Pressure Sensor Fabricated by Chemical Etching
An all-glass miniature (0125 |lm) fiber-optic pressure sensor design is appropriate for high-volume manufacturing. The fabrication process is based on the chemical etching of specially designed silica
Figure 3.33 Enlarged photograph of the strain sensor (left). Reflected spectrum of tuned sensor (right). (From Cibula, E. and Bonlagic, D. 2005. Applied Optics, 44(14), 2736-2744.)
Figure 3.34 Pressure sensor simplified fabrication process is performed over only three steps: (a) formation of the cavity, (b) fusion splicing with the lead-in fiber, and (c) creation of the diaphragm. (From Cibula, E. et al. 2009. Optics Express, 17(7), 5098-5106.) optical fiber and involves a low number of critical production operations. The presented sensor design can be used with either singlemode or multimode lead-in fiber and is compatible with various types of available signal processing techniques. The practical assembly of the sensor is achieved over only three steps, which are summarized in Figure 3.34.
In the first step, a cavity is micromachined at the tip of a cleaved optical fiber with well-defined depth and diameter. The cavity is created by the selective chemical etching of a custom-designed, sensor-forming optical fiber. This sensor-forming fiber has a germanium-doped step-index core with a diameter of 90 |lm and a refractive index difference of about 0.7% (corresponding to 7.42% mol of GeO2). When this fiber is exposed to 40% HF at 25°C, the doped region etches at a rate of about 2.8 p,m/min, while the pure silica cladding etches at a rate of about 1 |lm/min. The higher etching rate of the doped core region results in a cavity formation at the tip of the fiber, as shown in Figure 3.34a. The cavity depth is simply controlled by etching time. Depending on the desired cavity depth, the sensorforming fiber is drawn to a larger initial diameter in such a way as to achieve a standard 125-p.m fiber outer diameter after completion of the chemical etching phase. For example, when a target cavity depth
Figure 3.35 SEM photos of the etched cavity: (a) before fire-polishing and (b) after fire-polishing. (From Cibula, E. et al. 2009. Optics Express, 17(7), 5098-5106.)
of 12 Дш was selected, the starting (initial) sensor-fiber diameter corresponded to 138 |!m and the etching took 6.5 min in 40% HF at 25°C. Various cavity depths were produced by using different etching times and different initial outer diameters of the sensor-forming fiber.
In the second step, the etched sensor-forming fiber is fusion- spliced to the lead-in fiber, as shown in Figure 3.34b. Any desired type of lead-in fiber can be used depending on the signal processing technique. Fusion splicing is also used to fire-polish the inner surface of the cavity that becomes rough after chemical etching due to ring MCVD layers. The cavity end-surface qualities before and after firepolishing based on optimized fusion splicing procedure are shown in Figure 3.35.
In the third fabrication step, the sensor-forming fiber is cleaved near the fusion splice with the help of visual inspection under a microscope  and is polished to form a pressure-sensitive diaphragm (Figure 3.34c).
Figure 3.36 shows a picture of the produced sensor with a typical 550 nm/bar sensitivity, obtained using a SEM. The response characteristic at such high pressures becomes highly nonlinear, as shown in Figure 3.36.