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Fiber
Optics
in the majority of CCD applications, light reaches
the CCD through a lens- or mirror-based optical system.
However, in some situations it is advantageous to use
an image-preserving fiber optic bundle in place of conventional
imaging optics. Significant gains in the amount of light
collected can be achieved by directly coupling the light
source to the CCD using fiber optics. Depending on the
amount of demagnification, the gain in light collected
can exceed 10x that of a f/1.2 lens.
Imaging fiber optics are commonly used to couple
light from x-ray or neutron scintillator screens, chemiluminescent
markers, image intensifiers, or streak tubes. Fibers
can be bonded to most front-illuminated CCDs as well
as some back-thinned devices.
The Coherent Fiber Bundle
A coherent fiber
bundle is a collection of single fiber optic strands
assembled together so that the relative orientation
of the individual fibers is maintained throughout the
length of the bundle. The result is that any pattern
of illumination incident at the input end of the bundle
re-emerges from the output end with the image preserved.
Imaging fiber bundles can be made in a variety of shapes
and sizes, with the most common having a circular cross
section. Magnification can be achieved by the use of
tapered fibers in the bundle.
Proprietary Fiber Bonding Process
In order
to successfully couple light from an imaging fiber bundle
to the CCD, the CCD and  fiber
bundle must be in very close proximity. Light emerges
from the individual fibers at large angles, and a gap
between fiber and CCD will lead to a loss in resolution.
Princeton Instruments uses a proprietary bonding process
to minimize the distance without sacrificing CCD performance.
This process directly bonds the fiber to the CCD without
oil layers or the use of intermediate fiber stubs that
introduce losses in spatial resolution and transmission
efficiency. In addition, the bond is stable and will
survive the repeated thermal cycling that occurs
in HCCD camera systems. Princeton Instruments continuous
innovation in fiber bonding has extended available fiber
tapers to over 165mm in diameter, coupled fibers to
the largest commercially available scientific sensors,
and even mated fiber bundles to high efficiency back-illuminated
sensors.
Efficiency vs. Magnification
Besides the
transmission losses through a large piece of glass,
fiber-optic bundles have a transmission loss due to
changes in the fiber diameter as light traverses the
bundle. When light travels down a tapered fiber, a decreasing
reflectance angle results in some of the light paths
exiting the fiber. This appears as a loss in "effective"
numerical aperture (NA). The relative loss between fibers
with different magnifications can be estimated as the
ratio of their magnifications squared. The larger the
fiber bundle's magnification, the greater the reduction
in effective NA. Fiber bundles with a 1:1 magnification,
known as "stubs," provide the highest throughput.
Applications requiring the highest possible light collection
efficiency benefit most by using large CCDs to reduce
the amount of demagnification required.
Limitations of Imaging with Fiber Optics
A
disadvantage of fiber imaging systems is that field
of view is limited by the size of available fiber bundles.
Currently, the largest available fiber optic tapered
bundle is 165 mm in diameter at the large end. However,
to enable imaging of even larger areas, Princeton Instruments
can create a mosaic of fiber bundles which are connected
to multiple CCDs. This assembly can either be packaged
in a single camera head, or into multiple camera heads,
depending upon the number of bundles in the mosaic and
whether or not the bundles are tapered. A second limitation
of fiber optics is the introduction of distortion and
non-uniformity of response. These defects are introduced
during the fibers manufacturing process. Because these
defects are static, they can be corrected through image
processing. For example, response non-uniformity can
be handled in most cases by flat-field correction. Gross
distortion can be corrected by appropriate scaling and
warping of the image data. Shear distortion, sudden
dislocation in the alignment of adjacent fibers, is
more difficult to correct for due to its discontinuous
nature. Photometrics fiber defect specifications are
available for customers requiring detailed information.
Fiber Optic Options
Many of our cameras
are available with imaging fiber optics. Fiber bundles
range in magnification from 1:1 fiber stubs to large
6:1 fiber tapers, and in diameters up to 165mm. Supported
CCDs vary from 512 x 512 pixels to 4096 x 4096 pixels.
Fiber bundles are available with extramural absorption
(EMA) fibers to improve contrast, and low-thorium glass
to reduce background from radioisotopes. At a customer's
request, Princeton Instruments will also attach scintillating
fiber optic faceplates to the front of fiber optic tapers.
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