A Probe-Shaped Sensor With FBG and Fiber-Tip Bubble for Pressure and Temperature Sensing

A probe-shaped sensor for simultaneous temperature and pressure measurement was reported in this article. The effective length of the sensor was ∼2mm, consisting of a fiber Bragg grating (FBG) and a Fabry-Perot interferometer (FPI) with a nano silica diaphragm. The response sensitivities of the sensor for pressure and temperature were measured as −0.98 nm/MPa and 11.10 pm/°C, respectively. This sensor had an extremely low cross-sensitivity between pressure and temperature, which provided a significant potential in dual-parameter sensing.


Introduction
In the past two decades, optical fiber information technology has achieved a huge development. Optical-fiber-based sensors have attracted considerable attention due to their outstanding advantages of durability, flexibility, biocompatibility, high sensitivity, and electromagnetic interference immunity. For the indisputable superiorities, the optical fiber sensor is widely applied in many fields, such as industrial production, medical application, environmental protection, and monitoring quality of construction. A variety of optical fiber sensors have been fabricated to gather information of multifarious physical and chemical parameters, such as temperature [1], pressure [2], curvature [3,4], humidity [5], and refractive index (RI) [6]. In order to fabricate these sensors, various kinds of fibers are adopted, such as photonic crystal fiber (PCF) [7,8], polarization-maintaining fiber [9,10], twin-core fiber [11], sapphire optical fiber [12], micro-fiber [13], and D-shaped fiber [14]. The measurement principle of the sensors depends on the devices, such as the fiber Bragg grating (FBG) [15], long period grating [16], tapered fiber [17], Mach-Zehnder interferometer (MZI) [18], Fabry-Perot interferometer (FPI) [19], and Sagnac loop interferometer [20]. To address practical demands, many researchers have investigated optical fiber sensors to realize multi-parameter measurement. Hu et al. [21] presented a cascaded multi-mode fiber and FBG structure in fiber to develop a surface plasmon resonance (SPR) sensor for dual-parameter measurement. Li et al. [22] utilized few-mode fiber to fabricate an optical fiber distributed sensor, by analyzing the Brillouin frequency shift in each mode to measure and discriminate temperature and strain. A micro-tip FP cavity, fabricated by focused ion beam milling, was used to measure the temperature and RI in inaccessible locations [23]. Special filter with a dual-wavelength fiber laser was employed to measure RI and temperature by recording the wavelength shift and output power difference of two laser beams [24].
In this work, a probe-shaped optical fiber sensor which can be used for temperature and pressure measurement was proposed and experimentally demonstrated. The sensor is a hybrid structure consisting of an FBG and an FPI with a nano silica diaphragm on the tip, and the total length is less than 2 mm. Herein, the FBG and FPI respond to the environmental temperature and the pressure, respectively. Due to the pressure insensitiveness of the FBG, the cross-sensitivity between temperature and pressure of the FPI can be mostly eliminated by establishing the temperature compensation using the FBG temperature response. The probe-shaped all-silica micro sensor that can achieve dual-parameter measurement has a great potential in particular applications in harsh environments. Figure 1(a) shows a structure schematic of a probe-shaped sensor, which consists of an FBG and an FPI with a nano diaphragm. The fabrication process includes two steps. Firstly, a fiber-tip air bubble (FAB) for developing an in-fiber FPI was obtained by the fusion splicing technology which was an improved electrical arc discharge fabrication method to construct an all-silica FPI cavity with an air bubble and a silica nano diaphragm [25]. Secondly, the FBG with grating pitch of 1.089 μm, corresponding to a second-order Bragg resonance wavelength at 1 575 nm, was fabricated by a femtosecond (fs) laser, where a 40 mW laser was focused into the single mode fiber core by an oil immersed 63×objective (NA=1.4) during line-by-line FBG inscription. The corresponding FBG length and FPI cavity length were measured as ~1.7 mm and 90 μm, respectively. The reflection spectra of the probe-shaped sensor were recorded by an optical spectrum analyzer (OSA, Yokogawa AQ6370D) with wavelength ranging from 1 525 nm to 1 605 nm, as shown in Fig. 1(b). The resonance wavelength of the FBG was 1 575.2 nm and the free spectral range of FP interference was ~13.9 nm at 1 575 nm. The length of FP cavity was calculated as 89.2 μm. In the gas pressure measurement, the probe-shaped sensor was sealed in an airtight gas chamber. Chamber pressure was adjusted by coarse and fine valves, and a pressure meter was used to monitor the chamber pressure. The gas pressure response test results are shown in Fig. 2. The evolution of the FPI reflection spectra with the chamber pressure increasing from 0 MPa to 1 MPa and step of 0.1 MPa is shown in Fig. 2(b). The FPI dip shifted towards shorter wavelength as the gas pressure increased, which is because of the deformation of FAB nano diaphragm that leads to a decrease in the cavity length. The linear fitting [ Fig. 2(c)] of the pressure test result shows that the pressure sensitivity was -0.98 nm/MPa. The Bragg resonance peak had no distinct change in this process. As shown in Fig. 2(d), for the FBG, the change of pressure has negligible effect on the effective RI and the grating period, so the Bragg wavelength remains unchanged. Furthermore, to investigate the temperature response characteristics, the sensor was placed in a temperature control oven. The reflection spectrum was monitored as the temperature increased from 20 ℃ to 100 ℃ with step of 10 ℃. The spectra were recorded after 10 minutes of stabilizing at each temperature step. From Fig. 3(a), it can be observed that as the external temperature rises, the FBG peak shifts towards longer wavelength while the FPI dip has no obvious change. The evolution spectra of the FBG temperature response are shown in Fig. 3 Fig. 3(c). The corresponding temperature sensitivity was 11.1 pm/℃ which was similar to the result reported previously [26], and this parameter was mainly affected by the photo-thermal effect. The FPI dip shift shown in Fig. 3(d) is ~120 pm with a temperature sensitivity of 1.4 pm/℃. The low temperature sensitivity of FPI dip can be attributed to that an air cavity structure like FAB is insensitive to external temperature change.

(b) while its linear fitting is shown in
The temperature-induced error of the pressure measurement without temperature compensation was -1.428 6 kPa/℃ while the pressure-induced error of the temperature measurement was ~0 ℃/MPa. It can be seen that the probe-shaped sensor has low cross-sensitivity in simultaneous measurement of temperature and pressure.   Fig. 3 Temperature response of the probe-shaped sensor: (a) variation of the FBG peaks and FPI dips at 20 ℃ and 100 ℃, (b) wavelength shift of FBG peak with the temperature varying from 10 ℃ to 100 ℃, (c) the linear fitting of Bragg peak wavelength shift with a slope of ~11.10 pm/℃, and (d) wavelength shift of FPI dip with the temperature varying from 10 ℃ to 100 ℃, and the linear fitting slope is ~1.4 pm/℃.
To investigate the deformation of the probeshaped sensor under a particular gas pressure, a simulation was performed using commercial finite element analysis software. The simulation employed standard parameters of silica, i.e., silica density of 2 700 kg/m 3 , Young's modulus of 73 GPa, coefficient of thermal expansion of 5.5×10 -7 , and Poisson's ratio of 0.17. The morphology of the simulation model was extracted from the microscopic image shown in Fig. 1(a). Figure 4(a) illustrates the two-dimensional (2D) and three-dimensional (3D) deformation contours of the sensor, which was modeled under the gas pressure of 1 MPa. Notably, the modeling results show that when the sensor was subjected to 1 MPa pressure, only the thin diaphragm on the tip of the sensor was deformed while FBG was not affected by pressure, which contributed to the ultra-low, almost zero pressure-induced cross-sensitivity in temperature sensing. The influence of temperature was theoretically investigated by modeling the axial elongation of the sensor at 100 ℃. The simulation result is shown in Fig. 4(b). The whole sensor, including FBG and FAB, has inevitable axial elongation due to thermal expansion. The thermal expansion of silica is dominant in such structure, and the elongation of the bubble is far less than that of FBG due to less silica content of the bubble. Therefore, the sensor has low temperature-induced cross-sensitivity in gas pressure sensing.  (4) The pressure and temperature sensitivities of FPI and FBG were obtained by experiments. As a result, (4) can be further derived as FPI FBG 0 90.0901 1.020 4 0.1287 where the cross-sensitivity is basically removed.

Conclusions
In conclusion, an all-fiber hybrid sensor consisting of an FPI with silica diaphragm and an FBG was designed and demonstrated. The FBG was fabricated by the femtosecond laser inscription technique, and the bubble cavity was fabricated by arc discharge technology. The pressure sensitivity was -0.98 nm/MPa, and temperature sensitivity was 11.10 nm/℃. This sensor exhibited ultra-low crosssensitivities. The temperature-induced error of the pressure measurement was -1.428 6 kPa/℃ and pressure-induced error of the temperature measurement was ~0 ℃/MPa. Such kind of sensor with low cross-sensitivity is potentially suitable for various applications.