*5.1.1 Etched SMF sensor*

A 48% hydrofluoric acid (HF) (Sigma Aldrich) was used as an agent for chemically etching the SMF. For better holding of the optical fiber platform, both ends of the fiber are fixed using a metal racks so the fiber dangle into the vessel containing the acid used for the etching as shown in **Figure 3**. This is fixed inside a fume hood to prevent direct exposure to HF vapor and creation of aerosols. The etching process was started by filling 100 μl of HF acid in a container using a Pasteur pipette. To fabricate the etched optical fiber transducer, the stripped fiber is fixed as depicted in the figure to control its emersion in HF acid. The fiber is etched in two stages process to control its modification. The first stage includes immersing the stripped area in 48% HF acid at a specific time to produce different etched diameters. After that, the fiber was taken out and cleaned with deionized water for 30 minutes to remove the HF acid residual. The etched fiber is left to dry at room temperature. In the second stage, the fiber was immersed in HF with less concentrations of 12% to reduce the etching rate and hence, more control on the etching dimensions. A variety of parameters affect the etching rate of the cladding such as acid concentration, humidity and temperature. The humidity and temperature are fixed at 67% and 25 C, respectively. As the second stage of etching is finished, the fiber is immersed in deionized water for 30 minutes to remove the remaining of the HF acid as well as to prevent further etching due to residual HF [2, 11].
