> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
