II.

SCANNING LASER BASED INTERROGATOR

The interrogator module is based on a modulated grating Y-branch (MGY) tunable laser illustrated in detail in Fig. 1. The wavelength tuning of the laser in its spectral range spanning from 1528 nm to 1568 nm (C-band) is done over three input currents, namely the left and right reflector current and the phase current.

Inside the control FPGA (field-programmable gate array), a look-up table is stored wherein the relation between laser wavelengrhs and reflector currents is implemented. During a measurement cycle, the tunable laser scans through the full spectrum and the reflected light from the FBG array is measured by a photodiode. The reflected intensity signal is assigned to the output wavelength again in terms of a look-up table (LuT). Afterwards, the setup wavelength and the reflected signal are evaluated by the FPGA by means of dedicated algorithms. When the shift in Bragg wavelength is determined by the FPGA, the assignment to a physical temperature will be performed.

The measurement system can easily be adapted to more sensor strings by the implementation of additional photodiodes and passive optical couplers as long as the signal-to-noise-ratio (SNR) stays at an adequate level.

Fig. 1:

Fiber-optic interrogator based on a tunable laser with control FPGA, optical circulator, fiber-Bragg gratings sensors and photo amplifier (photodiode plus transimpedance amplifier).

III.

FBG MEASUREMNT PRINCIPLE

As fiber-optical sensor the fiber-Bragg grating (FBG) sensor is used. This sensor allows measurements of temperature and/or strain without electrical signals. A grating structure with a modulated refractive index along the fiber axis is written by a UV-excimer laser (193 nm or 248 nm) in a photosensitive fiber [10, 11]. Another manufacturing process is the writing of the grating by femto-second infrared (fs-IR) pulses [12, 3, 9].

After the writing process a wavelength dependent band-stop filter is formed in the fiber whereof the Bragg-wavelength can be approximated to

where λ_B is the Bragg-wavelength, n_eff is the effective refractive index and Λ is the grating pitch. Only light corresponding to the Bragg-wavelength λ_B is reflected back to the light source, light outside the Bragg-wavelength can pass the FBG without any loss. This is illustrated in detail in Fig. 2.

Fig. 2:

FBG with pitch period Λ and refelected light (top). Bottom, left to right: A broadband optical light is coupled into the FBG, and only light with the Bragg-wavelength is reflected (middle). The transmitted light shows a dip at the wavelength position where the reflection occurs.

The fact that light which does not correspond to the Bragg-wavelength can pass the FBG sensor is used to build a multi-sensor wavelength-division multiplexing (WDM) system. In this way many sensors can be read out with a single interrogator module and no additional switching components. The Bragg-wavelength of each sensor in the fiber must differ from each other and must be adequately spaced in wavelength to avoid a mixing of the sensor responses due to strong differences in sensor temperatures. The principle of forming a WDM FBG sensor array is shown in Fig. 3.

Fig. 3:

FBG sensor array is illuminated with a broad light source (top left) and the transmission (right side) and reflection spectra (bottom left) are shown.

The number of embedded FBGs is limited by the optical broadness of the illuminating light source and by the necessary spectral bandwidth of each sensor. The laser can be tuned within a bandwidth of 40 nm and by taking into account the fact that one sensor needs a bandwidth of approximately 4 nm to cover the full temperature range from –100°C to +100°C, the maximum number of sensors per string is given to ten sensors.

The shift in Bragg-wavelength according to equation (2) can be split up in two terms. The first term is affected by the applied strain to the sensor and the second term is affected by changes in sensor temperature. In silica fibers the change in the Bragg-wavelength is given per 95 % by the temperature dependent refractive index change dn/dT, whereas the change in the fiber length due to thermal expansion a amounts only to ≈5 %.

The presented design uses a laser with a central wavelength at 1550 nm, so the shift from the initial Bragg-wavelength due to strain changes Δε and temperature changes ΔT can be computed to [10] :

When the sensor is mounted on a strain insensitive platform on which only temperature changes affect the shift in the Bragg-wavelength, then a conclusion with respect to the temperature at the sensor location can be made. An appropriate transducer design is shown in the next section.

IV.

TRANSDUCER DESIGN

As seen before the FBG sensor is written into an optical fiber, which is general very thin and lightweight but also very sensitive to mechanical forces like twist, stress and shear strains. In addition the shift in Bragg-wavelength depends on temperature and strain applied to the grating. In order to gain mechanical robustness and due to the necessity for strain decoupling a special transducer was designed. A 3D-model is given in Fig. 4. The size of the transducer is 8 mm x 20 mm with a thickness of 1.5mm. The fiber will only be glued onto the middle paddle of the transducer at the fiber position where the grating is located. On the side grooves (shown in the picture in Fig. 4 on the right) only the fiber protection tube is glued. So if the structure where the sensor is mounted is bent, the middle paddle is not affected by this force and the FBG’s Bragg-wavelength stays constant for constant temperatures.

Fig. 4:

3D-model of the designed transducer with its Eigen frequencies (left) in the simulation with extreme values. Picture of the final transducer made out of aluminum with respect to the size or a One Euro coin (right).

The next step was to prove the concept of strain decoupling with the designed transducer. For this an FBG (FBG4) was glued onto the middle paddle and the complete transducer was glued on an aluminum plate. The reflected wavelength from the grating was measured during the plate was bent according to the four-point bending test. In addition to the transducer presented above two additional sensors where glued next to each other, FBG3 implemented in a PEEK hull with vacuum grease (experimental) and FBG5 glued directly onto the aluminum plate acting as reference.

The measurement result is given in Fig. 5 for all three sensors. The blue curve (bottom line) shows the result of the FBG sensor which is directly glued to the aluminium plate. The shift in Bragg-wavelength is quite high; a linear fit resulted in a value of 336.4 pm/mm. The shift of the designed transducer (red mark, middle line) is in the range of 0.8 pm/mm which is a comparable good result. For the transducer mounted into the PEEK hull with the vacuum grease the shift in wavelength is even lower, the linear fit shows a change in Bragg-wavelength of 0.2 pm/mm. The sensitivity of the wavelength shift to temperature can be approximated to 10 pm/K, so also a 0.8 pm shift corresponds only to a temperature error of 0.08 K.

The measurement campaign has shown that the transducer design works pretty good and decouples the mechanical strain from the structure to the FBG sensor. The transducer where the FBG is mounted with the vacuum grease showed also good results, but the implementations of the grease and the thereby possible pollution of other components during assembly phase is seen as risky.

Fig. 5:

Measurement results of the four-point bending test, FBG3 is the sensor glued directly on the plate, FBG4 is the designed transducer and FBG5 the experimental sensor implemented in the protection hull with vacuum grease.

Another important parameter of the transducer is the time constant. In the case here the time constant is defined as the time which is necessary to see 1 K temperature difference between middle paddle (sensor position) and outer frame (mounting interface). The simulation is carried out for aluminum (Al6061) and titan (Ti6Al4V), showing very high differences in time constants.

Fig. 6:

Simulation results for the time constant of the transducer made out of aluminum (Al6061) and titan (Ti6Al4V).

The notation “front”, “Mid” and “End” denotes the position of the middle paddle where “End” is the point most far away from the cross brace. The time constants are approximately 22 times larger for titan than for aluminum. By using aluminum also the sensitivity of the sensor is changed to higher values because aluminum has a higher thermal expansion coefficient than titan and glass (SiO₂). When the temperature changes, the aluminium transducer is stretched. This can be seen by the FBG as an additional temperature dependent strain. This is not the case when titan is used. General speaking, a higher sensitivity is advantageous in most applications.

The simulated equilibrium temperature as function of time is shown in Fig. 7 for aluminium (left) and titan (right). For correct interpretation of the simulation the value of the Mid-point must be taken because this is the location where the grating is mounted. For the simulation only conductive heat transfer was used, no radiative transfer.

Fig. 7:

Simulation of the time constant for the transducer made out of aluminum (Al6061, left) and titan (Ti6Al4V, right) respectively.

V.

RADIATION EFFECTS IN OPTICAL FIBERS AND FBGS

The developed sensing elements shall open also the possibility to mount the sensors outside of the satellite where an extreme high total ionizing (TID) dose up to 25 Mrad or 250 kGy is expected. For this reason a detailed review of radiation effects in FBG sensors and in optical fibers is given.

VI.

RADIATION TEST RESULTS

A.

Standard UV-Written FBGs

The first step in the project was to use commercially available UV-written FBGs and to carry out a first gamma radiation test. For those gratings the standard writing process with hydrogen loading is applied. The commercial gratings which are used for testing had different packages, #101 and #103 had an SS04 steel tube with an outer diameter of 4.7 mm and 3.1 mm respectively, where #107 had no mechanical package around the FBG. For each type of grating three samples where tested. All test where carried out with a Co-60 source located at Fraunhofer INT institute.

Fig. 8:

High dose rate (left) and low dose rate (right) TID test of commercial UV-written gratings for different packages.

The radiation test results given in Fig. 8 indicate that the degradation of the tested sensors is too high to use the sensors for the planned activity. The assumed TID of 25 Mrad behind the satellite’s panels would result in a shift of Bragg-wavelength of approximately 200 pm which corresponds to a temperature error of 20°C. It has also been shown that the packaging of the sensor (bare fiber, 3 mm thick stainless steel tube, 5 stainless steel tube) has no effect on the induced wavelength shift It is interesting that the FBGs irradiated with a lower dose rate (4.1 krad/h shown a shift of app. 10 pm after 250 krad whereas the tested FBGs with a higher dose rate (420 krad/h) have shown a shift of approximately 20 pm for the same TID. So a dose rate dependent effect was observed.

B.

Pure Silica Optical Fibers

As mentioned before pure silica based fibers would also be good candidates for the planned application. So first of all, the radiation performance of the fiber must be evaluated. For this four samples from different manufacturers where tested up to 20 Mrad (200 kGy, dose rate 0.8 Gy/s), the results are depicted in Fig. 9. Only one sample from one manufacturer showed a discrepancy to the other sample fibers, this type of fiber is excluded from the following grating inscription test and subsequent radiation test. Values of 30 dB/km seem very high but two facts must be considered. Firstly, the used length in the planned application is in the range of <20 meters which would result in an attenuation lower than 1 dB. And secondly, the test carried out was a high dose rate test. For low dose rates as they are expected in the planned application, the attenuation would be lower.

Fig. 9:

Radiation test results of the pure silica fibers from four different manufacturers.

VII.

FS-IR WRITTEN GRATINGS IN SELECTED FIBERS

After the two best samples with the lowest radiation-induced attenuation have been identified during the radiation test series, FBGs are written by the fs-IR technique. This step was necessary to prove if gratings can be written in this type of fibers. The gratings where written without any target specification just to test the writing process with this type of fiber. The writing is done by the Fraunhofer IOF institute in Jena by a special phase mask and a femto-second laser source. The test results are depicted in Fig. 10 for both fiber samples. The test has shown that it was possible to write the gratings into the selected fibers with a good spectral quality (especially for the spectrum shown on the left side), a high reflectivity of about more than 50 % and a flat transmission spectrum for light not corresponding to the Bragg-wavelength. This is very important when building WDM sensing systems with this kind of gratings. Again, it has to be noted that the gratings where written without a target specification to prove the concept.

Fig. 10:

Reflection spectra of FBGs written by fs-IR technique into two sample fibers.

VIII.

FURTHER APPLICATIONS

Once a fiber and a FBG writing technique has been selected which sustains the radiation loads, the first step towards photonically-wired panels for satellites is done. One might think about combining sensing fibers embedded into the satellite panels with data transmission functionality with data rates in the GBit/s range. So the fibers on the outside of the satellite build a neural system, which is able to sense physical parameters and can transport information as well.

IX.

OUTLOOK

The next step is to establish a target specification for writing the sensors in the selected fibers. In addition the polarization dependency of the test gratings will be measured, this impacts the interrogator design. Also the writing of FBGs with different wavelength must be investigated under an economic point of view. One multiple samples of FBGs are written, a second radiation test will be carried out to identify the by radiation induced Bragg-wavelength shift. After this test it can be decided if the gratings can be used in the target application.