assumed that the side-emitting effect is much stronger than light absorption and
other losses in thefiber, then the side-glowing radiation intensity ISthat is emitted
in any direction per steradian at a distance x from the light input end is
IS¼
I 0
4 p
expðÞkx ð 3 : 18 Þwhere I 0 is the input radiation intensity and k is the side-scattering efficiency
coefficient. Typical values of k range from 0.010 to 0.025 m−^1.
In biomedical applications where the glowingfiber lengths are on the order of 10
or 20 cm, the effect of an exponential decrease in side-emitted light intensity may
not be a major problem [ 5 ]. However, there are two methods for creating a more
uniform longitudinal distribution of the side-emitted light. One method is to inject
light into both ends of thefiber simultaneously, if it is possible to do so. In other
applications, a reflector element attached to the distal end of thefiber can produce a
relatively uniform emitted light distribution of the combined transmitted and
reflected light if thefiber is not too long.
Example 3.12Consider a 10 cm long side-emittingfiber diffuser that has a
side-scattering efficiency coefficient of k = 0.02 m−^1. What is the emitted
intensity at the 10-cm point relative to the intensity at the diffuser input point?
Solution: From Eq. (3.18)IsðÞ¼10 cm IsðÞ0cmexpðÞ¼kx IsðÞ0cmexp½ð 0 :02 m^1 ÞðÞ 0 :1m
¼ 0 :998 IsðÞ0cmThus at 10 cm the emitted intensity is 99.8 % of that at the diffuser input
point.3.12 Middle-Infrared Fibers
Amiddle-infrared fiber, or simply aninfraredfiber(IR fiber), transmits light
efficiently at wavelengths greater than 2μm[ 47 – 53 ]. Typically in biomedical
photonics, IRfibers are used in lengths less than 2 to 3 m for a variety offiber optic
sensors and optical power delivery applications [ 5 ]. Based on thefiber material and
structure used for their fabrication, IRfibers can be classified into glass, crystalline,
photonic crystal fibers, and hollow-core waveguide categories. This section
describes glass and crystallinefiber classes. Hollow-core waveguides and photonic
crystalfibers are described in Sects.3.8and3.9, respectively.
Glass materials for IRfibers include heavy metalfluorides, chalcogenides, and
heavy metal germinates [ 5 ].Heavy metalfluoride glasses(HMFG) are the only
3.11 Side-Emitting or Glowing Fibers 81