[Laser] pulse modulation

Sun, 29 Feb 2004 11:54:47 -0500

OK, I have the same question as James, N5GUI.

One certainly has to acknowledge that the average power is somewhat low for 
pulsed, but that the peak power is very high. But, how does one take 
advantage of the higher power peaks once the pulse reaches the destinations 
receiver??

I read Eric's email that explains the tremendous advantage of transmitting 
high power pulses...but I am still wondering how to gain an advantage in 
the receiver??

Now, I realize that we can build a narrowband receiver and receive the 
pulse with the narrowband receiver. But, in using this method, the 
receivers limited ability to response to a fast pulse merely averages the 
high pulse power-and you are no better off than using a qrp cw laser diode.

In order to take advantage of the fast rise and fall times of the very 
narrow pulse, the receiver has to have a wide bandwidth capability. Because 
the receiver needs to be many MHZ or even wider, then the receiver sees 
alot more noise. To me, it seems like there can be no advantage to 
receiving pulse signals. What am I missing???

I understand you can gate the receiver if you know what time you are 
expecting a pulse...but in order to respond to the fast rise and fall 
times, the receiver still needs to be a wideband receiver.

What is the advantage to transmitting pulses when all the instantaneous 
power gets buried in noise because the receiver needs to have 10 Mhz (or 
wider) bandwidth? The fact that a wideband receiver is going to hear more 
noise is a simple and basic physical law-how do we get around the extra 
noise pickup necessitated by a wider bandwidth receiver.

Hey Stewart, is there an easy answer for this???

Anybody have any thoughts about this?

Thanks,

Art

At 12:51 AM 2/29/04, you wrote:
>The comment by Joe KM1P on the short laser pulse ( Q switch YAG ) and pulse
>position modulation is interesting.
>
>The modulation technique can be demonstrated with just a short pulse on one
>of our experimental lasers.  It should be a neat educational 
>demonstration, and
>help developing the receiving systems for long distance communications (like
>the Moon).
>
>The suggestion that the time position of the pulse can represent data is
>true, but you need some way to know when the time slice starts, to know 
>which data
>element is sent.  On Earth we can synchronize clocks at each end, like is
>done for some other modulation systems for weak signals.  Another way to 
>solve
>that problem is to send a sync pulse every so often.
>
>When you are sending discrete information, the data for the next pulse is not
>related to the data in the next pulse.  If you are sending continuous analog
>information, such as voice, and particularly if the sampling rate is
>significantly higher than the minimum, the data in each sample will be 
>similar to the
>sample before and the sample after.  This leads to another way to encode
>information with pulses.  The system places the pulse in in a slot of a 
>fixed length
>frame and waits until the end of the frame before the next frame begins.
>Instead, start each new frame as soon as the pulse is sent.  The data is then
>encoded in the time between pulses.  For the (analog) experimenter 
>decoding the
>data can be as simple as generating a fixed length pulse when the received
>pulse is detected.  The signal generated by the fixed length pulse is passed
>through a low pass filter, and the analog signal is reconstructed.
>
>Demodulating analog data encoded in a fixed frame system would be a matter of
>starting a pulse at the beginning of a frame and quenching it when the
>received pulse is detected.  The signal is pulse width modulated, and will 
>also
>reconstruct the analog signal by sending it through a low pass filter.
>
>The equipment I am working with, Ramsey LBC6, uses a microprocessor to
>generate the pulses, be they fixed length pulses or variable pulses in a 
>fixed
>frame.  The laser is then used to transmit the "long" pulse, and the low pass
>filter is on the receiver side.  If you were going to try to model a short 
>pulse
>communications system, you would need to build the pulse stretching 
>circuits on
>the receive side.  On the transmit side, sending a short pulse on the falling
>edge of the existing modulation scheme would work.  ( For the fixed frame
>system, that is the right time slot.  For the fixed pulse length system, 
>rising or
>falling edge will work. )
>
>
>
>OK.  Now for you guys to put on your thinking caps.  The YAG laser with Q
>switching sends a brief but very powerful signal out.  Its average power is
>small.  Compare that to the work being done with weak signals, that listen 
>to low
>power signals for a long time to extract the data.  For the same amount of
>power, over the same amount of time, and with the same amount of data 
>transmitted,
>does the YAG laser have an advantage?  And what is that advantage.  The only
>thing that seems obvious to me is that a laser (and it would not have to be Q
>switched YAG ) can be made to send a much more narrow beam of light with a
>telescope of a more manageable size, than a radio wave.  Is it 
>theoretically the
>same amount of data that can be transmitted in a fixed amount of time with a
>specified amount of power?
>
>
>James
>N5GUI
>
>
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