Fiber-Bragg Grating Radiation Test Results

In this section the results of the FBG radiation test are presented. As discussed in section 6.2.2 two different types of FBG sensors were tested, UV written gratings in Ge doped fibers and fs-IR written gratings in PSC fibers.

UV-Written FBG

A total dose radiation test with gamma radiation was carried out for commercially available UV-written FBG sensors. For those gratings, the standard writing process with hydrogen loading is applied. The commercial gratings which were used for testing had different packages. Sensor #101 and sensor #103 had an SS304 stainless steel tube with an outer diameter of 4.7 mm and 3.1 mm respectively, sensor #107 had no mechanical package. For each type of grating three samples were tested. All tests were carried out with a Co-60 source at theFraunhofer INT institute in Euskirchen, Germany.

The radiation test results given in figure 6.21 on the left indicate a high shift in Bragg wavelength for the high dose rate test. The observed shift is too high to use the sensors for the planned activity for which a shift of±5 pm, corresponding to approximately±0.5 °C, is allowed. The assumed TID of 25 Mrad behind the satellite’s panels would result in a Bragg wavelength shift of approximately 200 pm, corresponding to a temperature error of 20°C. It was also observed that the packaging of the sensor (bare fiber, 3 mm thick stainless steel tube and 5 mm thick stainless steel tube) has only a minor effect on the induced wavelength shift. It is interesting that the FBGs irradiated with a lower dose rate (4.1 krad/h) showed a shift of app. 10 pm after 250 krad whereas the tested FBGs with a higher dose rate (420 krad/h) showed a shift of approximately 20 pm for the same total dose. So a dose rate dependent effect was observed and it can be assumed that the effect in the real environment due to lower dose rates is quite smaller as in the test here. Nevertheless, the risk that a shift higher than ±5 pm (corresponding to ±0.5 °C) will be observed during mission life time is seen as to high so the sensors and the writing technology were rejected.

Figure 6.21: Left: Wavelength shift at high dose rate gamma radiation test of UV-written FBGs protected with different packages. Right: Comparison between high dose rate and low dose rate test, a dose rate dependent effect is observable.

Femto-Second Infrared (fs-IR) Written FBG

The radiation test setup for the fs-IR written FBGs in pure silica core fiber is presented in figure 6.15. A reference grating is placed in a shielded and temperature stabilized loca-tion within the test facility. The acrylate protecloca-tion coating of the reference grating was removed and the bare FBG was glued with a stiff adhesive to the aluminum transducer.

The grating is queried in parallel to the FBGs under test with the same interrogation system.

The temperature variation of the reference grating was measured to approximately 0.7^◦C during the full test. The temperature sensitivity of the gratings which were glued to the transducer made out of aluminum was estimated to be approximately 40 pm/^◦C.

Hence the overall temperature induced wavelength shift for the reference grating is 28 pm.

The wavelength variation of the reference grating over time is shown in the bottom left diagram of figure 6.22 by the red line. As can be seen, the shift is in the range of 37 pm, larger than the shift calculated before caused by temperature effects. But by considering the temperature profile, a correlation between temperature at the reference sensor and Bragg wavelength shift can be found.

Highly interesting is that the two sensors placed in the low dose rate (LDR) test area follow the trend of the reference sensor. This can be seen by the blue and green curves in the bottom left diagram of figure 6.22. The shift in wavelength of the four sensors placed in the high dose rate (HDR) show identical behavior but with a higher total shift in wave-length. Most of the induced shift in wavelength occurs in the first 24 h of the irradiation time. This effect cannot be caused by radiation effects because also the reference sensor which was completely shielded shows the same effect.

0 2 4 4 8 7 2 9 6 1 2 0 1 4 4 1 6 8 1 9 2 2 1 6 2 4 0 2 6 4 2 8 8

Figure 6.22: Left: Bragg wavelength shift of sensors located in HDR area (top) and sensors in LDR area (bottom) respectively. Right: Wavelength shift of HDR sensors (top) and LDR sensors (bottom) respectively with subtracted shift of reference sensor.

By subtracting the wavelength shift of the reference sensor from the other sensor chan-nels the diagrams on the right side of figure 6.22 are obtained. The top diagram illustrates the deviation of the HDR FBGs, the bottom diagram the deviation of the LDR FBGs respectively. For the LDR FBGs the two curves show no correlation even if the sensors are manufactured identical and are placed in vicinity.

This might conclude that the effect measured here is not a radiation induced effect rather than a mechanical effect changing the birefringence in the fiber at the sensor posi-tion. This small shift in Bragg wavelength was expected, because the used measurement system based on an SLD and a spectrometer, does not include a depolarizer. It was as-sumed that a Bragg wavelength shift due to TID, if present, would be much larger than the observed shift due to birefringence effects. By taking into account the theoretical dis-cussion about birefringence in section 3.4.4, changes in the electrical permittivity tensor will affect the refractive index which consequently changes the Bragg wavelength. Me-chanical stress originating from the combination of stiff adhesive, bare fiber and aluminum transducer can have strong impact to the permittivity tensor. Due to radiolytic effects in the adhesive the effect can be enhanced.

When plotting the wavelength deviation as function of the total ionizing dose the dia-gram given in figure 6.23 is obtained. The blue and green curves indicate the shift of the LDR FBGs which were irradiated up to a TID level of 221.9 krad. For these two gratings the overall shift, including radiation and supposed mechanical effects, an overall value of

±10pm results. The sensors placed in the HDR zone are irradiated up to a TID value of 25 Mrad.

Figure 6.23: Bragg wavelength shift, compensated by reference sensor’s shift for FBGs in LDR and HDR zones respectively.

To proove that the fs-IR writing technique itself is suited for the target application a single FBG without any transducer and adhesive was tested up to a TID of 100 krad. The tested grating was made within the same process lot and showed the same characteristic parameters than the other tested fs-IR gratings, so a representative result is supposed.

This additional test was possible during a gamma radiation test series for the passive optical components and resulted in a maximum shift of 3.6 pm. So, it has been pointed out that the fs-IR written FBGs in the pure silica core fiber can be used for the target

application. Nevertheless, more investigations for the FBGs in the glued condition must be undertaken to identify the resulted high shift in Bragg wavelength. A further step would be to investigate different type if glue, a silicone based glue would eliminate the unknown coupling between structure and FBG sensor due its elasticity.