RE: [amsat-bb] Fiber Optics at RF

["Greg D." <ko6th_greg@xxxxxxxxxxx>]

Hi folks,

The difference between single and multi-mode has to do with how the light
bounces down the fiber cable from one end to the other.  Single mode fiber
has a very small (9 micron) core (where the light goes), and this constrains
the light to bonce all the same way.  What this does is keep the distance
travelled by the various light waves to all be about the same, so the signal
bits all arrive together.  If you are going a short distance, that's not a 
big
deal, but for long distances (beyond, say, 500 meters), you need single
mode fiber in order to get a clean signal out the other end.  If you don't,
all the bits get fatter as they go, and eventually you have mush instead
of a nice signal, or you have to slow the signal bit rate way down so
that there is more space between the bits.

The optics and transmitter/receiver have to be the right kind for the cable
you use.  Single mode is usually run at about 1300 nm (infra red), and
the optics are designed to put all the light into the small core of the 
fiber
and look for it in just that small area on the receive side.  Multi mode
is usually done with CD lasers or LEDs in the visible (red, usually) part
of the spectrum.  The fibers have larger core diameters (50 to 100 micron),
and optics at each end that are designed for that size light stream.

If you mismatch the core size and optics, you loose coupling efficiency.
Consider multi mode optics (big), shining light at the end of a single
mode fiber (small).  Most of the light won't get into the core and is lost.
The reverse is a problem as well; a multi mode fiber shining on a single
mode receiver will be shining its light on places where there is no 
detector.
And, of course, the transmit and receive need to be on the same wavelength.
I've never heard of anyone damaging a receiver with too much light, but
if the signal is too strong, the photo receiver could be overloaded and
not decode things right.  Usually this is done with a minimum length of
cable, although I've seen other methods to introduce loss.  Unless you
are using a link designed for 100km for a 2' patch, this shouldn't be a
problem with modern components.

Plastic fiber cables are used for really short (feet) distances, driven by
LEDs at low rates.  We once used a plastic fiber optic link between a
computer and the serial terminal (console) to pass an RFI emissions
test, since it removed the serial cable as an antenna.

Fiber optics aren't immune from RFI susceptability problems.  The
receive detector is very sensitive, and is hard to shielded well because
you need a hole to stick the cable into.  We're talking signals in the
microwave band here, typically.  Also, check out the fiber cable itself.
Some have a metal runner that goes along side the glass for protection,
and others have a high graphite content in the jacket.  Both can be
problems with Rf fields.

I hope this helps shed a little light (pun, sorry) on the subject...

73s,

Greg  KO6TH



>From: "Timothy J. Salo" <salo@saloits.com>
>To: amsat-bb@AMSAT.Org
>Subject: RE: [amsat-bb] Fiber Optics at RF
>Date: Mon, 21 Oct 2002 18:20:25 -0500 (CDT)
>
> > From: "Gary Gonnella" <gary@ridepix.com>
> > To: "Amsat-Bb" <amsat-bb@AMSAT.Org>
> > Subject: RE: [amsat-bb] Fiber Optics at RF
> > Date: Mon, 21 Oct 2002 08:53:39 -0700
> >
> > The first question needs to be, Are they designed for Single Mode or 
>Multi
> > Mode fiber?  One will not work with the other.
>
>Sort of.  We used to run devices made for single-mode fiber (i.e.
>lasers) over multi-mode fiber by putting attenuators between the
>device and the fiber.  We did however, have a light meter, so we
>weren't, for example, simply working off our computations of what
>attenuation ought to be required.
>
>If you are going to be playing fiber, I highly recommend a light meter.
>They aren't all that expensive.  I forget what we paid, but it might
>have been around a thousand bucks for the meter and various adapters
>for the different fiber connectors.
>
>I forget whether an unatennuated receiver can damage a receiver -- you
>might want to ask someone who knows about this stuff.
>
>-tjs
>----
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