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CHAPTER 2
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LENS PARAMETERS FOR FIBER INTERCONNECTS
The fiber optic tap presented in chapter one utilizes diffractive lenses to
couple light into and out of fibers. This chapter first reviews important fiber
parameters that need to be considered when designing lenses for this purpose.
The effects of lens size and chromatic aberration on fiber coupling efficiency are
then explored.
2.1 Optical Fibers
The characteristics of optical fibers must be taken into account when
designing a lens to be used for fiber interconnects. Important fiber parameters
to consider are numerical aperture (NA), normalized frequency parameter, and
output irradiance profile. 25
Optical fiber is designed with the index of the core being greater than the
index of the cladding so light will propagate down the fiber through total internal
reflection. The largest acceptance angle of the fiber, 0, can be defined in terms
of the fiber's NA,
NA = sind ~ Vnf -nf 2.1
where ^ is the index of refraction of the fiber core, and n 2 is the index of
refraction of the fiber cladding. Just as for any optical component, the NA of the
fiber is a measure of its light gathering power.
Another important fiber characteristic to consider is the normalized
frequency parameter or V number. The V number determines the number of