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Product Configurator/ Light Guiding
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In modern spectrocopy the most convenient
way to transport or deliver light is the use of light
guides based on optical fibers. Fibers can be made
of divers materials, for high-end spectroscopy use
mainly quartz based fibers are used. Light cannot
only be transported by light guides, it can also be
taylored. One typical example is the design of so-called
cross-section converters, based on a bundle of fibers
with a round configuration on one end and linearly
stacked fibers on the other side.
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The features of fibers allow to match
individual optical parts like spectrometers, light
sources or probes to be match. While designing light
guides the characteristics fo these functional units
have tbe kept in mind. The following considerations
have to be taken to design a light guide:
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Feature
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Consideration
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Information
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Funtionality/ Type
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What is the function
of the light guide: transfering light, splitting light,
creating/shaping illumination pattern
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Monofibers have
lowest losses due to the "undisturbed" optical
aperture area, and input equals output. To taylor
a Y type splitter or a cross-section converter bundles
of smaller fibers are required. Various diameters
as well as materials can be mixed. Bundles of thinner
fibers are less influenced by bending than monofibers
of same effective diameter, but show higher losses.
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Material/ Transmission
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What is the spectral
range to be covered, what is the numerical aperture
of the fiber
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Two main classes:
UV-VIS quality for 190 to 950nm, contains OH bands,
which has its first absorption band at 960nm, VIS-NIR
quality for 400 to 2500nm, at the long wavelength
end damping rises significantly
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Diameters
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Diameter of indifidual
fibers used (different diameters can be within one
strand), complete bundle diameter and diamter of indivudual
arms, shape, configuration of bundle
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Most typical diameters
are 80 to 100µm (for bundles) and 600micron
(monofiber extensions, but as well thinner, inbetween
or even 800µm fibers are available. The diameter
influences the reasonable bending radius. If the diameter
becomes too big, the fiber becomes a rod cannot be
bended at all. The total diameter of a fiber is the
core diameter as effective optical sensitive area,
in addition comes the cladding, typically 10% of the
core diameter.
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Length
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What is the total
length of the light guide, how long have the individual
arms to be
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Optical fibers
can be as long as several 100ms or even km. However,
to keep losses and cost at a mimimum it the shorter
the better.
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Connectors
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What type of connections
is required, has the light guide to be aligned in
repsect to position in z direction, angulat orientation?
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The (S-)SMA (905)
connector is the most wide spread. Limitations are
the diameter it can hold (approx. 2mm), and the z
position is fixed. Ferrules are more flexible and
overcome the two cited drawbacks, but they are not
standardized. A set-screw is required to fix the position.
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Sleeve
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How good has the
light guide to be protected? Are there limitations
in respect to diameter?
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Various sleeve
designs and materials are available. A typical robust
quality for process control contains a metal spring
with a rubber jacket.
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