Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 219
Figure 2. Optical absorption coefficients of principal tissue chromophores in the 0.1–12 µm
spectral region. (Reprinted with permission from Ref. [6]. Copyright 2003 American Chemical
Society)
transmission T is governed by Beer-Lambert’s law according to:
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Φ
T =
= exp(−µal) , (1)
Φ0(1 − Rs)
where Rs is the specular reflection of the sample and Φ is the radiant exposure
transmitted after travel through an optical path length l in a sample with absorption
coefficient µa [cm −1 ]. In the absence of scattering, the reciprocal of the absorption
coefficient (1/µa) defines the optical penetration depth δ and thus the characteristic
depth of laser energy deposition.
For most laser wavelengths used only a single tissue constituent (e. g., water or
collagen) absorbs the radiation. Therefore the spatial scales that characterize the
distribution of these constituents are vital to understanding the potential energy
transfer mechanisms between tissue constituents. The vast majority of tissue water
resides in cells and in the ground substance of the ECM in which the collagen fibrils
are embedded [6,10], and the spatial scale characterizing domains with different absorption
properties is given by the diameter and spacing of the collagen fibrils. The
fibril diameter is ≈ 30 nm in cornea and varies between 20 nm and 120 nm in dermis.
The centre-to-centre spacing is very regular (≈ 65 nm) in the transparent cornea and
exhibits more variations in other tissues [6,9].
Optical scattering arises from spatial variations in refractive index within tissue
that are particularly strong between collagen fibrils and ground substance. Typical
reduced scattering coefficients for tissues in the green are on the order of µ ′ s = 10–
40 cm −1 [11]. The wavelength dependence of the reduced scattering coefficient µ ′ s is