An optical fiber has interesting and useful characteristics, which are able to be applied to scientific measurement. Its phase detection characteristic based on refraction difference between two phases is applied to bubbles/droplets measurement. Recently, demands for measurement of tiny bubbles/droplets increase in research fields of steam injectors, sprays, automotive engines, fine chemical reactors, and so on. Meanwhile, laser science and engineering, particularly femtosecond pulse laser, has made remarkable advances lately. Their unique properties of the interaction between the femtosecond pulses and materials can be utilized for microfabrication. The optical fiber probe methods have been repeatedly improved in order to measure bubbles/droplets efficiently and reliably in gas-liquid two-phase flows. However, one has been taking it for granted that simultaneous measurement of their diameters and velocities needs at least two optical fiber probes. To break through this situation, we newly developed a Single-Tip Optical Fiber Probe (F-STOP) microfabricated by femtosecond pulse laser (fs-pulses) which realizes simultaneous measurement of diameters and velocities of tiny bubbles/droplets. In the F-STOP measurement, the following properties are used to realize the simultaneous measurement of diameters and velocities of bubbles/droplets: the relation between the reflected-light intensity at the wedge-shaped probe tip and the tip-surface area covered with a phase; reflected-light intensity at the groove microfabricated by fs-pulses. The first aim of the present study is to provide data for evaluation of the influences of surface tension and wettability on the bubble measurement in order to develop precise and reliable F-STOP method. The second aim is to describe the process to make F-STOP via fs-pulses. The third aim is demonstration of the newly developed probe in real measurement of bubbles/droplets. On the basis of these, the performance of the new probe is discussed.
Low-energy distortion-free femtosecond pulse laser inscription in coated, free-standing optical fiber