[Laser] pulse modulation

[laser@mailman.qth.net]

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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