Threshold current should not be a problem because if there's no data the
laser could go into "power saving mode".
As am modulation a simple buffer/r2r network DAC should do the job. The
signals to transmit are three: Tx, and two bidirectional.
Ilia.
Il 01/05/2016 10:27, Dr. David Kirkby (Kirkby Microwave Ltd) ha scritto:
Hi,
Several (many?) years ago National Geographic magazine show a picture
taken here in southern California of the state government sending red laser
signals between different mountain tops to keep track what was going on
near fault lines
There were no technical details on what was taking place. So it can be
done.
At a hamfest a few years ago I bought both a red and green 35 mW laser
pen for about $15 each. They do shine a long, long way.
35 mW is certainly unsafe to the eyes, so be very careful. There maybe
legal issues about doing this.
Whether these are powerful enough, or can be properly modulated for what
is needed, I have no idea.
You can pretty much modulate any laser diode. There are two important
currents to know about
* Threshold current I_th - below which it will not lase.
* Maximum operating current I_max - above which the device will be
destroyed.
You can AM modulate them by applying a DC current
I_th + (I_max - I_th)/2.
Then superimpose the modulation which has a peak value of (I_max - I_th)/2.
Those currents ensure that the laser is always lasing, and gets you
theoretically 100% modulation. For best lifetime, run at lower levels of
peak modulation current.
Watch out for transient currents - lasers make transistors look like
antisurge fuses!
For point to point contact you want a beam which diverges as little as
possible. IIRC the divergence is something like inversely proportion to
the cavity length. For this reason a diode laser with its short cavity is
not optimal. But of course they are cheap.
A veey long time ago I used to know quite a lot about lasers, but not
using them for years I realise that I have forgotten an awful lot!
FWIW, at university we had a 10 W argon ion laser. I think it took about 50
kW to produce those 10 W. When it was disposed of, it was sold to a company
that put on light shows.
Dave
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--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
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