[Ronja] Multiple receivers and transmitters

[Karel Kulhavy]

---- "J.D. Bakker" <jdb at lartmaker.nl> schrieb:
> At 23:41 +0200 23-09-2008, Karel Kulhavy wrote:
> >                                                                In the
> >daylight the noise is mostly created by the quantization of the ambient light.
> 
> Strictly speaking that's not what happens (or at best it's a major 
> oversimplification).
> 
> The photodiode is a semiconductor in which incident light causes an 
> electrical current to flow. In daylight a large part of that current 
> is effectively DC (compared to the signal bandwidth). Any DC current 
> flowing through a semiconductor junction induces shot noise as 
> individual electrons cross the barrier, which can be likened to the 
> noise of raindrops falling on a thin roof. This noise current is 
> indistinguishable from the wanted signal.

This is not true. If you irradiate the photodiode with some special
laser light (mode-squeezed or how it's called) you get less noise
than shot noise. The noise comes from the random times of arrival
of the photons.

But we cannot control the type of light the Sun produces so it
cannot be exploited for increasing the range anyway.

Karel
> 
> Shot noise increases with the square root of current flow, and thus 
> with the square root of incident light power. On a sunny day the 
> receiver gets 10^5 more light energy than on a moonlit night, but the 
> difference in noise level is 'only' 300x.
> 
> JDB.
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