[Laser] Re: sensor vs focal plane

[Tim Toast]

I looked tonight for some reference to that but
couldn't find anything... I think i got the idea in my
head from some stuff i've been reading lately about
optical heterodyne receivers, including this page at
coseti:
http://www.coseti.org/proffhet.htm  

scroll down to the link for "Optical Interference"

http://www.coseti.org/9008-030.htm 

It was in the section they have for coherent detection
and what not. Some other articles i read probably went
into more detail about it though. It was talking about
two beams focused on the surface of a photodiode
"interfering" or mixing there. And it was saying if
the local oscillator beam spot is larger than the
signal beam spot, the parts of the LO beam not
touching the signal beam would only contribute shot
noise to the resulting mixing products. Presumably the
same situation exists if the signal beam was larger
than the LO beam. Then, the parts of the signal beam
not touching the LO beam would be contributing the
extra shot noise.
In other words, to make the mixing process as
efficient as possible, among other things, the two
beams need to be the same size on the diode and
coincident. But not necessarily using the whole diode
surface. This probably translates in some ways into RF
mixer theory where the two beams/signals are mixed
properly.

I guess i assumed from that, any area NOT illuminated
by the beams would only contribute shot noise, when it
could at least be contributing to the total output if
it were illuminated. 

After thinking about it some more today, it does sound
fishy for there to be any difference between the light
being "bunched up" in a tiny spot or more evenly
distributed on the diode. The diode is converting the
light photons into electricity one photon at a time
anyway, so there shouldn't be any difference 'where'
the photon strikes, they would still be converted as
long as they strike it somewhere.

Really, all this "filling up the whole diode" amounted
to is; decreasing the field of view to try and fill up
the diode with nothing but a virtual image of the
distant transmitter's lens. Admittedly, de-focusing
the optics is a bad idea... It's better done by either
increasing the focal length of the receiver lens, or a
smaller area diode etc... At the expense of "breathing
room" and, in the process, almost eliminating any
background noise sources. 
Less noise, true, but not the way i had imagined it
happening in my last post. My fuzzy logic :)

>
>
>I am wondering if anyone has more information on the
issue of trying to illuminate a "large" area of a
photo sensor. Tim, if you can find a reference on
under-illumination, I would like to check into it
further...James N5GUI TWOSIG at aol.com 
>
> 


Tim Toast
http://www.aladal.net/toast/exp.html


		
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