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Abstract
In this paper, we evaluate different thermal treatments in order to stabilize fiber Bragg gratings written by a femtosecond pulsed laser in specialty highly birefringent micro-structured optical fiber, targeting pressure monitoring at high pressure and high temperature environments. We have obtained a pressure sensitivity of 3.30 pm/bar up to 1400 bar and 290 °C. An effective thermal treatment has been experimentally implemented, yielding a nearly unchanged reflectivity at high temperature in combination with stable temperature and pressure readings: a standard deviation of 0.42 bar in the pressure reading was observed over 7 days at 280°C.
© 2017 Optical Society of America
1. Introduction
Pressure monitoring devices for downhole applications can be categorized into two types of working conditions, according to the definitions provided in [1]. The first type are working conditions in ultra-high pressure and high temperature (Ultra HP and HT), which indicates that both the operating pressure and temperature can go up to 1379 bar and 175°C, respectively. The second type are working conditions up to 69 bar and 280°C, which are considered to be low pressure and ultra-high temperature (Low P and Ultra HT). The current market is still dominated by crystalline quartz based sensors that have several shortages such as lifetime reduction when exposed to higher temperature and increased pressure errors when encountering temperature transients [2,3].
Owing to numerous advantages of fiber Brag grating (FBG) based optical fiber sensors such as no electronics needed at the sensor side, the possibility of multiplexing, remote sensing and long operational life time, this technology appears as a strong candidate for usage in downhole applications [4,5]. The technology also has the potential to offer a solution for the shortcomings of the current technology and at the same time cover the full range of working conditions (ultra HP in combination with ultra HT). The necessary condition is that the gratings offer long term stability up to 280°C. Conventional FBG based sensors will face issues with stability at elevated temperature. They will exhibit a rapid power-law like decay in reflectivity within the first few hours [6]. Recently, alternative approaches have been developed that result into high temperature resistant gratings [7–9] based on femtosecond laser written gratings. These gratings rely on the defect formation of tightly focused laser pulses through a non-linear multiphoton absorption process [10,11]. To date, FBGs written with a femtosecond pulse duration laser operating in the UV or IR range using the phase mask [12–16] or point-by-point [17,18] technique have already been reported intensively. Depending on the material index modulation of the glass, Type I-IR and fs-UV grating formation written by a femtosecond pulse duration laser appear to have similar resistance to high temperature [19]. Moreover, as suggested in [19], Type I-IR and fs-UV gratings still behave similar and can be described by the color center model, which will allow us to study their thermal stability by performing accelerated aging experiment [6,20].
The FBG-based pressure sensors offer also an attractive solution with respect to the level of pressure sensitivity and pressure errors during temperature transients. Instead of adding an extra grating to recalibrate the temperature effect in pressure monitoring [21,22], micro-structured optical fibers (MSFs) have attracted a lot of attention owing to their design flexibility to either enhance the sensitivity to a certain measurands or to discriminate specific measurands [23–25]. A temperature independent measurement can for example be achieved when inscribing a grating in a MSF with a high waveguide birefringence [26,27]. Two reflected Bragg resonances, corresponding to the slow and fast orthogonally polarized axes behave identically to the temperature response whereas they behave differently with respect to pressure. In this way, pressure errors coming from temperature transients can be eliminated.
In this work, we combine the strength of the femtosecond laser written fiber Bragg grating together with the so-called Butterfly MSF to tackle the pressure monitoring in ultra HP / ultra HT environments. Due to the challenges for the inscribing beam to reach the fiber core region of the MSF [28], the femtosecond laser is operated at UV range for easier grating inscription in the GeO2-doped Butterfly MSF. To study the long-term stability of the MS-FBG sensor, different thermal treatments have been experimentally tested. In addition, to monitor the ability of the MS-FBG sensor to decouple pressure from temperature, the pressure response has been studied while cycling between low and high temperature.
This paper is structured as follows: In section 2 we explained the sensing principle and the production procedure of the MS-FBG sensor. In this section, a mathematical model to decouple the pressure response from the temperature response of the MS-FBG sensor was also included. Then, the pressure response of the MS-FBG sensor at different temperatures was presented in section 3. This allows us to obtain the coefficients related to the pressure response of the MS-FBG sensor in the aforementioned mathematical model. To further study the high temperature stability of the MS-FBG sensor, an accelerated aging procedure was applied on MS-FBG sensor. We experimentally tested several thermal annealing treatments on MS-FBG sensor. After each thermal treatment, the stabilities at the highest targeted temperature were also monitored in order to understand the effectiveness of different thermal treatment. With respect to different thermal treatment, we performed the temperature calibrations on the MS-FBG sensor and obtained the temperature response coefficients of the MS-FBG sensor as described in section 4. Furthermore, we investigated the effect of thermal transients on the pressure reading by presenting a calculated pressure readings in a slow and fast temperature variation environments. Finally, we concluded the performance of the MS-FBG sensor written in specialty fiber in an ultra-high pressure and ultra-high temperature environments.
2. MS-FBG sensor production
2.1 Sensing principle
The so-called Butterfly MSF is by design a highly birefringent fiber [29–31]. Its unique micro structure allows the stress to be asymmetrically concentrated at the fiber core region when the MSF is under a transverse load or a hydrostatic pressure. In this way, the phase modal birefringence (B) of the MSF that results in splitting the Bragg resonance, can be altered via the stress-optic effect. Hence, pressure information is encoded into the Bragg peak separation (Δλ), which is given by Eq. (1):
where λB1 and λB2 represent the slow and fast axis Bragg wavelength in the reflection spectrum and Λ is the grating period. These two wavelengths correspond to the light guided along the fast and slow axis of the Butterfly MSF, respectively. The temperature induced wavelength shift of the individual Bragg resonances is almost identical. In this way, a temperature insensitive pressure reading can be achieved.In order to decouple the pressure response from the temperature response of the MS-FBG sensor, we use the following mathematical model. The change of the individual Bragg wavelengths of the MS-FBG sensor can be linked to the contribution from the change of the temperature (ΔT) and the change in pressure (Δp) around the grating region as shown by the relation in Eq. (2). The parameters a, b, c and d correspond to the sensitivity as derived from individual calibrations. The pressure sensitivity of each Bragg wavelength is contained in a and c parameters while the temperature sensitivities are represented by b and d.
where λB1,0 and λB2,0 indicate the Bragg wavelength at reference pressure and temperature for fast and slow axis. This relation can also be expressed as in Eq. (3) after taking the mean and the difference of Eq. (2). Both the mean and the difference of the wavelengths can be indicated as and, respectively. In this way, the change of peak separation Δλ-Δλ0 can be associated with pressure and temperature through the coefficient a’and b´. These two coefficients a´ and b´ are referred to as the pressure sensitivity of the peak separation and the pressure-temperature cross sensitivity of the MS-FBG sensor. In case of the Butterfly MSF, the change of peak separation can be almost exclusively attributed to pressure changes. The coefficients c’and d’are the pressure and the temperature sensitivity of the mean wavelength of MS-FBG.whereandare the difference and the mean wavelength of two Bragg wavelengths at a reference pressure and temperature.2.2 Sample preparation
To manufacture the MS-FBG sensors, a length of 30 cm Butterfly MSF was first spliced to a standard Polyimide (PI) coated SMF which in turns was spliced to a standard SMF pigtail with FC/APC connector. The purpose of the PI-coated fiber is to integrate a section of this fiber in a high pressure feed through later on. The MSF was spliced after active alignment with a specialty fiber fusion splicer (FSM-100P + , Fujikura). A SLED light source was connected to the PI-SMF and the emitted light from the SMF was butt-coupled into the Butterfly MSF. Then, the guided light in the MSF was transmitted to an optical power meter. In this way, SMF to MSF core alignment could be optimized before applying the actual fusion splice. Apart from the alignment method, we have also optimized different splicing parameters to preserve the micro-structure both for the main electric arc and the re-arc and in order to minimize Fresnel reflections at the splice interface. During the re-arc step, we monitored the return loss or reflectance by means of the optical CW reflectometer testing method [32] until the reflection was reduced while minimizing the collapse of the micro-structure. Finally, we measured a round-trip insertion loss of ~12 dB for the SMF to MSF splice.
2.3 Grating inscription
The MS-FBG was inscribed with a femtosecond laser with the aim to achieve highly temperature stable gratings in the Butterfly MSF. As shown in Fig. 1, our femtosecond pulsed laser (Spectra-Physics) system is composed of a Mai Tai oscillator and a Spitfire PRO amplifier that delivers 3.5 mJ pulses with 120 fs duration and 1 kHz repetition rate at 800 nm [33]. These pulses are transmitted through a third harmonic generator (THG) that allows us to inscribe grating at 267 nm. The laser beam passed through a diaphragm with 6 mm diameter and a cylindrical lens with a focal length of 50 mm. Then, the tightly focused laser beam illuminates a phase mask with a pitch of 1095 nm that generates fringes in the Butterfly MSF.
The Butterfly MSF was locally stripped over a distance of 8 cm before mounting it in rotating fiber clamps at both sides. This allows us to optimize the fiber orientation with respect to the inscription laser beam. This is an important step, as the complex air holes structure in its cladding may disturb the inscription beam [28]. Next, we relied on monitoring the photoluminescence generated by the germanium dopants in the fiber core to focus the inscription laser beam into the fiber core. The optical power of the laser was kept low at this stage. Once alignment was achieved, the laser power was increased to ~100 mW for grating inscription. The reflection spectrum was carefully monitoring during the inscription to ensure that both Bragg wavelengths have a clear and a dominated Bragg resonances. Finally, the MSF end facet was sealed by collapsing the micro-hole structure with an electric arc from the fusion splicer. Sealing is needed for the hydrostatic pressure measurements.
2.4 Basic characterization results
The initial Bragg peak separation of the obtained MS-FBG sensors was around 0.59 nm, corresponding to a 5x10−4 phase modal birefringence. The average full width at half maximum (FWHM) and side-lobe suppression (SLS) were found to be around 0.16 nm and 12.6 dB for the produced MS-FBG sensors. Figure 2 shows a typical reflection spectrum of a MS-FBG sensor that was used in the following tests.
Fig. 2 Reflection spectrum of the MS-FBG sensor. Insert: Scanning electron micrograph of the cross-section of the Butterfly MSF and detailed image of the core region.
The reflectivity of the MS-FBG sensors was characterized from a measurement in transmission that shows transmission dips varying between −2.4 dB and −2.8 dB. The optical parameters listed above were characterized with an optical interrogator SM125-500 from Micron Optics in combination with a polarization scrambler from General Photonics. Due to the differences in splice quality from sample to sample, the noise levels of the reflection spectra from different MS-FBG sensors were different. Nevertheless, both the spectral shape and the SNR value of each Bragg resonance in these MS-FBG sensors provided consistent Bragg resonance with around 1-3 pm variation at a fixed temperature.
3. Pressure response on MS-FBG sensor
3.1 Sample preparations
The pressure calibration tests were carried out at Smart Fibres Ltd., where a pressure feedthough was built around the Polyimide coated SMF. A section of the PI-coated fiber was installed into a high-pressure stainless steel tubing of 6.35 mm diameter. In this pressure calibration, an air driven pressure controller (DHI PPCH) was used in combination with a Carbolite oven to control the temperature.
3.2 Pressure calibration of MS-FBG sensors
The pressure calibration was performed for 2 annealed MS-FBG sensors ranging from 0 to 1400 bar in steps of 100 bar for the first 2 cycles and 200 bar for the third cycle at 40, 160 and 290°C, respectively. The evolution of the Bragg peak separations and pressure values from the calibration at 290°C are presented in Fig. 3(a). Note that the normalized peak separation curves for two tested sensors were behaved identically.
Fig. 3 (a) Peak separation and (b) Bragg wavelength evolutions for 2 MS-FBG sensors during pressure calibration at 290°C.
A first observation is that the peak separation of both MS-FBG sensors responded to pressure in a reproducible way for different cycles, evidencing that the integrity of the micro-hole structure of the MSF was well-maintained under these ultra HP/HT conditions. Secondly, the MS-FBG sensor exhibited a positive pressure sensitivity as both peaks move away from each other for increasing pressure values. Figure 3(b) indicates the evolutions of the individual Bragg wavelengths for both sensors in this calibration. We observed a positive and linear pressure response for the slow Bragg peak wavelength. On the other hand, the fast Bragg peak wavelength showed a quadratic rather than linear response, resulting in a negative pressure sensitivity below 700 bar and a positive one above this threshold. This behavior may relate to the minor changes of microstructure that affect the stress distribution around the fiber cross-section. Nevertheless, the pressure response of the slow Bragg peak wavelength was much more profound than the fast Bragg peak wavelength. Therefore, the positive pressure sensitivity of the Bragg peak separation of the MS-FBG sensors is still maintained above 700 bar [30].
The coefficients a and c in the mathematical model can then be obtained after applying a linear curve fit to the pressure calibration, as shown from the results in Fig. 3(b). We have determined that the pressure sensitivity for fast and slow Bragg peak wavelength are 0.012 (a) and 3.36 (c) pm/bar, respectively. A linear approximation was still applied to the response of fast Bragg peak wavelength in order to simplify the complexity of the mathematical model that is used to describe the response of each Bragg wavelength. This first-order approximation can be accurate enough when we calculate the pressure reading in section 5 later on. The pressure variation and the pressure sensitivity of the Bragg peak separation at various temperatures are shown in Fig. 4.
Fig. 4 (a) The pressure sensitivity of Bragg peak separation (separated by 1 nm artificial offset) and (b) Its non-linearity at various temperatures.
Note that we introduced an artificial offset of 1 nm in order to clearly distinguish between the individual curves in Fig. 4(a). The peak separation showed a linear behavior (first order approximation) as a function of pressure, with a sensitivity that remained almost constant for the different temperatures. The sensitivity changed (Psen,Δλ) from 3.30 pm/bar at 40 °C up to 3.35 pm/bar at 290 °C. The temperature sensitivity of the Bragg peak separation (Tsen,Δλ) can be estimated from the Bragg peak separations at three different temperatures at constant pressure. At atmospheric pressure this is about 2.20x10−2 pm/°C. This value corresponds to a pressure-temperature cross-sensitivity of 6.06x10−3 bar/°C. If we consider an interrogator accurate to 1 pm (many of them are commercially available), the obtained pressure sensitivity corresponds to a minimum detectable pressure change of ~0.3 bar. Obviously, the temperature cross-sensitivity will mainly affect the reading of small pressure changes. For a pressure value of 0.3 bar, a temperature change of 100 °C will lead to a 0.6 bar pressure difference, yielding a strong impairment. For higher pressure values above 100 bar, this impairment is not significant. More importantly, using a mathematical model that takes into account both the temperature and the pressure response of the MS-FBGs, we can straightforwardly correct for a change of the pressure reading resulting from the temperature cross-sensitivity, as discussed in more details in section 5. As shown in Fig. 4(b), the non-linearity (difference between the experimental data and linear fit) scattered around 10 pm at each calibrated pressure and for each tested temperature. The shape of the non-linearity essentially follows the quadratic behavior of the fast Bragg peak wavelength in the pressure cycle.
The peak separation at zero pressure decreased slowly at 290°C when we compared those values at later stages during the pressure calibration with around 8 and 4 pm for the two tested MS-FBG sensors, respectively. This showed that the peak separation of the MS-FBGs are not thermally stable at 290°C even if the sensors had been pre-annealed at 290°C for 7 days. This can also explain the scattering of the non-linearity we have in this pressure calibration. Since the grating formation of the MS-FBG sensor is linked with color center model, accelerated aging can stabilize the grating at high temperature. In this case, the highest operating temperature is 280 °C as we target for pressure monitoring in Ultra HT environments. Therefore, a further study on temperature stability of MS-FBG sensor will be discussed as a function of different thermal treatments in the following section.
4. Different thermal treatments on MS-FBG sensor
4.1 1st and 2nd thermal treatment on MS-FBG sensor and its stability at 280 °C
The thermal treatment was done for two non-annealed MS-FBG sensors with a temperature calibrator (Fluke Metrology Well 9173). We have evaluated 3 different thermal treatments sequentially for the same samples: (1) annealing at 280°C for 7 days, (2) annealing at 400°C for 1 day and (3) annealing at 450°C for 7 days.
The 1st thermal treatment was chosen due to the fact that we can still preserve the Polyimide coating of the connecting fiber after annealing. Also, the 1st annealing period was prolonged to be longer than the required time for pressure calibration. The reflectivity and Bragg peak separation evolution in the first two thermal treatments can be seen in Fig. 5.
Fig. 5 Normalized reflectivities for each Bragg wavelength and Bragg peak separation evolutions of MS-FBG sensor for 1st and 2nd annealing treatments and its stability at 280°C.
In addition to the response to different treatments, the stability at 280°C was also monitored. In the 1st thermal treatment, after 7 days of annealing the reflectivity did not stabilize and decreased with approximately 20%. This result explains why the data from the pressure calibrations was drifting over time. To further stabilize the sensors, a higher annealing temperature will be needed. An empirical approach was used to determine the annealing temperature in this thermal treatment. Taking into account the quick decay in reflectivity at such high temperature, the annealing time was limited to 1 day only. In the 2nd thermal treatment, the reflectivity of the MS-FBG sensors was experiencing another 20% drop during the thermal treatment. To further validate the stability of the annealed MS-FBG sensors, we monitored the sensor response at 280°C for 6 days after the 2nd thermal treatment. This time, both MS-FBG sensors exhibited a stable reflectivity level but the peak separation gradually varied with about 6 pm in the 6 days test period.
4.2 3rd thermal treatment on MS-FBG sensor and its stability at 280 °C
To further improve the wavelength stability at 280°C, we performed a 3rd thermal treatment on the same MS-FBG sensors at 450 °C for 7 days, as it can be seen in Fig. 6. Similarly, to further verify the stability at 280 °C, we also monitored the reflectivity and peak separation evolution after the 3rd thermal treatment as shown together in Fig. 6. The 3rd annealing temperature was chosen due to the fact that the reflectivity of MS-FBG sensor was stable after 2nd thermal treatment and there was still minor drift in peak separation. Therefore, we increased the annealing temperature to 450 °C and prolonged the annealing period.
Fig. 6 Normalized reflectivities for each Bragg wavelength and Bragg peak separation evolutions of MS-FBG sensor for 3rd annealing treatments and its stability at 280°C.
Both the reflectivity and the peak separation did not stabilize during the 3rd thermal treatment. Nevertheless, the MS-FBG sensor exhibited an increased stability both in reflectivity and in peak separation at 280 °C. The stability of the peak separation at 280 °C had a standard deviation of approximately 1.4 pm, corresponding to 0.42 bar in pressure reading. Note that the scattering of the peak separation as shown in Fig. 6 may be misleading since the data distribution cannot be revealed in the figure. Apart from the stability check of the MS-FBG sensor, we also did perform temperature calibrations after each thermal treatment in order to determine the temperature sensitivity parameters. This is presented in the next section.
5. Temperature response on MS-FBG sensor
Temperature calibrations have been performed on the MS-FBG sensors for each thermal treatment. The calibration range was set from 55°C to 280°C in steps of 22.5°C for two full cycles with the temperature calibrator (Fluke Metrology Well 9173). A linear fit is used to determine the temperature response of each Bragg wavelength after plotting the wavelength variations as a function of temperatures. This allows us to obtain the coefficients b and d corresponding to the temperature response of fast and slow Bragg peak wavelengths, respectively.
5.1 Temperature calibration on MS-FBG sensors as respect to different thermal treatments
Table 1 presents the temperature sensitivities and the differential temperature sensitivities for individual Bragg wavelength after each thermal treatment.
Table 1. Obtained coefficients as respect to different thermal treatments (the applied temperature and annealing period are listed in brackets).
To differentiate the obtained coefficients, the step of different thermal treatments will be indicated in the subscript of each coefficient. In Fig. 7, we present the response of each Bragg wavelength after different thermal treatments.
Fig. 7 Temperature response of Bragg wavelength after (a) 1st, (b) 2nd and (c) 3rd thermal treatment, (d) Wavelength evolution for both individual Bragg wavelengths and peak separation in temperature calibration.
In Fig. 7(a, b and c), the experimental data were indicated in orange and blue dot for slow and fast Bragg peak wavelengths together with the fitted curves were indicated in black solid line. Furthermore, we also expressed the individual Bragg wavelength and the peak separation evolution during this temperature calibration as shown in Fig. 7(d). One can see that the differential temperature sensitivity approaches 2x10−2 pm/°C as the grating stability at high temperature improves.
5.2 Effects on pressure reading of MS-FBG sensors in thermal transients
To investigate the effect of thermal transients on the pressure reading, cycling between low and high temperatures has been performed. The cycle was set to last 1 day at 55 °C, 1 day at 280 °C and again 1 day at 55 °C for two times at atmospheric pressure. In this way, we can calculate pressure and temperature based on the obtained coefficients and the proposed mathematical model as indicated in Eq. (2). We only performed this thermal cycling for the MS-FBG sensor after 2nd and 3rd thermal treatment. First, we applied this thermal cycling to the sensor after 2nd thermal treatment. The wavelength variations at each temperature were reproducible with approximately 0.9 pm standard deviation, indicating that the temperature cycle had no influence on the absolute wavelength positions. Then, we evaluated the effect of the temperature cycling on the pressure readings. We calculated pressure and temperature by means of the deduced calibration parameters for pressure sensitivity (a and c) and temperature sensitivity (b2nd and d2nd), as shown in Fig. 3(b) and Fig. 7(b), respectively. There was no obvious difference in the pressure readings when cycling between these two temperatures but with around 2 bar variation in this test as shown in Fig. 8(a). Nevertheless, the used calibration parameters and the used linear model were accurate enough to correct for the observed changes in the peak separation.
Fig. 8 Calculated pressure (indicate in blue) and temperature (indicate in red) from the data obtained during the thermal cycling after (a) 2nd or (b) 3rd thermal treatment.
Next, the same thermal cycle was applied on the sensor after 3rd thermal treatment. Based on these obtained coefficients (a, b3rd, c and d3rd), we updated the calculated temperature and pressure readings in the thermal cycling test, as shown in Fig. 8(b). Similar result can be found with nearly unaffected pressure readings and with around 1 bar variation in this thermal cycling test. An even smaller pressure variation can be obtained, evidencing that we have acquired more accurate coefficients to calibrate MS-FBG sensor responses.
Finally, we validated the pressure monitoring with the MS-FBG sensor in the presence of rapid temperature transients by performing the thermal shock test. We monitored the response of MS-FBG sensors when we lowered into and taken out the sensors from the temperature calibrator which is stabilized at 280 °C under atmospheric pressure. We utilized the Hyperion Enhanced Visibility unit from Micron Optics with low degree of polarization source (LDOP) and capable of sampling at 10 Hz to monitoring the MS-FBG sensor responses. This test was performed for the MS-FBG sensors after 3rd thermal treatment. Similarly, we calculated the temperature and pressure readings in the thermal shock test as shown in Fig. 9.
Fig. 9 (a) Calculated pressure (indicate in blue) and temperature (indicate in red) from the data obtained during thermal shock test and (b) Zoom on the calculated data during taken out the sensor from the temperature calibrator.
We observed that the pressure reading varied around 1bar during the thermal shock test, a similar variation as the results in the thermal cycling test. Evidently, the effect to the rapid temperature transients is insignificant. Then, we determined the response time for the temperature to be stabilized (Tresponse) under a rapid temperature transient with around 0.03 minutes = 1.8 seconds as shown in Fig. 9(b).
6. Conclusion
In this paper, a method to stabilize femtosecond pulse duration inscribed gratings in Butterfly micro-structured optical fiber was implemented for pressure monitoring in Ultra HP / Ultra HT environments. We first demonstrated that the MS-FBG sensor provides a constant and a reproducible pressure sensitivity of about 3.30 pm/bar over a pressure range from 0 to 1400 bar and over an operating temperature range from 40°C to 290°C. Then, we studied different thermal treatments in order to stabilize the sensor in its operating temperature range by means of monitoring its absolute wavelength and peak separation stability at high temperature and evaluating the coefficients obtaining based on the first order approximation to the temperature and pressure. A thermal treatment consisting in annealing the MS-FBG sensor at 450°C for 7 days stabilizes the reflectivity and both Bragg wavelengths within the target operating temperature range. The obtained calibration parameters after this thermal treatment also show its capability to accurately reveal the pressure and the temperature when cycling at low and high temperatures. Furthermore, we obtained a pressure-temperature cross-sensitivity of (6.06x10−3 bar/°C). With the obtained calibration parameters and in the thermal shock test, it was shown that this kind of sensor is capable to accurately monitor pressure in the presence of rapid temperature transients.
Funding
European Space Agency (GSTP6.2); Fonds de la Recherche Scientifique (FNRS); Fonds Wetenschappelijk Onderzoek.
Acknowledgments
This research is carried out under the GSTP6.2 program funded by the European Space Agency and is supported by the Belgian Science Policy. C. Caucheteur is supported by Fonds de la Recherche Scientifique - FNRS (F.R.S.-FNRS). T. Geernaert is post-doctoral research fellow with the - Research Foundation Flanders (FWO). The authors would like to acknowledge financial support from Vrije Universiteit Brussel’s Methusalem programme and FWO’s Hercules programme, as well as the Belgian Science Policy Interuniversity Attraction Pole P7/35 and the assistance from Smart Fibres Ltd. for the pressure calibrations.
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