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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.
Photometrics' 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. Photometrics 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. Photometrics'
continuous innovation in fiber bonding has extended available fiber tapers
to over 145mm 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 160mm in diameter at the large
end. However, to enable imaging of even larger areas, Photometrics 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.
Photometrics Fiber Optic Options
Many of Photometrics' standard products 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 160mm. Supported CCDs
vary from 512x512 pixels to 2kx2k 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,
Photometrics will also attach scintillating fiber optic faceplates to the
front of fiber optic tapers.