Tech Briefs Magazine - November 2021 - PIT-15

Light beam
nair
Core
nclad
ncore
Cladding
the angular output of the fiber. The NA
is equal to the sine of the highest angle
that can enter and exit the fiber,
NA= nair sin( ) = ncore
2
ncladding
.
The NA of the fiber is determined by
the refractive index of the core and
Figure 2. A fiber taper is a common intermediate component used to alter the light at an interface
between a light source and endoscopic fiber bundle. As the taper diameter decreases, the ray angles
increase, resulting in a higher angle distribution at the exit than the entering angle.
blue light is absorbed by red tissue. The
high-blue signal in the spectra can lead
to challenges in the color tuning stage
and, potentially, images that look too
digitized or " fake looking " . A mixed
RGB LED approach can eliminate the
excess blue-light issues but requires
more complex optics in the light engine
in order to couple the three sources into
the system. If the system requires near
infrared (NIR) illumination, the NIR
LEDs are installed in the light engine as
well, compacting the design.
By housing the RGB and NIR LEDs in
the same housing, the light sources can
share the same fibers that deliver the
light to the tip. This maximizes the efficiency
of the endoscope's illumination
system. To transfer light from the light
engine to the tip of the endoscope, a
high-numerical (NA) aperture fiber
optics are required as well as an optical
system to relay light from the source to
the tip. The term to describe the angular
output of a fiber is numerical aperture,
or NA. The higher the NA, the higher
cladding of the fiber. The higher the
NA, the higher the angle light will exit
the fiber, illuminating a higher percentage
of the field of view.
To get the best performance from the
fiber optic cable, the design team must
consider the relationship between the
output of the light engine and the fiber
optic cable. A common solution is to use
a fiber taper to increase the angle of the
light entering the endoscope. The fiber
taper is typically installed on the proximal
end of the endoscope where the
light cable connects. The fiber taper
converts the large-area, low-angle output
of the light engine to a small-area, highangle
output.
The NA of the light exiting the light
box is typically on the order of 0.5 NA, the
angles associated with surgical robotics
can reach 0.87 NA or higher. The fibers
2
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Tech Briefs Magazine - November 2021

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