A novel optical fiber probe has been developed to provide mechanical stability to microbubbles generated in fluids, the tip of the fiber is etched with hydrofluoric acid to pierce a truncated horn that fastens the microbubbles to the fiber tip and prevents misalignment or detachment caused by convection currents, vibrations or shocks in the liquid. Microbubbles are photothermally generated on the etched fiber and used as Fabry-Perot cavity sensor. Two methods were used to interrogate the probe: the first one, in the wavelength domain, is suitable for calibration in static or quasi static situations; the second one, in the time domain, can be used in dynamic environments. Experimental results in the wavelength domain show that the microbubble size rises linearly with temperature and decreases with the inverse of pressure; the average slopes are 27.1 m/ 0 C and 88.3m/bar respectively. Dynamic variations of temperature have been measured in the time domain, temperature changes down to 0.007 0 C have been detected at a readout rate of 10 s -1 . Bubbles have been subjected to pressure shocks of 2.4 bar at a speed of 25 bar/s, pressure changes of 3.4 mbar have been resolved in the time domain at a readout rate of 20000 s -1 .
In the present paper, we show the experimental measurement of the growth of a microbubble created on the tip of a single mode optical fiber, in which zinc nanoparticles were photodeposited on its core by using a single laser source to carry out both the generation of the microbubble by photothermal effect and the monitoring of the microbubble diameter. The photodeposition technique, as well as the formation of the microbubble, was carried out by using a single-mode pigtailed laser diode with emission at a wavelength of 658 nm. The microbubble’s growth was analyzed in the time domain by the analysis of the Fabry–Perot cavity, whose diameter was calculated with the number of interference fringes visualized in an oscilloscope. The results obtained with this technique were compared with images obtained from a CCD camera, in order to verify the diameter of the microbubble. Therefore, by counting the interference fringes, it was possible to quantify the temporal evolution of the microbubble. As a practical demonstration, we proposed a vibrometer sensor using microbubbles with sizes of 83 and 175 µm as a Fabry–Perot cavity; through the time period of a full oscillation cycle of an interferogram observed in the oscilloscope, it was possible to know the frequency vibration (500 and 1500 Hz) for a cuvette where the microbubble was created.
We demonstrate that a cone pierced in a fiber tip can stabilize microbubles photothermally generated in liquids. Bubbles can stand and monitor pressure shocks over 3.3 bar with a sensitivity below 7 mbar.
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