat-treated piece of the KS4V fiber differed from the as-drawn piece in the amount of hydroxyl and NBOHC.

The first run of irradiation allowed us to observe the pre- treatment effect on the 670 nm transient absorption band in KS-4V fibers (Fig. 2). This band turned out to be totally suppressed in the H2-Jloaded piece. The piece that was subjected to H2-loading and heat treatment showed a somewhat weaker band than the as-drawn piece, and this is also consistent with the strained-bond model proposed by Griscom [7]. However, the fiber piece that was heat-treated without H2-loading exhibited a broader absorption band than the untreated piece, although heat treatment was expected to relieve strain in Si-0 bonds and hence to weaken the 670 nm absorption, This experimental fact appears to be in conflict with the'above model for the transient absorption in KS-4V fibers.

1600 I-' I

+loaded & heat-treated

-e.....-..--&

0,5 0,6 0,7 03 089 wavelength, pm

Figure 2: The transient 670 nm absorption band in KS-4V fiber pieces which underwent different treatments and were then irradiated to 297 Gy.

477

During the three days of irradiation run #2 a large share of H2 molecules entered into the glass net through radiolytic reactions. This is manifested by a tremendous rise of the OH- group absorption at 1.38 pm in the KS-4V fibre, from 45 to 1600 d I 3 h (Fig. 1). Note that this effect is not observed in the case of low-dose irradiation of H2-loaded fibers. We also notice an absorption band of radiolytic Si-H bonds centered at 1.53 pm which merges with the OH-group absorption band. At the same time, there is no more molecular hydrogen in the fiber; the 1.24 pm band is absent.

After irradiation run #2, the absorption in the visible spectral region in the H2-loaded fiber pieces, both with KS- 4V and KU cores, turned out to be more than an order of magnitude lower than the absorption in the untreated fiber pieces (Fig. 3). Concurrent with the drastic radiation hardening in the visible and near-IR regions, the H2-loaded fiber pieces featured a higher induced absorption in the W region, which was probably due to the E 7 ~ center [8] known also as H(1) center [ 131. This center is an E’ center modified by a nearby hydrogen, and its absorption band peaks in the UV region. The UV tail in the induced absorption is larger for the KS4V fibre. This fact can be explained if we recall that the precursors of the E’P center are oxygen-deficient centers (oxygen vacancies), whose concentration in KS4V silica is larger by an order of magnitude than that in KU silica.

As we saw, the stockpile of molecular hydrogen in the fibers was exhausted during irradiation run #2. Nevertheless, the H2-loaded fibers retained clear superiority over their untreated counterparts after irradiation run #3 as well (Fig. 3). Losses in H2-loaded fibers after run #3 still lie below losses in the corresponding untreated fibers after run #2, which is a spectacular demonstration of the radiation hardening effect previously revealed in [ 101 and [ 1 11. The losses after run #3 make it clear that the fibers can be hardened to a greater extent, if the H2-loading and pre- irradiation procedures are repeated several times. In fact, behavior of the H?-loaded KU fiber shows that -

E m Y

U . A

1. I

0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 wavelength, pm

Figure 3: Total loss in the fibers tested measured after irradiation ~ u n s #2 (denoted ”1.7 MGy”) and #3 (”3.4 MGy”). Symbol “Hz“ signifies that the fiber was hydrogen-loaded before irradiations without preliminary heat treatment.

the NJ3OHC precursors, such as peroxy linkages and strained Si-0 bonds that do not recombine on being disrupted by radiation, make a major contribution to NBOHC. At the same time, the effect of radiolytic splitting out of hydrogen from OH groups is not so significant. By repeating H2-loading and irradiation again, it may be possible to eliminate the first two types of NBOHC precursors completely, while the resultant increase in OH-group concentration must not strongly affect radiation-induced absorption. Therefore, cyclic pre-treatment of fibers via Ha-loading and irradiation as well as optimization of the pre-treatment regimes (dose, dose rate, etc.) become an interesting matter for subsequent experiments.

The possibility of radiation hardening of fibers via cyclic H2-loading and irradiation returns us to the question of the best suited fiber core material. Quantitatively, the H2-loaded fibers of the both types behave similarly both at 1.7 and 3.4 MGy (Fig. 3), the KS-4V fiber being preferable in the red region, while the KU fiber, in the blue region.

Finally, note that the behavior of the untreated fibers after irradiation run #3 was in a good agreement with the previous observations [2], [3]: the loss in the KS-4V fiber remained practically unchanged, while the NBOHC absorption in the KU fiber had grown further. The heat-treated pieces of the KS-4V fiber showed practically the same loss spectra after runs #2 and #3 as the untreated piece; therefore, the corresponding loss spectra are not depicted in Fig. 3.

IV. Conclusion H2-loading is shown to be a very efficient tool for

radiation hardening of pure-silica-core fibers and fibroscopes with KU and KS-4V cores intended for use at megagray dose levels in the visible region. H2-loading totally suppresses the 670 nm transient radiation-induced absorption band in fibers with a KS-4V core. After y-irradiation to 1.7-3.4 MGy(Si), H2-loaded KS-4V and KU fibers demonstrate comparable losses, which are far lower than the losses in unloaded fibers. At the same time, Ha-loading increases radiation-induced absorption in the UV region and slightly in the blue region, owing to the H(1) centers.

In the process of irradiation of H2-loaded fibers to a megagray dosage, a large share of hydrogen enters into the glass net, thereby eliminating the precursors of radiation color centers. As the result, a pre-irradiated H2-loaded fiber demonstrates much weaker absorption after subsequent irradiations even in the absence of molecular hydrogen. A cyclic pre-treatment procedure consisting of several steps of H2-loading and irradiation is expected to be a particularly promising radiation-hardening technique for fibers and fibroscopes to be used in the nuclear industry.

V. Acknowledgment Work by A.Tomashuk was supported by Russian Basic

Research Foundation (grant 97-02- 16552).

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