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We compared the sensitivity to X-rays of several fiber Bragg gratings (FBGs) written in the standard telecommunication fiber Corning SMF28 with different techniques. Standard gratings were manufactured with phase-mask and UV lasers, continuum wave (cw) at 244 nm or pulsed in the nanosecond domain at 248 nm, in a pre-hydrogenated fiber. Others gratings were written by exposures to a femtosecond IR-laser (800 nm), with both phase-mask and point by point techniques. The response of these FBGs was studied under X-rays at room temperature and 100°C, by highlighting their similarities and differences. Independently of the inscription technique, the two types of fs-FBGs have showed no big difference up to 1 MGy(SiO2) dose. A discussion on the causes of the radiation-induced peak change is also reported.
© 2015 Optical Society of America
1. Introduction
A fiber Bragg grating (FBG) is a periodic modulation of the refractive index along the fiber length, performed by exposing some core regions to an intense laser beam, through different techniques, as the phase-mask (PM) [1] or point by point (PbP) [2]. This index perturbation causes the reflection of a narrow band of wavelengths centered on a value, called Bragg wavelength (λB) and defined as [3]:
where neff is the effective refractive index of the fundamental mode, Λ is the grating period and m is the diffraction order. The Bragg wavelength depends on external parameters, such as temperature and strain, and as a consequence these components are often used as sensors, even in harsh environments associated to nuclear industry or space [4]. However, the ionizing radiation present in these environments influences the Bragg peak response: the most important effect is the radiation-induced Bragg wavelength shift (RI-BWS) [5], which causes an error on the sensing parameter measurement and depends on a lot of parameters, such as fiber composition, pre-treatments, writing and irradiation conditions.
For this reason, in this paper we compare the radiation-sensitivity of gratings written in the most widely used standard telecommunication Germanium-doped single-mode fiber, the Corning SMF-28, with different techniques.
The gratings written with ultraviolet (UV) lasers, continuous wave or pulsed ones in the nanoseconds width domain, are the oldest and the most commonly used FGBs. The refractive-index change, for these gratings, is associated with the generation of defects, through the Kramers-Kronig dispersion relation. In particular, the photosensitivity of non-hydrogenated Ge-doped fibers is mainly related with the UV-induced bleaching of the absorption band around 244 nm of germanium oxygen deficient centers. If a H2-loading pre-treatment is used to enhance the fiber photosensitivity before the grating inscription, more processes are activated leading to H-bearing species. Moreover, if the laser emits nanosecond pulses, not only one-photon but also two-photon processes become very likely [6].
The use of ultrafast UV or infrared (IR) lasers (femtoseconds pulse duration) makes it easier to produce high peak power densities and, as a consequence, the defect formation can result from multi-photon absorption processes [7]. However, depending on the used laser power density, the refractive index change can be also associated with a structural change, i.e. densification or stress redistribution, as we tried to demonstrate in one of our recent publications [8]. Their high-temperature stability is linked to these particular inscription conditions [9].
Recently, it was observed that by inscribing a grating point by point with fs-radiation at 800 nm the power density obtained at the focus area is one order of magnitude higher than that for the grating inscribed with the phase-mask technique [10]. This leads to refractive index modifications with complex morphology, consisting of sub-micron sized voids in a shell of densified glass [11, 12], and a higher thermal stability [13].
Until today, several research groups focused their attention on the radiation-effects on the different grating types (see references in the review paper [5]), however, the latter type of gratings (written point by point with fs-laser in the IR) is relatively new and only a recent work [14] presents some preliminary results about the BWS induced by γ-rays on this FBG type. In this paper, we will compare the radiation-induced effects on all these types of gratings, including the last one.
2. Experimental details
The gratings were written in either the fiber Corning SMF-28 or SMF-28e. The main difference between these two fibers is the OH content, which is slightly lower in the SMF28e to permit a wider spectral transmission for metropolitan networks [15]. The gratings were written by three different laboratories, with different lasers and techniques.
The iXfiber SAS in Lannion (France) used two UV lasers and two different phase masks (PM) for the grating writing in the fiber SMF28. The KrF laser emits 10 ns long pulses at 248 nm, with a repetition rate of 200 Hz. The frequency doubled Ar-ion laser is a cw laser working at 244 nm. The used fiber shows low photosensitivity at the UV laser’s wavelength, because of its low Ge-content (about 5 wt%). As reported in [6], the refractive index modulation (Δnmod) induced in such a fiber by a laser light at wavelengths longer than 193 nm is typically ~ for a cumulated fluence of 100 kJ/cm2, but it can be enhanced by pre-inscription H2-loading. For this reason, before the grating writing, the fibers were loaded with hydrogen at pressure of 200 bars and temperature of 80°C, for 60 hours.
The laboratory Hubert Curien of Saint Etienne (France) writes gratings with a phase mask and an ultrafast Ti:sapphire laser emitting 150 fs long pulses at 800 nm with a repetition rate of 1 kHz and 560 µJ energy. The ultrafast laser was focused on the core of the SMF28e fiber by the use of a cylindrical lens having a focal length of 10 mm and, to homogenize the induced refractive index in the core, a scan of the beam over the whole fiber core was performed at 2.5 Hz frequency and 12 µm range.
Other FBGs were written point-by-point using focused laser pulses at 800 nm, with pulse duration lower than 120 fs and 230 nJ pulse energy. A detailed description of the inscription setup and technique is given in [16]. The fs laser pulses were focused within the fiber core by an oil-immersion objective lens with numerical aperture (N.A.) of 0.8.
It is well known that the temperature dependence of gratings is slightly nonlinear over a large range [3]; however, it can be considered as linear for [25°C – 100°C] temperature range and the corresponding coefficient of all the gratings is (10.17 ± 0.09) pm/°C, independently of the inscription technique.
All the gratings are 10 mm long, with the exception of one grating type written with the 248 nm laser light, whose length is 1 mm. An overview of the grating inscription parameters is reported in Table 1, together with an estimate of Δnmod obtained from the initial value of the peak amplitude. We note that all the gratings are characterized by a Δnmod value of about 10−4, except for the 1 mm long grating written with 248 nm ns-pulsed laser. The Δnmod of this FBG is about 10−3 and it was obtained by increasing both the pulse energy and the writing time, compared with the 10 mm long grating written with the same laser.
Table 1. Inscription parameters for the different gratings. 𝚫nmod is the value of the modulation amplitude of the refractive index, calculated from the peak amplitude recorded before irradiation. It cannot be calculated for the PbP grating, as it has a very non-uniform refractive index profile.
Moreover, to compare the radiation-response of the gratings, independently of the fiber H2-loading, they were all subjected to the standard thermal treatment usually performed by the iXfiber SAS. It consists of a first annealing step at 220°C for 10 minutes to facilitate the outgassing of residual H2, when present, and a second one at 120°C for 8 hours to ensure the grating thermal stability at the irradiation temperatures. The duration of this latter treatment was estimated from the data present in literature on the thermal stability of UV-grating [17], known to be less stable than the fs-ones [18]. The gratings were placed in the oven, once its temperature was already stable at the required value. The gratings were not recoated before irradiation.
The irradiation tests were performed by using the 10 keV X-ray machine Probix at CEA in Arpajon (France), both at room temperature (RT, around 27°C) and 100°C, with a dose rate of 50 Gy(SiO2)/s. The accumulated dose is fixed to 1 MGy or 3 MGy, depending on the experiments. We showed in previous publications through several spectroscopic techniques that these X-ray tests are representative of γ-ray tests for optical fibers and well adapted to the irradiation at high doses of small lengths of optical fibers [19].
During an irradiation test, gratings of each type were studied simultaneously: they were fixed on the heating plate of the irradiation system and their transmission spectra were recorded in situ by using a tunable laser (Tunics Plus, NetTest) and a high performance optical tester with a wavelength resolution of 1 pm (Yenista Optics CT400). Since the acquisition device has only four independent channels and the number of studied gratings was five in total, two FBGs with distinct Bragg wavelengths were connected in series. To monitor the temperature during the tests, two thermocouples were fixed on both sides of the gratings: the temperature variations, for all the irradiation conditions, were lower than 0.5°C.
The Bragg wavelength position has been determined by using a third order polynomial approximation (for a possible peak asymmetry) on N points of the transmission spectrum near the main dip, with N greater than 70 and depending on the peak amplitude. In the case of the fs-PbP-FBGs, whose peak is saturated, the result of this polynomial fit is not adapted. Therefore both flanks around 3 dB above the peak have been fitted with straight lines, to determine the wavelength values at which the transmission value is 3 dB above the peak and then to calculate the Bragg wavelength as their average value, since it was verified that the peak shape of these gratings does not change during the experiments. The grating amplitude is calculated as the difference between the value of the measured and the reference spectra, both calculated at the Bragg wavelength. The reference spectrum is the transmission spectrum of the fiber without a FBG and it is simulated by a high-order polynomial performed over all the spectrum range without the portion of about 10 nm around the peak wavelength. This procedure eliminates the effect of the fiber transmission degradation.
3. Results
Figure 1 reports the Bragg wavelength shift induced by the radiation at the fixed temperature of 27°C.
Fig. 1 Radiation-induced Bragg wavelength shift (RI-BWS) at RT associated with a period of X-ray exposure up to 1 MGy dose followed by a recovery period, for gratings written with different techniques and lasers. The vertical dashed lines indicate the start and the end of the irradiation.
The UV-FBGs show a larger radiation induced red-shift with respect to the fs-FBGs. However, these FBGs seem to present a saturating behavior at a value that depends on the grating inscription parameters. The saturating RI-BWS value is lower for the 10 mm long grating written with 244 nm cw laser than for that produced with 248 nm ns-pulsed laser light (60 pm and 150 pm, respectively). Moreover, between the two gratings written with the 248 nm pulsed laser, we observe that when Δnmod decreases almost by a factor of 10, the BWS induced by the radiation at a 1 MGy dose decreases by a factor of almost 2. Their radiation-sensitivity is clearly dependent on the different inscription parameters. The RI-BWS of the gratings written with fs-radiation is lower than for the UV-FBGs; as a consequence, to better highlight the small variations, Fig. 2 shows these gratings’ BWS.
A similar response to radiation is observed for the two gratings written by fs-radiation at 800 nm with two different techniques, phase-mask and point by point. The Bragg wavelength shift rapidly increases (i.e. red-shifts) up to 20 pm at the accumulated dose of 40 kGy and then remains constant. During the recovery, the BWS reduces and then stabilizes at 6 pm and 13 pm for fs-PM and fs-PbP, respectively.
The radiation does not only influence the peak position but also the peak amplitude, as shown in Fig. 3. The peak amplitude increases during the irradiation for the UV-FBGs, up to 3 dB for the 10 mm long grating written with 248 nm laser source, whereas the amplitude remains constant within 0.5 dB for the fs-FBGs. We note that a peak amplitude increase of 3 dB for the 248nm-10mm FBG corresponds to an only 6% increase of Δnmod.
Fig. 3 Variation of the peak amplitude as a function of the time from the irradiation start at 27°C up to 1 MGy dose. The vertical dashed lines indicate the start and the end of the irradiation.
The same experiment was performed on other pristine gratings at a higher irradiation temperature of 100°C. Figures 4 and 5 report the corresponding effects on both the peak position and amplitude. The RI-BWS was calculated by subtracting to the Bragg wavelength value the one obtained when the grating temperature was stable at 100°C, just before the irradiation start.
Fig. 4 Radiation-induced Bragg wavelength shift at 100°C associated with a period of X-ray exposure up to 1 MGy dose followed by a recovery period, for gratings written with different techniques and lasers. The vertical dashed lines indicate the start and the end of the irradiation.
Fig. 5 Variation of the peak amplitude as a function of the time from the irradiation start, for irradiation around 100°C up to 1 MGy dose. The vertical dashed lines indicate the start and the end of the irradiation.
The 10 mm long UV-FBGs, written with the two lasers at 244 and 248 nm, show the same behavior for the BWS during X-ray exposure at the high temperature of 100°C: the Bragg wavelength shifts towards longer values with a saturating behavior around 70 pm. The peak amplitude is more reduced for the FBG written at 244 nm than at 248 nm, of about 3 dB. As for the irradiation at RT, the 1 mm long FBG written with the laser source at 248 nm is characterized by a BWS that is about 2 times bigger than the value of the 10 mm long grating: at 1 MGy dose, the BWS is around 120 pm.
In comparison to the UV-FBGs, the peak of the fs-FBGs shifts towards shorter wavelengths during both irradiation and recovery, without showing a change of the slope. This suggests the hypothesis of a possible instability of these gratings at a temperature as high as 100°C; in this case, the observed blue-shift should be caused mainly by the high temperature held during the experiment and not by the radiation.
The fs-PbP-gratings are known for being stable at high temperatures [13] and, indeed, no peak shift of the pristine grating was observed during a thermal treatment at 100°C (data not reported here). As a consequence, the blue-shift observed for the fs-PbP-FBG is not caused by the high irradiation temperature: it should be a temporary effect of the combined radiation and high temperature. A few days after the irradiation, another thermal treatment at 100°C was carried out on the already irradiated sample and no peak shift was observed, as for the pristine one. However, the fs-PbP-FBGs, already known to be very stable at high temperature, show the lower shift induced by X-rays at both RT and 100°C.
Concerning the other fs-FBG, its stability at 100°C is strongly dependent on the pre-irradiation thermal treatment, as for the classical UV-gratings. To demonstrate that the standard thermal treatment performed before irradiation was not enough to stabilize the grating to work at 100°C for 1 day, a new pristine sample was subjected to a longer thermal treatment at 120°C (for 50 hours). Figure 6 shows the comparison of the radiation effects on the fs-FBGs subjected to the standard annealing and to the longer treatment at 120°C (this grating is indicated with the adjective new in the graph) and it highlights that the previous behavior was due to the grating instability at high temperatures.
Fig. 6 Comparison between the radiation effects on the fs-PM-FBGs subjected to the standard thermal treatment or the longer annealing (50 hours at 120°C) and the fs-PbP-grating. The irradiation was performed at 100°C up to 1 MGy dose. As for the other graphs, the vertical dashed lines indicate the start and the end of the irradiation.
Independently of the inscription technique, during the irradiation the Bragg wavelength of the fs-gratings (fs-PbP and new-fs-PM, the latter annealed for 50 hours) shows an initial fast red-shift up to about 15 pm at 50 kGy, followed by a slower blue-shift down to −10 pm at 1 MGy dose. During the recovery, for the fs-PbP-FBG the BWS continues to reduce (down to −35 pm), whereas for the new fs-PM-FBG a slight blue-shift still appears but it seems to stabilize around −25 pm.
It is well known that the grating radiation-sensitivity depends on the pre-thermal treatment, as we have recently shown for the fs-PM-FBGs [20]. For this reason, all the gratings presented above (except for the new one), underwent a same thermal treatment. However, two non-thermally treated fs-PbP-gratings were subjected to two irradiation runs of 1.5 MGy each, at the same two irradiation temperatures, RT and 100°C, the dose-rate being 50 Gy/s. Figures 7 and 8 display the BWS induced on the non-treated fs-PbP-FBG by the irradiation at RT and 100°C, respectively.
Fig. 7 Bragg wavelength shift induced by the radiation at RT, as a function of the time from the first irradiation start, for a non-pre-treated fs-PbP-FBG. The vertical dashed lines indicate the start and the end of the irradiation runs. In the upper part of the graph the total dose reached after a run is shown, the dose-rate being 50 Gy/s. In the inset, comparison of the radiation-effects at RT on two different fs-PbP-gratings: one was not thermally treated and irradiated up to 1.5 MGy dose (black curve) and the other was subjected to the standard annealing and irradiated up to 1 MGy dose (red curve, results already shown in Fig. 1 and 2).
Fig. 8 Bragg wavelength shift induced by the radiation at 100°C, as a function of the time from the first irradiation start, for a non-treated fs-PbP-grating. The vertical dashed lines indicate the start and the end of the irradiation runs. In the upper part of the graph the total dose reached after a run is shown, the dose-rate being 50 Gy/s. In the inset, comparison of the radiation-effects at 100°C on two different fs-PbP-gratings: one was not thermally treated and irradiated up to 1.5 MGy dose (black curve) and the other was subjected to the standard annealing and irradiated up to 1 MGy dose (red curve, results already shown in Fig. 4 and 6).
As shown in the inset of Fig. 7, during the irradiation up to 1 MGy dose at RT, a pre-treatment of this grating does not influence the radiation-resistance of the fs-PbP-grating: independently of the pre-treatment, the peak shifts towards the red by 17 pm at 1 MGy dose. Thanks to the second run of 1.5 MGy, a positive effect of the pre-irradiation is highlighted; indeed, the BWS increases by about 15 pm after the first 1.5 MGy dose and by less than 5 pm after the second one.
Concerning the irradiation at 100°C, the inset of Fig. 8 compares the RI-BWS on two fs-PbP-FBGs, one thermally annealed (results already shown in Fig. 4 and 6) and another one non-treated. At this irradiation temperature, the BWS induced at the 1 MGy dose is reduced from −22 pm to −5 pm by the pre-irradiation thermal treatment. As for the irradiation at RT, a positive effect of the pre-irradiation is highlighted in Fig. 8: the BWS induced by the first run of 1.5 MGy is about 30 pm towards the blue, whereas it is less than 10 pm after the second run. However, as already mentioned, the most important conclusion that can be inferred from this figure is the combined effect of radiation and temperature on the fs-PbP-FBG peak. Indeed, its Bragg wavelength does not change during a thermal treatment at 100°C, whereas it shifts slightly towards the red during the irradiation at RT (the RI-BWS is lower than 20 pm up to a dose of 3 MGy). During irradiations and recoveries at 100°C, instead, the peak shifts towards the blue.
4. Discussion
The first clear result is that the radiation induced BWS of UV-FBGs is larger than for the fs-gratings. The main difference in the grating inscription is the H2-loading used to enhance the fiber photosensitivity to the UV-light. Indeed, the H-bearing defects induced during the grating inscription in the H2-loaded fiber, such as GeH, GeH2 and OH groups, transform into GeE’, H(II) and NBOHCs during the X-rays irradiation, thus increasing the refractive index of both bright and dark fringes, which entails the increase of neff and Δnmod, consequently of the peak position and amplitude [6]. When the refractive index of the bright fringes saturates, Δnmod and so the amplitude start decreasing, as shown for the 248nm-10mm FBG. In particular, by comparing the 10 mm long gratings written with the UV lasers, larger increases of λB and of the peak amplitude are observed for the grating written at 248 nm than at 244 nm. This implies that more precursors are created by the two UV photons processes induced by the 248 nm pulsed laser than by the one-photon processes induced by the cw light at 244 nm. However, among the UV-FBGs, the largest Bragg wavelength red-shift and the lowest amplitude increase are observed for the 1 mm long grating written with the 248 nm laser. Since the Bragg wavelength does not depend on the grating length, the different behavior under radiation of the two gratings written with the ns-pulsed laser at 248 nm is due to the different inscription conditions. The 1 mm long FBG is characterized by a higher initial Δnmod with respect to the other UV-FBGs by one order of magnitude. As a consequence, the refractive index of the bright fringes is almost saturated and the increase of neff is due to the more efficient defect transformation in the dark fringes.
The fs-FBGs showed higher radiation-resistance than the UV-FBGs. The multi-photons processes induced by the femtosecond laser can generate refractive index changes of the order of magnitude of 10−3 [6], without the need of any technique for the photosensitivity enhancement. The absence of hydrogen eliminates the dynamics linked to H-bearing species, we have already talked about. Moreover, the high peak power density values obtained thanks to the ultra-short pulse duration probably transform all the precursors in defects during the grating inscription, not leaving precursors to be transformed by the X-rays.
The irradiation tests were performed at two different temperatures, to study also the influence of this parameter on the grating radiation-resistance. Some studies reported in literature show that the higher the irradiation temperature the lower the radiation-induced BWS [21]. A similar dependence on the irradiation temperature has been observed also for the radiation-induced attenuation (RIA) in most of fiber types [22]. In particular, for the Ge-doped fibers, it was observed that, after a transient period, the thermal bleaching and the radiation-induced generation of defects reach a stationary regime around a defect concentration that is no longer dependent on time and that decreases with increasing temperature [22]. By applying this reasoning to the peak evolution under radiation, the RI-BWS should show a saturating behavior as a function of the dose (or irradiation time, as in our figures), around a value that decreases by increasing the irradiation temperature, as reported in literature [21] and as observed here for both the FBGs written with the 248 nm laser.
Before both X-ray irradiations all the gratings underwent a thermal treatment at 120°C, higher than both the irradiation temperatures, about 27°C and 100°C. Consequently, the defect precursors present during the irradiations are thermally stable and the RI-BWS dependence on the irradiation temperature can only be associated with the stability of the species generated by X-rays. The saturation behavior is the result of a competition between generation and annealing of the radiation-induced defects [23].
Unexpectedly, the irradiation temperature does not influence the response of the grating written with the 244 nm laser. This effect could be associated with the nature of the writing laser: the conversion of the UV-generated defects under X-rays is not influenced by the irradiation temperature.
Concerning the fs-PM-FBGs, we cannot compare the results obtained at RT and 100°C, as we had to carry out a different pre-irradiation thermal treatment to obtain a new fs-PM-FBG stable at the highest temperature.
Finally, the radiation-response of the fs-PbP-grating was studied on-line at two different irradiation temperatures, RT and 100°C, up to the maximal dose of 3 MGy. This grating is well known for its stability in temperature [13] and it was demonstrated in this paper that it is also stable under radiation at RT: in this case, a very fast increase of the BWS was observed with a saturating behavior around only 20 pm after a dose of 3 MGy. However, during and after the irradiation at 100°C, the peak shifts towards the blue, without showing a saturating behavior at least during the performed 2 days lasting monitoring. The induced shift at 3 MGy dose is about 50 pm. Since this FBG is stable in temperature and, indeed, no grating peak shift was observed during a thermal treatment at 100°C, it seems that the induced blue-shift is not due to the high temperature held constant during the tests but is a consequence of a combined effect of radiation and temperature. More tests have to be performed to understand its origin.
5. Conclusion
We have investigated the influence of the inscription techniques on the radiation resistance of FBGs written in a standard Ge-doped fiber, the Corning SMF-28. The UV-FBGs showed the highest radiation sensitivity, mainly associated with the H2-loading performed before grating inscription to enhance the fiber photosensitivity to the UV light. However, the RI-BWS shows a saturating behavior around a value that depends on the inscription and irradiation conditions. The fs-gratings exhibited a good radiation resistance, independently of the inscription technique. Under radiation at 27°C, the Bragg wavelength shifts by 20 pm after a dose of about 40 kGy and then it does not change anymore up to the total dose of 1 MGy. Under radiation at 100°C, with an appropriate pre-irradiation thermal treatment to assure the grating stability at high temperature, the Bragg wavelength blue-shifts by about 10 pm at the total dose of 1 MGy. By considering these gratings as a temperature sensor, the error on the temperature measurement performed under radiation at a temperature between 27°C and 100°C, is lower than 2°C. However, even if the radiation-effects on the grating peak are negligible within an error of 2°C, the limitation of these sensors is the choice of a standard fiber, doped with germanium and characterized by a RIA value of around 200 dB/km at a dose of 1 MGy and dose-rate of 50 Gy/s [24]. To obtain a good radiation-resistant sensor, the grating should be written with the fs-radiation in a radiation-resistant fiber [20].
Finally, an unexplained instability was observed for the fs-PbP-gratings under radiation at 100°C up to the accumulated dose of 3 MGy.
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