Re: [amsat-bb] Fiber Optics at RF

[Art Goldman <aegoldm@xxxxxxxxxxxx>]

Greg:

Nice description of how light travels through fiber....

One comment about not hearing if a receiver can be damaged
by too much light: after we 'inadvertently' coupled a +8 dBm
transmitter to one of our OC-48 receivers (sensitivity to
-28 dBm), the receiver no longer worked. We determined that
the detector was indeed fried. Good thing the board was on
maintenance, otherwise it would have been about $90k to
replace.

Art
N3OY

"Greg D." wrote:
> 
> 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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> 
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