Chemical Etching (Doped Fibers)
Strain Sensor Fabricated by Chemical Etching
Chemical etching micromachining technique could be used to fabricate FFP sensor in a special-doped fiber, which is based on the much
Figure 3.31 In-line optical fiber strain sensor. (From Cibula, E. and Donlagic, D. 2007. Optics Express, 15(14), 8719-8730.) higher etching rate of doped silica when compared to pure silica. Such an FP sensor is generally used as a strain sensor. The sensor design is shown in Figure 3.31. A short air cavity between the lead-in and lead-out SMF acts as an FPI, where light waves, reflected from both cavity surfaces, interfere.
The fabrication of a short-cavity strain sensor includes three steps: concave cavity formation at the tip of the lead-out fiber, splicing the fibers to perform an in-line sensor, and, finally, tuning of the sensor’s length to achieve the desired operating point.
First the short segment of the standard 62.5/125 gradient index MMF is attached to a standard SMF by splicing and cleaving at the desired distance from the splice, in our case 25 |lm, as shown in Figure 3.32a. The appropriate tools were used to achieve repeatable cleaves with 1 p,m resolution . This initial 25 p,m distance is shorter than the target cavity length, as the fusion splicing process
Figure 3.32 Strain sensor fabrication procedure. (From Cibula, E. and Donlagic, D. 2005. Applied Optics, 44(14), 2736-2744.) causes the elongation of the cavity. Next, the prepared fiber tip is immersed in 40% HF acid at room temperature (Figure 3.32b), where the core of the MMF is etched away in the form of a concave cavity, as illustrated in Figure 3.32c. To assure high reflectivity of the structure, it is essential to terminate the etching process at the moment the acid solution reaches the MMF-SMF boundary at the center of the fiber. They adopted the etching procedure described in Reference 31 where the reflectivity is continuously measured during etching using an appropriate optical system. The fiber is removed from the acid at the moment when the reflectivity reaches maximum value. In the following step, the prepared fiber is spliced to another flat-cleaved SMF, thus forming the sensor, as shown in Figure 3.32d.
Splicing is performed using a standard fusion splicer (Ericsson FSU 925 PM-A). While the first splice between the MMF and the SMF is achieved using standard parameters for MMF-MMF splicing, the second splice required careful adjustment of fusion parameters to ensure high reflectivity and high interference contrast simultaneously with high tensile strength. An enlarged photograph of the produced cavity is shown in Figure 3.33. The reflected spectrum of the tuned sensor is shown in Figure 3.33.