**4. Degradation of optical fiber tip**

Laser-induced hydrodynamic effects in water and bio-tissues can lead to the significant degradation of the fiber tip (Yusupov et al., 2011a). The most significant degradation of the

It is known from (Bagratashvili et al., 2006) that the mechanical action on cartilages in the hertz frequency range actively stimulates the synthesis of collagen and proteoglycans even at relatively small amplitudes. The above estimations show that the pressure on biotissue provided by the vapor-gas bubbles can reach tens of kilopascals. In accordance with (Buschmann et al., 1995; Millward-Sadler & Salter, 2004), such pressures in the hertz frequency range can lead to regenerative processes in cartilage owing to the activation of the interaction

Note an interesting phenomenon in the experiments on the generation of bubbles in the vicinity of the blackened tip surface of the fiber in the water cell: bubble microjets can be generated at a laser power of less than 3 W (Fig. 11) (Yusupov et al., 2010). The lengths of the microjets (Fig. 11a), which always start in the immediate vicinity of the fiber tip, reach several millimeters, the transverse sizes normally range from 10 to 50 μm, and the sizes of the bubbles that form the jets range from several to ten microns. The lifetime of the microjets ranges from a few fractions of a second to tens of seconds. A microjet that emerges at a certain spot on the tip surface remains attached to this spot and exhibits bending relative to the mean position. Bubble microjets didn't use to be continuous from start to end, the discontinuities used to appear on them, which used to restore quite often. The observations show (Yusupov et al., 2010) that the discontinuities are always related to the hydrodynamic perturbations and are caused by relatively large bubbles that move in the vicinity of the microjet. The appearance of quite a large bubble attached to the fiber tip caused the bubble microjet bending around large bubble (Fig. 11b). Thus, we conclude that two conditions must be satisfied for the generation of the bubble microjets. First, a hot spot must be formed on the tip surface. Second, the neighborhood of such a spot must be free of the centers that provide the generation and detachment of large bubbles. Note that the possibility of bubble microjets in the vicinity of a point heat source is demonstrated in (Taylor & Hnatovsky,

of the extracellular matrix with the mechanoreceptors of chondrocytes (integrins).

Fig. 11. Bubble microjets in the vicinity of the tip surface of optical fiber.

Laser-induced hydrodynamic effects in water and bio-tissues can lead to the significant degradation of the fiber tip (Yusupov et al., 2011a). The most significant degradation of the

A part of the blackened fiber tip is sown at the right upper corner.

**4. Degradation of optical fiber tip** 

**3.3 Laser-induced generation of bubbles microjets** 

2004).

fiber tip surface occurs in the regime of channel formation when the fiber is shifted inside the wooden bar that mimics the biotissue. In this case, we observe substantial modifications and distortion of tip surface. The comparison of the sequential photographs (Fig. 12) shows a significant increase in the volume of the fiber fragment (swelling) in the vicinity of fiber tip.

Fig. 12. Modifications of the profile of the blackened fiber tip surface (side view) for regime of channel formation (the channel is formed by the fiber that moves inside the wooden bar with water and the radiation power is 5 W). The left-hand panel shows the original fiber just after its blackening (Yusupov et al., 2011a).

SEM images (Fig. 13) show that the laser action in the regime of the channel formation in the presence of water causes substantial modifications of the working surface: the sharp edge is rounded and surface irregularities (craters) emerge on the tip surface. The image shows that a thin shell (film) with circular holes is formed at the tip surface of the optical fiber. Multiple cracks pass through some of the holes. In addition, we observe elongated crystal-like structures on the surface (Fig. 13b). Looking through the largest hole in the film on the tip surface (at the center of the lower part of the fragment at Fig. 13a), whose dimension in any direction is greater than 10 µm, we observe the inner micron-scale porous structure.

Fig. 13. The microstructure of the fiber tip surface after laser action. **a -** SEM image of a fragment of the fiber end surface; **b -** magnified SEM image of a fragment of the end surface with the crystal-like structures on the surface (Yusupov et al., 2011a).

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 109

The fiber tip surface is blackened before laser irradiation with 0.97 µm wavelength.

cases: in a bath of free water (**a**) and in a water-filled capillary (**b**).

feature, and low-frequency modulation period is about 2 s.

Arrows show the moments of laser on and laser off.

Fig. 15. Fragments of acoustic response to 3 W laser irradiation of water for two different

filled capillary (Fig. 15b). In the case of the bath with free water, the short random laserinduced acoustic spikes take place. At the same time, the acoustic response to laser irradiation in the case of water-filled capillary (which imitates situation in real water-filled biotissue channel) is different (Fig. 15b). Acoustic signal is amplitude-modulated by its

Fig. 16 demonstrates acoustic response to laser irradiation of nucleus pulposus *in vivo* when optical fiber was moved forward (regime of channels formation in the course of laser healing of degenerated disc). The acoustic signal is non-stationary by its nature. The shortpulse intense acoustic spikes take place and the signal itself is amplitude modulated

(similarly to that in water-filled capillary) with a modulation period of about 3 s.

Fig. 16. Acoustic response to 3 W laser irradiation with 0.97 µm wavelength of nucleus pulposus *in vivo,* when optical fiber was moved forward in the intervertebral disc.

Typical micron-scale circular holes on the film surface (Fig. 13a) can be caused by cavitation collapse of single bubbles. It is well known that cavitation collapse of bubbles in liquid in the vicinity of the solid surface gives rise to the high-speed cumulative microjets which can destroy the solid surface (Suslick, 1994). Apparently, this effect leads to multiple cracks on the film and the formation of the porous structure (Fig. 13a), since the cumulative microjets can punch holes, cause cracks in the film, and destroy the structure of silica fiber tip.

Collapse of cavitation bubble apart from high pressure generation (up to106 MPa) can cause overheating of gas up to temperatures as high as 104К. Such high values of water pressure and temperature can result in formation of supercritical water (critical pressure of water is Рc=218 atm, critical temperature - Tc =374ºС), which can dissolve silica fiber (Bagratashvili et al., 2009).

Fig. 14 shows Raman spectra of some areas of laser irradiated fiber tip surface (curves 3-5) compared with that of graphite (1) and diamond (2). Raman bands at 1590 cm-1 and 1590 cm-1 to diamond and graphite nano-phases correspondingly (Yusupov et al., 2011a).

Fig. 14. Raman spectra from different areas of laser fiber tip surface (curves 3, 4 and 5) compared with that of graphite (1) and diamond (2) (Yusupov et al., 2011a).

Formation of diamond nanophase at a fiber tip surface in this case is rationalized by extremely high pressures and temperatures caused by cavitation processes stimulated by laser irradiation (Yusupov et al., 2011a).
