EFFPI Based on Fiber End and Diaphragm
In order to enhance the sensitivity of pressure sensors, another EFFPI structure was proposed and also widely used. The EFFPI structure was formed by a cleaved fiber end face and a diaphragm. A general theory for diaphragm-based EFFPI can be found in Reference 39.
This kind of structure can be fabricated by the chemical etching method . The fabrication process is shown in Figure 2.8. A section of MMF was first fusion spliced with a SMF, and the MMF was cleaved with a residual length of 40 p,m similar to that of the final air cavity length. Then the MMF was chemically etched by HF acid. The etching rate of the MMF core is much faster, about 10 times than that of the pure silica cladding, enabling the selective etching of the MMF core. The etching process was stopped when the SMF core was exposed to the solution. The air cavity can also be formed by fusion splicing and precisely cleaving a section of hollow-core fiber. Then the SMF with an air hole at the tip was spliced to another section of SMF. The second SMF was cleaved and then polished so that the SMF length was controlled down to 3-5 |lm, acting as the pressure- sensitive diaphragm. The polishing process was very important for the sensor performance.
Figure 2.8 Fabrication of EFFPI with a diaphragm. (a) Fusion splicing an MMF to SMF and cleaving the MMF, (b) chemical etching the MMF, (c) fusion splicing a SMF to the etched MMF and cleaving the SMF, and (d) polishing the SMF to the desired thickness of the diaphragm.
Another simpler method is by using a fused silica ferrule with a V-shaped hole at one end . A cleaved SMF was inserted into the silica ferrule, and a fused silica diaphragm was attached to the silica ferrule by heating fusion bonding. An EFFPI structure was formed at the V-shaped end of the silica ferrule. The V-shaped hole is important for enhancing the pressure sensitivity as it introduces a large effective diameter of the diaphragm. The thickness of the diaphragm is often optimized for the pressure detection with a certain range. A thick diaphragm is better for large pressure sensing, while a thin diaphragm is so for small pressure sensing. A similar structure was employed for the detection of the refractive index of a glass substrate. The fringe contrast of the two-beam interference from one cleaved fiber end face and the sample glass surface was used for the detection. Four different glass materials, that is, SiO2, BK7, SF10, and SF11, were measured . The standard deviation of the refractive index measurement for 20 times was less than 0.003. The method is simple, cost effective, easy to fabricate, and also easy to use. It was also used for small displacement measurement, with a resolution of 16 nm at an initial distance of 30 |lm .