Communication+Physics (was Re: Conventions that don't suck?)

In article <e6vqd6$mnc$1@xxxxxxxxxxxxxxxx>,
Keith F. Lynch <kfl@xxxxxxxxxxxxxx> wrote:
[...] How
much modulation can you put on a laser before it ceases being even
close to monochramatic?

I'm not sure exactly how to parse that, but if you want to know
what bit rate you can put on a carrier, the answer is half the
carrier frequency, at least with modulation schemes I'm familiar
with. I think it's one of those mathematical hard limits, but
now that I'm writing this it occurs to me to wonder whether you
could combine AM and PM on the same carrier to send two data streams
at the same time, each at the Nyquist limit, for a total bandwidth
equal to the carrier frequency ... ?

(I don't want to think about how to do FM with a laser, which is
why I suggested PM -- not that I'm certain how to do PM on a laser
either, but it sortakinda sounds reasonable that it might be possible),
though FM seems easy enough with microwaves. What's the theoretical
upper limit using PWM? Is that still half the carrier frequency, or is
it slower?)

Caveat: I'm not a EE. Which should be obious to any EEs by this
point anyhow ...

And can you ever have more bits than photons?

Hmm. The "speed" I'm talking about above would be the baud rate.
With FM (and PWM), you can have each transition convey more
than one bit, simply by having more than two distinct levels to
transition between, though attenuation and other losses will hit
you a lot harder the more levels you try to encode. Can you do
more than two values with PM (for example, but shifting in 90 degree
increments instead of 180 degrees), or does that not work?

I'm not sure about AM with a laser -- does "louder/brighter" mean
more photons at a time? I'm guessing that it does, since "higher
energy photons/lower energy photons" would make it FM instead. So
I guess you can't have more bits than photons using AM. (See previous
caveat.)

Urk. There's a modulation with a 'K' in the abbreviation as well,
right? Or is what I'm failing to remember just a synonym for phase
modulation?
--
D. Glenn Arthur Jr./The Human Vibrator, dglenn@xxxxxxxxx
Due to hand/wrist problems my newsreading time varies so I may miss followups.
"Being a _man_ means knowing that one has a choice not to act like a 'man'."
http://www.radix.net/~dglenn/ http://dglenn.livejournal.com
.

Relevant Pages

  • Re: modulateable laser diode drivers
    ... Hi - can anybody lend me any guidance as to how to build a laser ... You are talking about modulation, and not 100% on/off driving, ... and add a modulation network at the laser diode. ... the bias-T isolates the 50-ohm ...
    (sci.electronics.design)
  • Re: Can one passively mode lock a green laser pen?
    ... a laser pen might be a good place to start in terms of components. ... in the battery supply lead. ... Then I'd turn up the rf drive _very_ slowly and look for a small amount of rf modulation on the green output -- and having seen it, then measure the modulation depth vs frequency, looking for a resonant peak, to tune in To the exact c/2L round-trip frequency. ...
    (sci.optics)
  • Re: Radio and Aliasing
    ... Because of the reference to sampling I believe the question is meant ask the highest modulation frequency that can be transmitted over a 150 kHz AM broadcast. ... One of the implications of this is that the highest frequency component of the "data" must be less than one half the carrier frequency to avoid aliasing effects. ...
    (sci.electronics.basics)
  • Re: Amplitude characteristic and harmonic spectrum
    ... amplitude characteristic and harmonic spectrum? ... In modulation theory, no method of imposing information on a carrier ... Bthe carrier frequency, you get a produce with three compnents, ... Any simple spectrum analyzer will demonstrate this for you. ...
    (sci.physics)
  • Re: The differences between LET, SRT and IRT
    ... Laser beam sources. ... Kerr cell shutter and driver electronics. ... lack features required for continuous modulation such as ...
    (sci.physics.relativity)
Loading