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

Lokar, Žiga (author), Petelin, Jaka (author), Horvat, Darja (author), Agrež, Vid (author), Petkovšek, Rok (author)

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.

• Language: English
• Keywords: shockwave, fiber optic hydrophone, cavitation bubbles, interferometry
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FS - Faculty of Mechanical Engineering
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: 9 str.
• Numbering: Vol. 172, art. 111655
• PID: 20.500.12556/RUL-175896
• UDC: 532.528:533.951:621.37
• ISSN on article: 0894-1777
• DOI: 10.1016/j.expthermflusci.2025.111655
• COBISS.SI-ID: 257013763
• Publication date in RUL: 13.11.2025

Record is a part of a journal

• Title: Experimental thermal and fluid science
• Shortened title: Exp. therm. fluid sci.
• Publisher: Elsevier
• ISSN: 0894-1777
• COBISS.SI-ID: 170523

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: udarni val, hidrofoni, optična vlakna, kavitacijski mehurčki, interferometrija

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: P2-0270
• Name: Proizvodni sistemi, laserske tehnologije in spajanje materialov

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: L2-4485
• Name: Merjenje absorpcije črnega ogljika s sistemom na osnovi optičnih vlaken