[Laser] Re: Is speed of photodiodes inversely proportional to sensitivity?

[Tim Toast]

Could it be that part of the apparent 10dB better
sig/noise with the larger diode is due to the fast F1
optics causing the focal "point" to be larger than the
smaller diodes surface area? ..and then the larger
diode is just seeing all the light?  I know generally
a diode's speed goes down with increasing sensitivity
but speed in this case isn't really an issue.
Also i thought part could be due to the larger area
diode being less affected by the air disturbances?,
bubbles etc? 
On the other hand, I had read somewhere that
under-illuminated diodes (when the focal spot is
smaller than the total area of the diode) would have
more noise than if you used the whole surface area -
this would mean you could de-focus the optics slightly
to ensure the whole surface area was being used. But
then you would have a wider field of view and would
pick up more noise theoretically (not so much at night
though). 

Some actual measurements would be more telling as to
what's really going on there. In any case, noise or
not, 100 miles+ with LED's is very encouraging to say
the least :)

>
>
...The surprise on 19 February is that the el-cheapo
BPW34 gave us about (by ear estimation) 10 dB better
sig/noise than the expensive IPL10040DW. In both
cases, the optical system, alignment and pre-amp were
identical. Has anyone an explanation? Are fast
photodiodes processed with a thinner, less sensitive
silicone layer? ...Chris Long (Melbourne Australia). 
>
>



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


		
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