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

[Phanumas Khumsat]

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
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>