[Laser] 5 mW lasers & LEDs - Detectors

[Dave]

On 11/5/2013 12:00 PM, laser-request at mailman.qth.net wrote:

> Date: Tue, 05 Nov 2013 08:50:12 -0800
> From: "Steve J. Noll" <sjnoll at ix.netcom.com>
> To: laser at mailman.qth.net
> Subject: Re: [Laser] 5 mW lasers & LEDs - Detectors
> Message-ID: <527921C4.900 at ix.netcom.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> On 11/1/2013 10:39 AM, Zack Widup wrote:
>> Once upon a time, probably several years ago, there was some discussion
>> about what detector diode device was best for laser reception. I remember
>> there was one device in particular that was recommended and that it was
>> somewhat difficult to come by.
>>
>> Things may have changed since then with advancing technology. Are there any
>> suggestions?
>>
>> 73, Zack W9SZ
>
> I'd like to see that discussion too!

I vaguely remember it.  It ought to be stashed somewhere in someone's
archives.  Maybe.

> I believe one may want to look at it two ways - setups "capable of
> communications at a range of at least 1 km." or setups for balls-out
> record setting distances.

I don't know that there's a hard and fast line between the two.  For
that matter, there are even "toy" systems good for a few hundred feet
or so.  Those are good to introduce people to the concept, and can be
evolved into longer range systems.

> For the latter, really can't beat a photomultiplier tube. Next would be
> an avalanche photodiode.

PMTs are nice, but they require a rather high voltage supply.  The
prototypical example is the 931A, although I seem to remember that
Hamatsu has some more recent designs that may even be superior.

> I just retired from being senior test engineer at Advanced Photonix
> where we made large area Silicon APDs, 5mm to 16mm dia, the biggest in
> the industry.

For some reason, I have a fuzzy memory of an even larger APD, somewhere
around 25 mm (which is absolutely HUGE).  But, I can't remember where I
saw those.

> Unfortunately, they're really expensive, even as surplus, as they're not
> easy to make.

One would expect that wafer processing houses could produce some really
huge APDs.  Then, again, getting decent yield and consistent
characteristics across the entire chip may be a challenge.

> Also require ~1800V.

Wow!

> As the operating point is temperature sensitive they're usually TE cooled.

Not too surprising.  I know that for radio-astronomy, Dry Ice (Solid
CO2) and LN (Liquid Nitrogen) cooling are not too uncommon, and some
have even resorted to LH (Liquid Helium) cooling.  One would expect
similar techniques to be used for optical systems.  One problem, though,
with optical systems is that condensation must be avoided on the optics.
Additionally, one wants to avoid a huge temperature differential, since
that can result in swirling air currents, which can distort the optical
path.

> These were supposed to be a PMT-killer, they haven't been.  They're the
> sensors in the Optos scanners at your better-equipped eye doctor.

It's hard to beat a good PMT, since good PMTs are capable of single
photon counting.

> Then there's small area APDs. We made those for long-range barcode
> scanners. 100V to 200V.
> I see on eBay right now First Sensor (formerly Pacific Silicon Sensor)
> AD500 small area APDs for $16 from China. How someone can sell these for $16
> when they're $135 from Mouser is a mystery, one hopes they're not
> rejects.

They very well could be rejects, or they could be counterfeit.  Or, they
could be surplus due to the cancellation of a project.  Then, again, the
materials cost for an APD is pretty small (sand), and, when you throw in
an almost zero labour cost, well, they can be made pretty cheaply.

> Their spectral response curve is not great. About 155V bias
> requirement which is also temperature dependent.
> The biggest disadvantage is the tiny active area (500 um.) Can you get
> most of your receive signal on that 20 mil spot?

That depends upon the design of the optics, as well as the dispersion in
the atmosphere.  Thus, it may be better to go with a slightly larger APD
to handle the spot wandering due to atmospheric turbulence.

> If not then I'd go with a regular silicon photodiode which you can get
> with much larger active areas. And they're cheap.

Maybe that's what I was thinking of above.  The sensitivity of a
photodiode isn't nearly as great as with a APD, but the larger surface
area available (economically) may allow a greater signal to noise ratio,
by capturing more of the optical signal.  Maybe.

One wonders how much background/cosmic radiation interferes with
optical detectors.  Does shielding the detector improve the S/N ratio?

> 73,
> *Steve J. Noll, WA6EJO
> http://www.qrz.com/db/WA6EJO
> http://www.linkedin.com/in/stevejnoll

Dave