# High-Pressure Sensors Based on Laser-Manufactured Sintered Silicon Carbide
## 2 Materials and Methods
### Low-Finesse Fabry-Perot Pressure-Sensor Structure and Measurement Set-Up  
The schematic configuration of the fiber-optic Fabry-Perot pressure sensor is illustrated in Figure 5a. A single-mode fiber (SMF, SMPF0215-FC, with a mode field diameter = 10 \(\upmu\)m @ 1550 nm; a cladding diameter = 125 \(\upmu\)m; a coating diameter = 245 \(\upmu\)m, by Thorlabs Inc., Newton, NJ, USA), is coupled to the laser-formed S-SiC membrane. FP cavity is represented by the fiber end-face and the inner surface of S-SiC diaphragm. As described in the previous section, S-SiC membrane is obtained by means of laser-ablation process of pristine cylindrical samples. As depicted in Figure 5a, laser light propagating into the SMF is partially reflected at the SMF facet (red arrow, \(A\)), whereas light transmitted at fiber output is reflected by the SiC diaphragm surface (orange arrow, \(B\)). Reflected beams propagating back into the fiber will generate an interference spectrum. Laser-engraved S-SiC sample was fixed with epoxy to a \(\frac{3}{4}\) inches full brass male cap holder used for mounting in a high-pressure-chamber used for sensor characterization. To assure SMF-membrane alignment, the brass holder was machined to create a through-pass hole (\(\Oamond 1.8\) mm) and a 0.5 mm deep engrave for sample housing. The SMF, terminated with a glass ferrule (1.8 mm in diameter), was then inserted into the though-pass hole and positioned in front of the diaphragm surface. Finally, fiber was fixed to the brass sensor holder with epoxy. Figure 4: (**a**) An example of laser-treated S-SiC sample (top). Hole diameter is around 3 mm. The structure of the bottom of the hole reveals the cross method adopted during laser treatment. The 2D profile reconstruction obtained by means of digital microscopy analysis is reported on the bottom. (**b**) 2D profile reconstruction for a sample with a membrane with \(\sim\)2 mm diameter and \(\sim\)150 \(\upmu\)m thickness. Dotted lines represent glass ferrule-terminated SMF inserted in the engraved S-SiC sample (see Section 2.1). For samples with a hole diameter around 2 mm, the ferrule was inserted until it touched the side walls of the engraved S-SiC cylinder. As shown in Figure 4b, due to the V-groove shape of the laser-treated cylinder, SMF face-end membrane distance was around 100 \(\upmu\)m, assuring a good collection of reflected beams. Conversely, for samples with hole larger than ferrule diameter a different approach has been adopted. In this case, ferrule-terminated fiber was inserted into the brass holder/S-SiC system until it touched the surface of the membrane. Then, with a micrometric handling system, the fiber head was removed by 100 \(\upmu\)m and finally fixed with epoxy resin. An example of mounted SMF-terminated S-SiC sample is reported in Figure 5b. The experimental arrangement for characterizing the S-SiC pressure sensor is shown in Figure 6. The optoelectronic system is composed of a fiber-coupled laser diode operating at 1550 nm, an InGaAs photo-diode, and a 3-ports optical circulator (LPSC-1550-FC, FGA01 FC, and 6015-3, respectively, by Thorlabs Inc., Newton, NJ, USA). Laser light enters in port 1 of the circulator and is emitted toward S-SiC sensor whose SMF is connected to port 2. Reflected light, hence interference signal, exits from port 3 and it is detected by the fiber-coupled photodiode. A current-to-voltage converter (PDA200C, Thorlabs Inc., Newton, NJ, USA) and a digital voltmeter (Keithley 2700 series, Tektronix Inc., Beaverton, OR, USA) are used for interference light intensity acquisition. As described in [39], for sensor characterizations, a hydraulic table top test pump is used to impinge a stationary pressure on sensor membrane and measuring the inlet pressure with a digital test gauge connected to the pump reference port. Figure 5: (**a**) Schematic of sintered SiC sensor. Single-mode fiber (SMF) end-face and diaphragm represent the Fabry–Perot cavity. Interference signal is generated by reflected signal at fiber end (\(A\)) and SiC membrane (\(B\)).