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hlmark
Posted on Friday, November 04, 2005 - 12:31 pm:   

Heinrich - good point. In fact, it reminded me of a paper I read once, where a fiber was used as what was effectively an internal-reflectance probe. All that was done was to coil up a length of fiber, a section of which had no cladding, and dip it into the sample.

Howard

\o/
/_\
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Heinrich Pr�fer (H_Pruefer)
Posted on Friday, November 04, 2005 - 12:10 pm:   

Nuno and colleagues,

Please allow me a few very late remarks on this topic.

I have been working with low-OH step-index silica fibers in NIR online applications for a couple of years. Some of the observations I remember are:

1) When fibers with transparent coating are used in the dark room to transmit e.g. intensive red laser light, you can really see that with smaller bending radius more light leaves the fiber. So curves affect the signal because they shift a certain amount of transmitted rays beyond the angle of total reflection.

2) There are effects of macro- and microbending which can be found experimentally. Microbending introduced by vibrations due to large process pumps has been observed with fiber bundles as well as with single fibers as periodic modulations on the spectra.

3) Even more interesting: fiber bending can add variance with band structures. An example is shown in this plot where a 5 m fiber as sample was measured against a 1 m fiber from the same preform as reference in a dual-beam AOTF spectrometer.
application/pdffiber spectra
fiber_bending.pdf (7 k)

One spectrum was measured with the straight sample fiber, the other one when it was rolled-up. (Both spectra are corrected with the same baseline.)
I believe that this effect is due to a larger amount of cladding modes reaching the detector.

So it should be considered that optical fibers themselves can act as "sensors" and have some influence on the resulting spectra.

Best regards,
Heinrich
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hlmark
Posted on Friday, September 30, 2005 - 8:44 pm:   

Nuno - one possible explanation is that when you bend the fiber, micro-cracks will develop at the surface. While there's no actual absorption, the micro-cracks could reflect or scatter the light away from the axis of the fiber, allowing it to escape and giving the effect of reducing the light throughput.

Howard

\o/
/_\
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W. Fred McClure (Mcclure)
Posted on Friday, September 30, 2005 - 6:04 am:   

Nuno,

Your advisor is correct. There is a similarity to mirrors and to ATF crystals. The more times you bounce the energy between mirrors the less you will end up with. The more you curve a fiber the more reflectios you get. Hence, you get less energy out the other end.

If you could bend the fiber at right angles without breaking it, you would reflect (stop) the remaining energy totally.

I used plastic fibers up to 12 feet and found curving was very critical. In addition, I found that it was best to "fix" the cable so it would not move during measurements.

Good luck,

:-)

Fred
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Nuno Matos (Nmatos)
Posted on Friday, September 30, 2005 - 5:34 am:   

Dear members,

I'm working with a NIR probe and a 15m fiber optics extension. Someone (that works with fiber optics for a very long time) told me that the more curves the cable has less signal we will get.

This statement doesn't seem logical to me. Does any one have a thought about it?

Thank You in advanced

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