Biophotonics_Concepts_to_Applications

(b) From Example7.3b, the radius of the bundle is 375/2μm = 187.5μm.

Then the total cross sectional area of the bundle is

Total area¼pð 187 : 5 lmÞ^2 ¼ 1 : 10  105 lm^2 ¼ 0 :11 mm^2

The ratio of the active area to the total cross-sectional area of the bundle

then is

0 : 055 = 0 : 11 ¼ 0 : 50 ¼ 50 %

7.3 Optical Fiber Tip Geometries

An important design factor for optical probes is the tip geometry of thefiber at its
distal end [ 3 ]. The tip shape controls the light distribution pattern on a tissue sample
and also determines the light collection efficiency for viewing the scattered or
fluorescing light emitted from the irradiated tissue sample.
When selecting the probe tip geometry, parameters that need to be considered
include the sizes of the illumination and light-collection areas, the collection angle
(related to thefiber NA), and thefiber diameter [ 3 ]. Another key point to remember
is thatbiological tissue has a multilayered characteristic from both compositional
and functional viewpoints. Because specific biological processes and diseases take
place at different depths within this multilayered structure, it is important to ensure
that the probing light penetrates the tissue down to the desired treatment or eval-
uation layer.
A multitude offiber probe tip configurations have been analyzed, designed, and
implemented [ 16 – 22 ]. The basic configurations when using a single opticalfiber are
illustrated in Fig.7.9. The simplest end face is aflat surface that is orthogonal to the

(a)(b)

Acceptance

angle θA

Beveled

angle

Illumination areas

Optical

fiber

Flat-polished core

fiber tip Angle-polished

fiber tip

Fig. 7.9 Basic opticalfiber probe tips:aflat-polished end face;bangle-polished end face

206 7 Optical Probes and Biosensors