[time-nuts] Using lasers for data transmission

Ilia Platone info at iliaplatone.com
Sun May 1 11:22:25 UTC 2016


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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