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

[Chris L]

Dear Tim and light comms people,

No, my fresnel focusses a spot to about 0.75 mm by 0.75 mm which is a much 
smaller spot than any of my photodiodes. The BPW34, as I mentioned before, 
seems to REFLECT less light that the IPL PD, the BPW34 looks "blacker" if 
you get what I mean, and the IPL diode looks more metallic, more 
mirror-like... And the focussed dot from the fresnels doesn't perceptibly 
wander with turbulence, the fresnel resolution is WAY too poor for that... 
That kind of resolution would be more a liability than an asset, 
obviously...

In the days when I used photomultipliers with semitransparent photocathodes 
about 3.5 cm diameter, you could purchase an additional doughnut-shaped 
magnet to plonk on top of the photocathode glass to reduce the effective 
area of the photocathode. This reduced photoemissive noise from the cathode 
periphery. The toroid-shaped magnet apparently deflected photoelectrons from 
the outer part of the photocathode, stopped them hitting the first dynode, 
thereby 'deadening' the unused peripheral photocathode area if you didn't 
need it to detect a focussed spot. With the price of those PM tubes it sure 
was a blessing to have this accessory and not have to replace the tube! Mind 
you, we did a lot of scrounging in electronic disposals stores and managed 
to find them, usually scrapped from scintillation counters (S11 cathode) or 
from old flying spot telecine chains...

As for defocussing a received spot to spread light over the photocathode, a 
better idea would be to put an iris or pinhole the same size as the focussed 
spot in the focal plane, then allow the defocussed light passing through 
that aperture to hit your whole sensitive area. In that way you retain 
maximum directivity while illuminating more of your sensitive area, if you 
need to.

Theoretically, if all parts of the sensitive area are equally sensitive, 
defocussing should produce no difference in photocurrent - but in practice 
the edges of a photodiode's sensitive area have slightly different spectral 
response (colour response changes). But in practice, that's difficult to 
measure - you should hear no difference irrespective of which way you 
operate.

So: it seems from your comments and my experience, that the old slow BPW34 
must be one of the most sensitive commonly available photodiodes! Trust the 
Germans to make 'em! I must buy a few. At roughly US$2 each a few are worth 
keeping in stock for folk who want to play with light comms. I find a major 
problem today is that it's increasingly hard to find photodiodes without 
that infernal 'black' IR filtering that makes them useless for visual 
light... Is it the same in your country? I find that many people have not 
seen a PD without IR filtering, therefore few have actually seen the 'works' 
inside a PIN diode. With a jeweller's loupe, they're fascinating to 
examine...

BTW the chromatic aberration of fresnels should not be a concern for LED or 
laser operation, as both should be essentially about as monochromatic a 
source as you'll ever find.

Mike VK7MJ is now back in Hobart and is writing an article on our 
experiments... I'm concurrently preparing a web page (or series of same) 
which will eventually (not for a few weeks yet) appear piggy-backing on the 
page of Tony Sanderson VK3AML at:

www.bluehaze.com.au

Have a look at his multimedia page for a recording of our 'death by 
lightbeam' contact back in 1976, when we were using EMI photomultipliers and 
modulated high pressure mercury arc lamps for a link... but the subject 
matter is a bit silly, as we also were at age 21 or so... we were for some 
reason talking about Egyptian mummification and the techniques of embalming 
on that night... all relayed on 160 metres a.m. for the titillation of the 
vulgar mob (us!) As I come closer in age to the date of actually needing an 
embalmer myself (I'm 51) I find the subject a little too close to home to be 
completely comfortable with it (!)

All the best,

Chris Long, Melbourne Australia



>From: Tim Toast <toasty256 at yahoo.com>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: laser mailinglist <laser at mailman.qth.net>
>Subject: [Laser] Re: Is speed of photodiodes inversely proportional 
tosensitivity?
>Date: Thu, 24 Mar 2005 16:58:19 -0800 (PST)
>
>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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