[Laser] Re: Laser Digest, Vol 19, Issue 1

[Glenn Thomas]

Continuing on the thoughts about laser optics...

At 05:10 PM 1/12/2006, James N5GUI wrote:

<snip>

>My point is that a bigger aperature is not always the answer.  It may be a
>simple way to get more light in if there is no problem with light noise
>sources.  Particularly for use in daylight, you may want a narrower 
>field of view
>to improve your signal to light noise ratio instead of your signal plus light
>noise to system (electronic) noise ratio.

<snip>

I suspect that there are very few cases where a bigger aperture will 
not improve a laser comms system. The main disadvantage is that as 
the beam divergence is reduced, beam pointing will require more 
precision. On the other hand, a larger aperture will reduce the 
effects of atmospheric turbulence on the received signal, thus 
reducing one source of channel noise. Of course, as the system 
approaches being diffraction limited, SNR improvement doesn't come as easily.

On receive, I get the impression that most folks try to place their 
sensor at the focal point of the receive optics. It's true that 
they'll have all the energy collected by the optics available at that 
point. However, as has been pointed out, some of that energy is noise 
that we'd rather not have.

Instead of looking for signals at the focal point, we're much better 
off to look for them on the focal plane. Like the focal point, nearly 
all of the signal energy received is concentrated in a small area. 
Unlike the focal point, the noise energy is spread over the entire 
focal plane. Thus (assuming a sensor area that is small compared with 
the focal plane area) the optics system provides a spatial filter 
that can remove most of the noise energy. A further advantage comes 
from the fact that we can do very fine pointing adjustments by moving 
the sensor in the focal plane to better align it with the image of 
the transmitter. Even better, the fine pointing adjustment requires 
only that we move the sensor, the rest of the optical system can 
remain stationary.

The disadvantage of this is that we become more concerned with the 
quality of the received image than we have been. A poor focal plane 
image will result in a larger (blurred) signal image that has a lower 
SNR due to the blurring of noise signal. The signal image may also be 
larger than the detector area, reducing the amount of signal 
available to the detector and thus degrading the SNR with respect to 
sensor and electronic noise.

Since there are lot of BIG Fresnel lenses out there, the obvious 
question is, how good an image do they produce? I dunno. Has anybody 
experimented along these lines?

73 de Glenn Thomas WB6W



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