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

[KY1K]

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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