Author(s): Vogel, Alfred; Venugopalan, Vasan | Abstract: The mechanisms of pulsed laser ablation of biological tissues were studied. The transiently empty space created between the fiber tip and the tissue surface improved the optical transmission to the target and thus increased the ablation efficiency. It was found that the structure and morphology also affect the energy transport among tissue constituents.

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Mechanisms of pulsed laser ablation of biological tissues.

Laser-tissue interactions

Fiber optic probes are a key element for biomedical spectroscopic sensing. We review the use of fiber optic probes for optical spectroscopy, focusing on applications in turbid media, such as tissue. The design of probes for reflectance, polarized reflectance, fluorescence, and Raman spectroscopy is illustrated. We cover universal design principles as well as technologies for beam deflecting and reshaping.

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Fiber optic probes for biomedical optical spectroscopy.

LASER ablation propulsion (LAP) is a major new electric propulsion concept with a 35-year history. In LAP, an intense
laser beam [pulsed or continuous wave (CW)] strikes a condensedmatter surface (solid or liquid) and produces a jet of vapor or plasma.
Just as in a chemical rocket, thrust is produced by the resulting reaction force on the surface. Spacecraft and other objects can be propelled in this way. In some circumstances, there are advantages for this technique compared with other chemical and electric propulsion schemes. It is difficult to make a performance metric for LAP, because only a few of its applications are beyond the research phase and because it can be applied in widely different circumstances that would require entirely different metrics. These applications range from milliwatt-average-power satellite attitude-correction
thrusters through kilowatt-average-power systems for reentering near-Earth space debris and megawatt-to-gigawatt systems for direct launch to lowEarth orbit (LEO). 
We assume an electric laser rather than a gas-dynamic or
chemical laser driving the ablation, to emphasize the performance as an electric thruster. How is it possible for moderate laser electrical efficiency to givevery high electrical efficiency? Because laser energy can be used to drive an exothermic reaction in the target material controlled by the laser input, and electrical efficiency only measures the ratio of exhaust power to electrical power. This distinction may
seem artificial, but electrical efficiency is a key parameter for space applications, in which electrical power is at a premium.
The laser system involved in LAP may be remote from the
propelled object (on another spacecraft or planet-based), for
example, in laser-induced space-debris reentry or payload launch to low planetary orbit. In other applications (e.g., the laser–plasma microthruster that we will describe), a lightweight laser is part of the propulsion engine onboard the spacecraft.

Review: Laser-Ablation Propulsion

Underwater and water-assisted laser processing: Part 2—Etching, cutting and rarely used methods

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 μm of the CO2 laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 μs and a long pulse of 250 μs. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO2 radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm3. Histological examination revealed a minimal zone of 10–15 μm of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 μm is lower than at 10.3 and 10.6 μm. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO2 radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm3.

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d=300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T=280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At lase...

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

An experimental study was made of the heating of metal targets by CO2 laser pulses of intensity 1–100 MW/cm2 at various air pressures (10–5–1 atm). Initiation of breakdown in the vapor of the target material or of low-threshold breakdown in air increased considerably the deposition of the laser energy in the target. A size effect was then observed: the effective absorptivity of metal targets increased when the ratio of their radius to the radius of the illuminated spot was made larger. The results were explained by the contact of the breakdown plasma with the target surface in the course of the former's expansion into the ambient medium.

https://iopscience.iop.org/article/10.1070/QE1979v009n01ABEH008568/pdf

Heating of metals by CO2 laser radiation pulses

The effect of a breakdown plasma in air and of oxidizing reactions occurring on the surface of metallic targets on the temperature dependence of their effective absorption coefficient was investigated. A lowering of the threshold for the breakdown of air by pulse-periodic CO2 laser radiation close to a copper target was discovered when the target was heated. A comparison was made between the kinetics of heating metals in air by means of cw and pulse-periodic laser radiation.

http://iopscience.iop.org/article/10.1070/QE1978v008n07ABEH010465/pdf

Investigation of the absorption coefficient of metal targets irradiated by pulse-periodic CO2 laser radiation in air

A high speed camera was used to record time-resolved the development
and the shape of the holes drilled by Er:YAG laser pulses of high
intensity. The experimental results suggest that above a critical
radiant exposure the dynamics of the drilling mechanism changes and
driven by a high vapor pressure gradient, hydrodynamic instabilities
within the hole occur. They are evidenced by dramatic local
variations of the channel diameter and by a decrease of the
normalized recoil momentum. These instabilities influence strongly
the extent, and velocity of the ejection. It is shown that, when
cutting soft tissue, the extent of the zone of thermal damage is
determined by the amount of hot liquefied material that remains in
the hole and acts as a heat source.

Alexander S. Silenok

Papers

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

Heating of metals by CO2 laser radiation pulses

Investigation of the absorption coefficient of metal targets irradiated by pulse-periodic CO2 laser radiation in air

Dynamics in laser cutting of soft media