Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 7

Cladding (n cl )

Air (n i )

q i q r

Core (n co )

Fig. 1.3 Reflection, refraction, and light propagation in the fiber. i and r are the incident angle
in the air and the refraction angle in the fiber. Light propagation is governed by the internal total
reflection. n co and n cl are the refractive index of the core and cladding

the refractive index of core or decreasing the refractive index of the cladding,
the numerical aperture will increase, which will increase the acceptance angle
and the field of view of the optical fiber. Currently, the most commonly used
silica/silica (core/cladding) fibers have an NA of 0.22. Some specialized fibers such
as silica/hard polymer or silica/Teflon have an NA of 0.37 and 0.66.
To minimize light loss, the NA of the fiber needs to match the coupling optics.
The light loss due to NA mismatching is given by

D 10 log NA 1
NA 2

2

Loss NA ; (1.2)

where NA 1 and NA 2 are the NA of the coupling optics and fiber, respectively.
When NA 1 > NA 2 , there is light loss because the coupling is overfilling the fiber.
Theoretically, there is no light loss when NA 1 NA 2 because all light from the
coupling optics is accepted by the fiber. But in practice, in order to design high-
throughput optical system, the ideal situation is to make the coupling optics and the
fiber to be perfectly NA matched (NA 1 D NA 2 ). The choice of the fiber determines
the coupling lens by NA D D=.2f e /,whereD is the diameter of its entrance pupil,
in most cases the diameter of the lens, and f e is the effective focal length of the lens.
For example, an NA

D 0:22 fiber requires a D D 25 mm and f e D 56:8 mm lens

for perfect NA matching.

1.2.2.1 Single Mode

Single-mode fiber is a kind of fiber that there is only one mode, the primary mode,
of light that can be transmitted in the fiber. The core size of single-mode fiber is
usually within 4-10 m. The mode of the light travels through the fiber with much
smaller distortion compared to multimode fiber. Single-mode fiber is the choice of
long-distance data and voice communication. Because the core size is small and
cannot afford to high optical power, it is generally not suitable for power delivery.
Single-mode fibers have been used in confocal Raman spectroscopy applications