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

[Jim Moss]

One way to get around the "noise" away from the source is to add a tube
blocking side inputs to the system.

Maybe we could use a CCD if we could address individual pixels (and if they are
fast enough and sensitive enough).

Jim
N9JIM/6

--- Glenn Thomas <glennt at charter.net> wrote:

> 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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> 
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