[Laser] Is a typical laser pointer (635nm) fast enough for 10Mbpsethernet link?

[John Matz]

Hello
The longer distances use much lower equivalent data rates.  Voice is maybe
equivalent to about 10 kilobits per second.  Morse code or PSK31 is more
like about 30 bits per second.   The bandwidths required are proportionally
smaller ... like 4 kHz for voice and 50 hertz for slow speed data.  At a few
kilometers, the usual optical receiver is only capturing about a millionth
of the transmitted optical power, even with laser transmitters and 30 cm
receiver lenses.  The receiver noise goes directly as receiver bandwidth
which goes directly as data rate.  The received signal has to be louder than
the receiver noise to recover the message error free.  That means long
distance paths usually mean slow data rates ... Ronja LEDs are focussed by a
transmitter lens to a beamwidth similar to a laser's ... no advantage there.
John Matz KB9II

----- Original Message -----
From: "Phanumas Khumsat" <phanumas at gmail.com>
To: "KY1K" <ky1k at pivot.net>
Cc: "Free Space LASER Communications" <laser at mailman.qth.net>
Sent: Monday, May 09, 2005 5:46 AM
Subject: Re: [Laser] Is a typical laser pointer (635nm) fast enough for
10Mbpsethernet link?


> Hello there,
>
> Thanks for your answer.
>
> We would like to have a free-space optic link of about 5km for 10Mbps
> (or higher, i.e. 100/1000) ethernet operate all day and night. Of
> course, it doesn't have to be laser, anything that can deliver what we
> want is absolutely fine. We see on the web that a few people have
> already achieved a range of more than 10km fairly easily, not an
> ethernet link, though. So we think 5km FSO link range for 10Mbps could
> be feasible. Probably the best home-brew FSO (LED-based) is Ronja at
> http://ronja.twibright.com but its range is rather well below our
> requirement, so we think using laser pointer might increase link
> distance.
>
> You have mentioned cheap optic-fibre transceiver modules to be
> modified for FSO, can you recommend any good ones for 10/100/1000
> ethernet. We have another project in which the link range is no
> important (about 1km) but speed is a more critical issue ... we need a
> FSO system for ethernet link at 100Mbps at minimum. Probably your
> suggestion in modifying an optic-fibre transceiver module could come
> into rescue.
>
> And we plan to build things ourselves ... finger crossed :-)
>
> Thanks in advance for your advice.
>
> Don
>
>
> On 5/8/05, KY1K <ky1k at pivot.net> wrote:
> > Yes and maybe......
> >
> > The speed (generally) is not an issue with the stock laser pointer
driver
> > or the diode itself.....although laser diodes built into fiber optic
> > transmitter modules can be driven much faster.
> >
> > To use your laser pointer at high speeds, turn it on. After the output
> > settles, use a fast current sink to steal current from the laser diode
> > during times you do not want any output.
> >
> > This can provide medium turn on and turn off times, probably allowing 10
Mbps.
> >
> > Regarding range...you have left quite a few questions unanswered.....
Are
> > you asking about fiber optic or free space links? Are you willing to use
an
> > APD, photomultiplier tube or garden variety photodiode? Do you need a
24/7
> > link, or will it only operate when it's dark outside? Do you want a
simple
> > inexpensive solution, or do you have a big budget? What are your size
> > limitations on the transmitter/receiver pair? Can you build your own
gear,
> > or do you need to use building block assemblies that you can buy already
> > built? Do you really need a laser, or will an LED provide enough
> > range---LED's can give great performance over fiber if the distances
aren't
> > to great.
> >
> > I think Ethernet slows down automatically if the link becomes marginal,
is
> > this acceptable or do you need a bone crushing signal strength so that
the
> > highest speed is always used?
> >
> > There are some ready built free space data laser home built projects on
the
> > web, look them up.
> >
> > If you want to modify ready built transceiver modules, you can buy them
> > dirt cheap these days and convert them from fiber to free space fairly
> > easier-this gives you the receiver and transmitter that is already
> > assembled. These transceivers usually use some sort of FM, which has the
> > advantage of a constant duty cycle to the laser..which is much easier to
> > control from the laser driver aspect.
> >
> > For free space, there is no amount of laser power and receiver
sensitivity
> > that will guarantee a 24/7 link....it rains, snows and gets foggy. If
you
> > want to go over a half mile with any reliability, you probably want an
rf
> > solution rather than laser.
> >
> > Laser transmitters and receivers that transmit video are becoming very
> > inexpensive to purchase as well. EBAY has several video LED links, which
> > can be modified for longer range by converting to laser and/or putting a
> > lens in front of the receiver for additional range.
> >
> > Perhaps others will comment.........
> >
> > After you get a better idea of the issues above, let us know so the
group
> > and make suggestions regarding whether your project is likely to succeed
> > and give you some general guidance so that you don't end up with a
design
> > that has no chance at all to function.
> >
> > Unless you really need ethernet, a USB link is easier to implement. Some
> > IRDA hardware is easily modifiable in order to extend the range.
> >
> > Regards,
> >
> > Art
> >
> >
> > At 10:12 AM 5/8/2005, you wrote:
> > >Hello there,
> > >
> > >Is a typical laser pointer (635nm) fast enough for 10Mbps ethernet
> > >link? What is the expected longest link range achieved by such
> > >pointer?
> > >
> > >Thanks,
> > >
> > >Don
> > >_______________________________________________
> > >Laser mailing list
> > >Laser at mailman.qth.net
> > >http://mailman.qth.net/mailman/listinfo/laser
> >
> >
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