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2.4 Output Beam Propagation
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The lens discussed as a coupler above must also be used to "collimate"
the output of a fiber in the fiber tap. Since the output of the fiber is Gaussian, it
will expand as it propagates away from the lens. If arrays of lenses and fibers
are to be used for interconnects, beam expansion must be minimized to reduce
optical crosstalk between neighboring fibers. Beam expansion is determined
by the microlens focal length ana diameter, which in turn depend upon the
spacing between fiber centers.
The minimum possible fiber spacing in an array will be equal to the
diameter of the fiber cladding, a typical value being 125 nm. Assuming that the
output of the fiber fills the lens, the ratio of lens diameter to beam waist as a
function of distance may be calculated as in Figure 2-3 for several different lens
diameters. It is clear from the figure that the beam divergence, and thus
crosstalk between channels, increases with reduced lens diameter. Hence, a
microlens diameter of 500 |am was chosen to minimize this problem. Table 2-1
summarizes the resulting lens parameters that were calculated based on these
factors.
diameter
500 p.m
NA .16
focal length 1.5625 mm
F# 3.125
Table 2-1 Microlens design parameters.