This is called Hi-jacking and thr subject line should be changed.<g>
----- Original Message -----
From: "Marcus Johansson" <macce@ENEA.SE>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, August 02, 2000 4:35 PM
Subject: Re: [OT]: Laserdome


> This thread went from a lasertag-game just carrying the killer id to the
> discussion of high-bandwidth laser links :-)
>
> /Marcus
>
> Jurva-Markus Vehasmaa wrote:
>
> > Michael Rigby-Jones wrote:
> > >
> > > > Harold Hallikainen wrote:
> > > >
> > > > >         I've heard that there was recently an article about
modulating
> > > > cheap
> > > > > laser pointers with voice. Anyone have any ideas about modulating
one
> > > > > with high speed data as an "across the parking lot" LAN link?
> > > > >
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: Marcus Johansson [SMTP:macce@ENEA.SE]
> > > > Sent: Wednesday, August 02, 2000 8:29 AM
> > > > To:   PICLIST@MITVMA.MIT.EDU
> > > > Subject:      Re: [OT]: Laserdome
> > > >
> > > > I have been thinking about that aswell, it's pretty much like a
> > > > megabit-range
> > > > laserdome isn't it :-) A standard laserpointer reaches some 100
meters I
> > > > think,
> > > > and that would be enough for many purposes. However, it might be
> > > > troublesome to
> > > > modulate that kind of devices with very high frequencies by just
"turning
> > > > the
> > > > button on and off". Has anybody reverse-engineered a pointer device
like
> > > > that?
> > > >
> > > > /Marcus
> > > >
> > > You probably wouldn't be able to get decent data rates by simply
switching
> > > power to the standard laser drive circuit on and off with e.g. a
transistor.
> > > The slow switch-on times would give a very poor eye and would
considerably
> > > limit bandwidth. You would really need to make (or buy) a proper laser
> > > driver designed for modulation.  For fequencies of a few Mbit/s you
wouldn't
> > > need any exotic components.
> > >
> > > The more difficult bit is at the other end.  You need a suitable
detector
> > > that has a good responsivity at the laser wavelength you are using.
You
> > > will need to screen the detector from daylight, by e.g. mounting it at
the
> > > end of a black tube.  You need a high gain, high bandwdith, low noise
amp
> > > after the detector, followed by some form of clock recovery.  The RX
will
> > > almost certainly have to be AC coupled to compensate for changes in
optical
> > > power input, this limits the minimum bit rate you can put through the
system
> > > i.e. there will be a maximum number of consecutive 1's or 0's you can
send.
> > >
> > > Of course the snag with this scheme is the accuracy you need in
aligning the
> > > TX and RX, and keeping them aligned.  Rain, snow and mist/fog would
also
> > > affect the system quite badly, as would birds/insects flying through
the
> > > beam.  That could be solved, or at least reduced by using some form of
error
> > > correction.  It would make an interesting, if somewhat challenging
project
> > > though.
> >
> > If usign tcp/ip channel, errors would be corrected. Use lasers only as
> > fysical carrier layer.
> >
> > > Mike
> > >
> >
> > J.M. Vehasmaa
> > --
> > Jurva-Markus Vehasmaa (vehasmaaATdlc.fi)
> > ICQ: 69535169
> > http://www.saunalahti.fi/vehasmaa/index.html
> >
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