On 2008-12-03, Wild_Bill wrote:
Thanks for the example Mark. Do you know of any sources of online
information (or manuals, books) that describe interfacing lenses to fiber
optic cable/bundles not just for light transmission, but the fibrescope type
applications?
Do you know of sources for a visual image optic cable approximately 0.68mm
(.027") in diameter (enclosed in a sheath)?
I have a used fiberscope with a significant number of broken fibres. I'm
wondering if the cable can be replaced, because I suspect that it's unlkely
that it could be shortened or repaired.
The light carrying fibres (a loose bundle) in the fibrescope are fairly
coarse by comparison, looking more like they could actually be worked with
by hand.
The overall size of the cable is 3mm diameter x about 3.3M.
I can see a very small optical window/lens element at the tip, but I know
nothing about the end termination methods involved with fibreoptics.
The fibers are typically two different glasses, selected for
different index of refraction, one solid as the center, and the other
hollow around the first. They are heated and the ends drawn apart, and
the outer sheath collapses onto the inner core and they fuse together.
I have seen rigid fiber optic devices which have been made by
drawing down the center of a bundle, then cutting it in the middle,
grinding polishing it. The result is a bundle which will enlarge or
shrink an image. You can get these from Edmund Scientific or Edmund
Optics (I forget which sells that) as demonstration pieces.
You can also get ones where the fibers have been fused together
and then twisted while still hot enough for the glass to bend, so the
image is twisted 180 degrees.
I've also seen fiber optic bundles fused together in a
hex, then the center drawn out to make a smaller hex bundle, gathered
with more hex bundles to farm a larger one, then drawn again through
about three cycles -- then cut into thin slices and the inner core is
removed by chemical etching leaving a honeycomb prior to coating (vacuum
evaporation of metal) to form a channel for electrons as part of an
image intensifier tube. This was called a microchannel, and was used to
intensify the tiny signals which were common in serious astronomy. The
electrons bouncing along the tubes from side to side, and accelerated by
a voltage difference between the ends of the tubes, keep kicking out
multiple additional electrons every time they hit, thus increasing the
signal strength.
Before microchannels -- larger intensifier tubes were made with
rigid fiber optic bundles ground to a curve to match the electrostatic
focusing in the tube, and ground to flat on the end -- joined to two
other similar intensifier tubes by a silicone grease to couple the fiber
images together.
The limited information that I've discovered about mating lenses in optical
equipment involves Canadian balsam (?) or special grades of epoxies.
This is normally for joining the surfaces (usually curved) of
different glasses to build a lens with just the needed index of
refraction. You'll find a lot of these joints in the more complex
camera lenses (faster and zoom lenses), and even the early Zeiss Tessar
design had four elements -- the rear two cemented together, then a space
for the iris diaphragm and shutter, and then two more elements which had
an air gap between them -- and for folding cameras typically had the
spacing between these two lens elements adjustable for focusing.
I haven't seen the interface at the eyepiece end yet, and I'm curious about
how that's accomplished.
Well ... typically the bundle for the image scopes is made by
laying out fibers one at a time into a precise pattern, then fusing the
ends (leaving the majority of the length loose for flexibility) and
polishing the ends. The objective lens focuses the image on the flat
polished surface at one end, and the eyepiece picks it up from the other
end.
Aside from the occasional broken fibers (which increase with
use), there are also occasionally misplaced fibers which move a dot from
one place to another.
If *I* were to try to make an optimum quality image fiber
bundle, what I would try to do is to lay the fibers in a loop and
optically weld the ends together. (I've seen the device which does this
with fiber optic lines used for telephone and network here in this
neighborhood, and it is a neat device.) Anyway -- the welds would be
scattered around the bundle, and the clamped group of fibers would be
fused together (again two glass types -- the center for the optical
signal, and the outer to protect the inner fiber and to fuse to adjacent
ones). Once about two inches or so of fiber is fused into a rigid
block, I would then diamond saw through the block and polish both ends
to optically flat. This way, the fibers could not shift where the images
are formed, and it does not really matter how much they shift in
between, as long as they don't get broken. That's what the sheath is to
prevent.
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But I *think* that the original mention of how small a radius
bend you can put in a glass fiber was to suggest that thin HSS (which
you had said was as brittle as glass) could be bent to an equally tight
radius if it was no thicker than the glass fiber's diameter.
IIRC -- when someone in this thread mentioned solid HSS bandsaw
blades, he did say that they were thinner than the usual carbon steel
blades.
And from bending which I have observed in a HSS (actually cobalt
steel) parting blade with a maximum thickness of 1/8", I would say that
that can be bent into a circle of about ten feet diameter or so. And
that is a *lot* thicker than any bandsaw blade which I have ever used.
Enjoy,
DoN.
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