Biophotonics_Concepts_to_Applications

melanin, blood, and the epidermis are larger than that of water, they have relatively
small concentrations compared to water and thus account for a comparatively small
amount of absorption in the therapeutic window.
Light absorption characteristics of tissue for regions outside the therapeutic
window are important for implementing functions that depend on high optical
power absorption, such as drilling, cutting, bonding, and ablation of tissue. As
Chap. 4 discusses, a wide variety of optical sources can be used to carry out these
functions. For example, as is indicated in Fig.1.6, ultraviolet light from ArF or KrF
lasers emitting at wavelengths of 193 and 249 nm, respectively, is strongly
absorbed in the surface of a tissue and thus can be used for many surgical appli-
cations. In the infrared region, 2940 nm light from an Er:YAG laser is strongly
absorbed by osseous minerals, which makes optical sawing and drilling in bone and
teeth possible. Also in the infrared region, strongly absorbed light from CO 2 lasers
are used in applications such as surgery, ophthalmology, and cosmetic treatments.

1.5 Signal Attenuation

Some type of optical link configuration is used for many biophotonics imaging,
therapeutic, monitoring, and tissue evaluation techniques. Such a link typically
contains a light source, an opticalfiber, a photodetector, and various intermediate
optical components. As an optical signal passes through such a link, the signal loses
a percent of its power within each constituent of the link. An understanding of the
degree of signal attenuation is important for assuring that the correct optical power

Wavelength (μm)

Absorption coefficient (cm

-1

)

KrF

ArF 249 nm

193 nm

Er:YAG

2940 nm

Diagnostic or

therapeutic

window

Water

Whole

blood

Melanin

Water

CO 2

Proteins

10.6 μm

Fig. 1.6 Absorption coefficients of several major tissue components as a function of wavelength
with example light source emission peaks

12 1 Overview of Biophotonics