Biophotonics_Concepts_to_Applications

fiber axis, as shown in Fig.7.9a. This end face will distribute the light emerging
from thefiber in a circular pattern, which is determined by the core size and the
numerical aperture. A handheldfiber-breaking tool that simply cleaves thefiber can
be used to create the end face or the end face can be polishedflat with a standard
fiber-polishing machine. In a slightly more complex tip configuration shown in
Fig.7.9b, the exit surface can be polished at an angle relative to thefiber axis to
deflect the light to a selectable area.

Example 7.5 Consider a conventional silica glassfiber that has a 50-μm

core diameter and for which the NA = 0.20. If thisfiber is used to illuminate

a tissue sample, what is the diameter of the light spot on the tissue at a

distance 1 mm from the end of thefiber?

Solution:From Eq. (3.3) the acceptance angle in air (n = 1.00) is

hA¼sin^1 NA¼sin^10 : 20 ¼ 11 : 5 

As shown in Fig.7.10, at a distancedfrom the end of thefiber, the light

will be projected onto a circular area ofπ(a+x)^2 where x =dtanθAandais

thefiber radius. Therefore the diameter Dspotof the light spot on the tissue

1 mm (1000 μm) from the end of the fiber is Dspot=2[a+d tanθA]

= [50 + 2(1000) tan 11.5°]μm = [50 + 407]μm = 0.457 mm.

If the end face angle is beveled at the critical angle for total internal reflection as
shown in Fig.7.11, then the light will leave thefiber through its side [ 3 ]. This is the
basis of what is called aside-firingfiber. For biomedical procedures the side-firing
tip can be used with a needle (e.g., 0.5–0.9 mm diameter) for treating interstitial
tumors. Other applications that involve directly using the side-emitted light
emerging from thefiber include treating atrialfibrillation, prostate enlargements,

x

2 a

Core-sized

illumination area

Annular ring

illumination area

Fig. 7.10 Illuminated spot
size of diameter 2(x +a)ona
tissue sample

7.3 Optical Fiber Tip Geometries 207