Effect of gamma radiation on the reflectance spectrum of fiber Bragg gratings

Optik 124 (2013) 2246–2250

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Effect of gamma radiation on the reflectance spectrum of fiber Bragg gratings Gongliu Yang, Song Lin ∗ , Jing Jin, Ningfang Song National Key Laboratory of Inertial Technology, Beijing University of Aeronautics and Astronautics, Beijing 100191, China

a r t i c l e

i n f o

Article history: Received 9 February 2012 Accepted 24 June 2012

Keywords: Fiber Bragg gratings Gamma rays Temperature sensitivity coefficient Radiation effects

a b s t r a c t The effect of grating fabrication on radiation sensitivity of the fiber Bragg grating has been investigated experimentally. The FBGs were fabricated in different processes and GeO2 concentrations. H2 -loading was applied to change the radiation sensitivity of the FBGs. The FBGs were fabricated in photosensitivity fiber and coupling single mode fiber with GeO2 concentrations in a range from 6 to 23 mol%. The temperature sensitivity coefficient variation during radiation was studied and temperature-induced Bragg wavelength shift was compensated. 3 types mathematic model were used for fitting curve of radiationinduced Bragg wavelength shift. The lowest Bragg wavelength shift (about 13 pm after a total dose of 50 kGy) was obtained by a grating written in photosensitive optical fiber PSF-GeB-125 with 20.2 mol% GeO2 concentration and without H2 -loading treatment. © 2012 Elsevier GmbH. All rights reserved.

1. Introduction



Fiber Bragg grating sensors (FBGs) are increasingly used in health monitoring of large space structures and nuclear power plants because of their characteristics of low mass, small dimensions and immune to electromagnetic interference [1]. In order to be used in spacecraft, FBGs must perform with high stable properties during the whole mission time. However, as a factor, radiation will intimidate the stability of FBGs [2]. Motivated by the need of improving radiation tolerance of FBGs in space applications, effort of radiation on fiber Bragg gratings have been studied for more than decade [3,4]. The effect of radiation on an optical fiber is well-known to be an increase of the transmission loss [5,6]. The advantage of a FBG is that information on the measured parameter (temperature or strain) is wavelength-encoded and is therefore insensitive to radiationinduced loss. The sensing mechanism of the Bragg grating is well known [7,6]. Basically, the periodic perturbation of the refractive index along the fiber within the FBG acts as an in-line stop-band optical transmission filter. The largest optical reflection coefficient for the filter occurs at the Bragg wavelength given by B = 2neff 

(1)

dneff dneff d d dB =2 (n ) = 2 + 2neff = 2 + 2neff ˛ dT dT eff dT dT dT

∗ Corresponding author. Tel.: +86 10 82316906 821; fax: +86 10 82316906 818. E-mail address: [email protected] (S. Lin). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.06.083



= B

 KT = B



1 dneff + ˛ T neff dT

(3)



1 dneff +˛ neff dT

(4)

where T = T − T0 is the temperature variation in working environment, T is the current temperature value, T0 is the initialization temperature value, B = B (T) − B (T0 ) is the Bragg wavelength shift (BWS), ˛ is the thermal expansion coefficient, (1/neff )(dneff /dT) is the thermally induced refractive index change, and KT is the temperature sensitivity coefficient. As a temperature sensor, the return Bragg wavelength B will shift proportionately to the variation of the temperature. The shift of the Bragg wavelength as a function of temperature is given by B (T ) = B (T0 ) + KT (T − T0 )

(5)

where KT is the temperature sensitivity coefficient. If the values of B (T0 ) and KT is given, the temperature of the FBG can be computed as: T = T0 +

(2)



dB 1 dneff T = 2neff  B = + ˛ T neff dT dT

B (T ) − B (T0 ) KT

(6)

When FBG works in the radiation environment, defects generated by ionizing radiation affect the effective refractive index of gratings, thus resulting in a change of the Bragg wavelength B (T) → B (d, T), where d is the total radiation dose. The application of FBGs for

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Table 1 Parameters of the fiber Bragg gratings. Grating label

Fiber

Core dopant (mol%)

H2 -loading

Wavelength (nm)

Length (mm)

Reflectivity (%)

FWHM (nm)

1-1 1-2 2-1 2-2 3-1 3-2 4-1 4-2 5-1 5-2

PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 CS 1060 CS 1060

GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2

No Yes No Yes No Yes No Yes Yes Yes

1520 1520 1530 1530 1540 1540 1555 1555 1550 1545

10 10 11 11 10 10 11 11 12 12

87.125 86.358 84.605 89.102 83.126 88.132 86.304 87.140 84.396 85.265

0.26 0.22 0.25 0.23 0.27 0.25 0.25 0.25 0.24 0.23

(20.59) (20.59) (20.37) (20.37) (20.2) (20.2) (23) (23) (12.1) (6)

temperature sensing in radiation environments depends on to what extent B (T) and KT are sensitive to radiation [6]. Considering the effect of radiation and temperature on FBGs simultaneous, the shift of Bragg wavelength as a function of radiation and temperature is given by B (d, T ) = 2n(d, T )(d, T )

(7)

where d is the total radiation dose, T is the current temperature value. Perform Taylor series expansion to formula (7), and ignore temperature dependent high order terms, then only terms up to second order in the radiation induced wavelength shift are preserved, one can obtain



B (d, T ) = 2n(d0 , T0 )(d0 , T0 ) + 2 



∂n ∂ +n ∂d ∂d



d d=d0 ,T =T0



∂2 n ∂2  2 + 2  2 + n 2 (d) ∂d ∂d



+2 

∂n ∂ +n ∂T ∂T

2.2. Irradiation conditions



T

(8)

d=d0 ,T =T0

where B (d0 , T0 ) = 2n(d0 , T0 )(d0 , T0 ), formula (8) could be transformed to



∂n ∂ B (d, T ) = 2  +n ∂d ∂d







d + 2  d=d0 ,T =T0



Hydrogen loading (one week at 2 kMPa and 50 ◦ C) was used to increase the photosensitivity of CS 1060 before grating inscription. Four gratings written in PSF-GeB-125 are also with H2 -loading treatment, which were used for comparing the effect of H2 -loading on the radiation sensitivity of FBGs. The gratings were fabricated using a phase mask method with an excimer laser (BraggStar Industrial Series, 193 nm, by Coherent Inc.). The stabilized pulse energy density is 7 mJ/cm2 . The acrylate coating was mechanically stripped before grating inscription and all the gratings were recoated after the fabrication. After recoating, the gratings with H2 loading were annealed for 12 h at 120 ◦ C to accelerate out-diffusion of the remaining hydrogen. The grating lengths of FBGs were about 10–12 mm, the reflectivity were about 83.126–89.102% and the full-width at half-maximum (FWHM) were about 0.22–0.25 nm. The Bragg wavelengths of the gratings were from 1520 nm to 1555 nm. Table 1 shows the parameters of the FBGs.

∂2  ∂n ∂ 2 +n +n 2 (d) + 2  ∂T ∂T ∂d

∂2 n ∂d2



The FBGs were irradiated using a Co60 source at a dose-rate of 0.1 Gy/s up to a total dose of about 50 kGy. A ferrous sulfate dosimeter was used to measure the dose-rate before the experiment. It is 10−3 mol/L ferrous sulfate solution from 0.4 mol/L preparation of sulfuric acid. Under the radiation of the ␥-rays, Fe2+ is oxidized to Fe3+ . By measuring the number of Fe3+ formed and using of the known G(Fe3+ ) value, the absorption dose can be measured. 2.3. Experimental setup

T

(9)

d=d0 , T =T0

The first two terms of formula (9) are radiation induced wavelength shift, and the third term is Bragg wavelength shift induced by temperature variation. In this paper, the effect of grating fabrication on radiation sensitivity of FBGs was investigated experimentally. 10 FBGs were fabricated in different fibers and with different GeO2 concentration, respectively. H2 -loading treatment was applied to change the radiation sensitivity of the FBGs. The variation of temperature sensitivity coefficient and Bragg wavelength shift were addressed. 2. Experimental procedure

The FBGs were mounted on a 5-mm-thick aluminum plate. In order to compensate the effect of the temperature fluctuations during irradiation, three Pt1000 temperature sensors were placed on the aluminum plate near the FBGs. The temperature measured circuit was placed behind of a 5-cm-thick lead brick, and the accuracy of the measured circuit is 0.1 ◦ C. The gratings were kept unstrained during the irradiation to avoid complications related with the temperature-strain cross-sensitivity. The schematic view of the irradiation setup is shown in Fig. 1. It relies on a FBG interrogation system SM125 from Micron Optics with a working wavelength range from 1510 to 1590 nm, wavelength resolution is 1 pm, wavelength stability is 1 pm, wavelength repeatability is 0.5 pm at 1 Hz. To reduce the number of

2.1. Grating fabrication 10 FBGs which were written in two different fiber types were used in the experiment. 2 FBGs were written in coupling single mode optical fiber CS 1060, with the GeO2 concentrations between 6 and 12 mol%. 8 FBGs were written in photosensitive optical fiber PSF-GeB-125 with GeO2 concentrations between 20 and 23 mol%. Both the CS 1060 and PSF-GeB-125 are manufactured by Yangtze Optical Fibre and Cable Company Ltd. (YOFC, China). All the ten FBGs were fabricated by Micro Optics Int. (MOI).

Fig. 1. Schematic view of the irradiation setup.

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measurement channels, spectrally separated Bragg gratings were spliced in chains with a distance of ∼0.3 m between the gratings. The spectral separation of the gratings in a chain was ∼10 nm with the Bragg wavelength in a range of 1520–1555 nm. 3. Experimental results and discussion 3.1. Temperature variation during irradiation The temperature variation during the irradiation experiment is shown in Fig. 2(a). The temperature variation from 25.8 ◦ C to 26.5 ◦ C, the difference is smaller than 1 ◦ C. The curve in Fig. 2(b) shows the Bragg wavelength shift of FBG2-2 during irradiation experiment. The positive values represent the peak shift toward the longer wavelengths. The graph shows an increase of the Bragg wavelength during the irradiation. To get the Bragg wavelength shift induced by irradiation, the temperature-induced wavelength shift should be eliminate from the B (d, T). Before that, the influence of radiation on the temperature sensitivity coefficient should be studied.

Fig. 3. The Bragg wavelength shift of FBG3-1 during temperature cycling experiment before radiation.

3.2. Temperature sensitivity coefficient variation during irradiation To get the change of temperature sensitivity coefficient of the FBGs during the radiation experiment, all the gratings were put in an temperature controlled oven to carry a temperature cycling by heating up to 50 ◦ C and then cooling down to 0 ◦ C slowly, with an incremental step of 10 ◦ C. The temperature cycling was carried out before the irradiation and after the end of irradiation to address a possible radiation effect on the temperature sensitivity coefficient. The Bragg wavelength B (T) was considered as a function of the temperature measured with Pt1000 temperature sensors. The temperature sensitivity coefficients KT were calculated by least squares method. A typical Bragg wavelength shift during temperature cycling experiment is shown in Figs. 3 and 4. The temperature sensitivity coefficient fitting line is shown in Fig. 5. The baseline is the fitting line before radiation, and the dashed line is the fitting line after radiation. It can be see that the temperature sensitivity coefficient after radiation is little higher than the one before radiation. The variations of the temperature sensitivity coefficient of the FBGs are summarized in Table 2. The fitting accuracy of the temperature sensitivity coefficient is ±0.20 pm/◦ C. The temperature sensitivity coefficient variations of FBGs are between −4.27% and 5.76%, without noticeable dependency of fiber dopants.

Fig. 4. The Bragg wavelength shift of FBG3-1 during temperature cycling experiment after radiation.

Fig. 5. The temperature sensitivity coefficient fitting line of FBG3-1.

From Table 2, all the temperature sensitivity coefficient variation of FBGs were within ±6%. The temperature fluctuations caused by radiation-induced temperature sensitivity coefficient variation were within 0.06 ◦ C, less than the temperature accuracy (0.1 ◦ C) of the temperature sensor Pt1000. Therefore, the temperature sensitivity coefficient variation could be neglected during the radiation, and the sensitivity coefficient measured before radiation could be used directly for the temperature-induced Bragg wavelength shift compensation. 3.3. Temperature-induced Bragg wavelength shift compensation The total Bragg wavelength shift of FBGs in radiation environment could be described by B (d, T ) = B (d) + B (T ) = B (d) + KT TB

Fig. 2. (a) The temperature variation during irradiation experiment. (b) Bragg wavelength shift of FBG2-2 during irradiation experiment.

(10)

The temperature effects during the irradiation were compensated by subtracting the temperature-induced Bragg wavelength shift B (T) from total Bragg wavelength shift B (d, T) to obtain the radiation-induced Bragg wavelength shift B (d). Curve 1 in

G. Yang et al. / Optik 124 (2013) 2246–2250

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Table 2 Change of temperature sensitivity coefficients of FBGs. Grating label

Fiber

Core dopant (mol%)

H2 -loading

Temperature sensitivity coefficient (before irradiation) (pm/◦ C)

Temperature sensitivity coefficient (after irradiation) (pm/◦ C)

Change of temperature sensitivity coefficient (%)

Max Bragg wavelength shift (pm)

1-1 1-2 2-1 2-2 3-1 3-2 4-1 4-2 5-1 5-2

PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 PSF-125 CS 1060 CS 1060

GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2 GeO2

No Yes No Yes No Yes No Yes Yes Yes

10.8958 11.9389 10.8543 12.7122 10.9334 11.8010 10.4027 9.4232 12.0525 11.9434

11.4258 12.3819 10.4611 12.3097 11.5633 12.0511 10.2044 9.2678 12.6463 11.4332

4.86% 3.71% −3.72% −3.17% 5.76% 2.12% −1.91% −1.65% 4.93% −4.27%

15 40 14 31 13 26 25 46 36 32

(20.59) (20.59) (20.37) (20.37) (20.2) (20.2) (23) (23) (12.1) (6)

Table 3 The radiation-induced Bragg wavelength fitting expressions. Model

Expression

Quadratic polynomial model

B quad (d) = a × d + b × d + c

Exponential model

B exp (d) = a × exp(b × d) + c × exp(e × d)

Power model

B

Fitting parameters 2

power (d)

= a × db

a = −1.551e−005 b = 0.001361 c = 0.004481 a = 0.02554 b = 0.006871 c = −0.02602 e = −0.105 a = 0.006223 b = 0.4585

Fig. 6 is the total Bragg wavelength shift B (d, T), and curve 2 is the radiation-induced Bragg wavelength shift B (d). 3.4. Radiation-induced Bragg wavelength shift The radiation-induced Bragg wavelength shift of the FBGs is summarized in Table 2. The highest Bragg wavelength shift (about 46 pm after a total dose of 50 kGy) was obtained by FBG4-2, with 23 mol% GeO2 concentration and H2 -loading treatment. The lowest Bragg wavelength shift (about 13 pm after a total dose of 50 kGy) was obtained by FBG3-1, with 20.2 mol% GeO2 concentration and without H2 -loading treatment. 8 FBGs were used to investigating the influence of H2 -loading on the Bragg wavelength shift. Both the FBG1-1 and FBG1-2 have the same GeO2 concentration and the difference is FBG1-2 with H2 -loading treatment. The relationship between FBG2-1 and FBG22, FBG3-1 and FBG3-2, FBG4-1 and FBG4-2 are the same with the FBG1-1 and FBG1-2. According to the Bragg wavelength shift listed in Table 2, it can be conclude that FBG with H2 -loading treatment had a higher radiation sensitivity. The result is in agreement with the conclusion reported in [8,9]. To study the regularity of Bragg wavelength shift, quadratic polynomial model, exponential model and power model were

Fig. 7. The radiation-induced Bragg wavelength shift and fitting curve (FBG5-1).

used for fitting curve according the experimental data. The fitting expressions are listed in Table 3, and the coefficients were within 95% confidence bounds. A typical radiation-induced Bragg wavelength shift and fitting curve are shown in Fig. 7. The residual error of exponential model is small than the residual error of quadratic polynomial model and power model. The regularity of radiationinduced Bragg wavelength shift is approximately of the exponential form. 4. Conclusion

Fig. 6. Temperature compensation during irradiation (FBG1-2).

The purpose of our work was to investigate the effect of radiation on the temperature sensitivity coefficient of fiber Bragg gratings during ␥-irradiation and to study the radiation-induced Bragg wavelength shift of FBGs. Such studies could also improve the understanding of the formation mechanisms of fiber Bragg gratings. The temperature sensitivity coefficient variations of FBGs are between −4.27% and 5.76%, without noticeable dependency of fiber dopants. The temperature fluctuations caused by radiationinduced temperature sensitivity coefficient variation were within

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0.06 ◦ C, less than the temperature accuracy (0.1 ◦ C) of the temperature sensor Pt1000. Therefore, the temperature sensitivity coefficient variation could be neglected during the radiation, and the sensitivity coefficient measured before radiation could be used directly for the temperature-induced Bragg wavelength shift compensation. The experiment data confirm previous findings that gratings written in the H2 -loading fibers show a higher radiation sensitivity, and H2 -loading can increase the Bragg wavelength shift by about 14–25 pm. The regularity of radiation-induced Bragg wavelength shift is approximately of the exponential form. The lowest Bragg wavelength shift (about 13 pm after a total dose of 50 kGy) was obtained by a grating written in photosensitive optical fiber PSF-GeB-125 with 20.2 mol% GeO2 concentration and without H2 -loading treatment.

[2]

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Acknowledgement This research is supported by the National Natural Science Foundation of China (Grant No. 61007040).

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