Laser induced cavitation bubble and shockwave measurements with fiber optical hydrophone very close to origin

• Cavitation phenomena measurement using fiber optical hydrophone at short distances. • Sectional signal filtering approach to separate shockwave and interference waveforms. • Interference signal is used to measure bubble growth. • Shockwave signal is used to measure shockwave attenuation in water. • A transient pressure above 1GPa measured extremely close to the source. Laser induced breakdown in water generates shockwaves and cavitation bubble, both originating from the breakdown of the plasma. The dynamics of a shockwave propagating from a point source changes significantly with distance. In spherical geometry without energy loss, the energy spreads out in accordance with the inverse square law, the pressure decreases proportionally with distance. Thus, closer to the shockwave source a more pronounced shockwave dynamics is expected. A fast, robust and precisely positioned sensor with a small detection area is needed to accurately measure the shockwave pressure in such conditions. A single mode optic fiber-based hydrophone meets these requirements. In addition to measuring the shockwave induced change in light reflectance from the fiber tip, the fiber optic hydrophone, when positioned very close to the breakdown bubble, picks up some of the light which exits the sensor and is reflected from the bubble wall. Due to bubble growth, this results in an oscillating interference signal. We introduce signal processing to distinguish between the two signals, originating from different phenomena but captured in a single oscilloscope trace. While using only FFT processing to remove the oscillations from the signal does not preserve the shockwave properties properly, the introduced local filtering procedure allows for correct separation of the two overlapping contributions to the waveform. The transient interference signal allowed for extraction of tens of bubble wall velocity data points in the first 100 ns of a single bubble growth event, showing the very early bubble growth behaviour. After this contribution was eliminated from the signal, the shockwave pressure trace was successfully assessed at very small distances, down to the situation where the hydrophone nearly touched the plasma edge. Pressure trace allowed for extraction of various parameters such as maximum pressure, risetime, shockwave energy and pressure impulse. The shockwave energy was measured to decay as 6.5 dB/mm for distances above 50 μm.