[Laser] 5 mW lasers & LEDs - Detectors

[Mike]

Hi Clint,

I'm probably preaching to the choir but the RCA 7102 is centered at 950 
and the RCA 6032 Image tube might be a good candidate also for IR 
viewing/detecting. Hey, I like your HF idea since I do have both of 
these tubes. Although the 6032 does need at least 15k for good viewing.

Mike C.
(soon to be on Sand Mountain, GA)


On 11/5/2013 3:42 PM, Clint Turner wrote:
> Having messed a bit with both APDs and PMTs, I can definitely say that 
> the PMTs are more sensitive - but as has been inferred, finding one 
> that works well at "Red" or longer wavelengths is a significant 
> challenge!  Shorter wavelengths that fall into the optimal range of 
> most PMTs (e.g. wavelengths <green - particularly blue/UV) tend to 
> fall victim to atmospheric effects more rapidly - a limiting factor 
> for really long distances.
>
> For that, the only real choices are some of the MultiAlkali and GaAs 
> units:  The former are more readily available surplus (I've seen them 
> as low as $25 on EvilBay, but they are usually closer to $100 or much 
> higher).  The GaAs are much better for longer wavelengths, but they 
> are very much harder to find and extremely fragile and tolerate no 
> abuse at all compared to standard PMTs - which are fragile enough.
>
> As for a PMT supply, that's easy enough if all you need is 1000-1300 
> volts:  A modified inverter from a <$5 Harbor Freight electronic 
> flyswatter (polarity-reversed, more robust output filter cap, a simple 
> feedback regulator) works nicely and is inherently self-limiting in 
> its current supply which helps protect the PMT from abuse.
>
> Barry, G8AGN, has done some in-field tests using the '931 PMT and its 
> variants and found them to work miserably at "red" - probably worse 
> than a good PIN photodiode detector.
>
> * * *
>
> As for APDs, I have an APD-specific receiver that I've tested in the 
> field:
>
> http://www.modulatedlight.com/optical_comms/optical_apd_rx1.html
>
> This uses the Pacific Silicon Sensor AD1100 APD - and I paid full 
> price for it:  It is about the same price as the gas used by everyone 
> on an expedition where we have several people driving hundreds of 
> miles round trip, so if it makes the difference between success and 
> failure, it may be worth the cost.
>
> I was able to achieve 6-10dB improvement with it over a conventional 
> photodiode-based receiver, but as was the case for the PIN-based 
> receiver, the bandwidth is limited and this is a fundamental 
> limitation of the physics.  I noted - as was also pointed out on an 
> APD app note from Hammamatsu - that best APD S/N occurs in the area 
> where M (multiplication gain) is in the area of 3-10 - well below the 
> maximum "gain" of the APD, but this it so often the case in an 
> amplifying mechanism where the noise floor rises faster than the 
> gain!  In this case the "intercept" point (e.g. where further 
> increases in gain caused a reduction in signal/noise ratio) occurred 
> at an operating voltage of about 35-45 volts.  What this ultimately 
> means is that if it's both the ultimate high bandwidth and high gain 
> you want, you are still looking at a PMT.
>
> I've not attempted to cool either a PMT or an APD and it's likely that 
> some benefit would arise from doing so, but even the plain, old PIN 
> Photodiode in the "Version 3" detector is capable of yielding speech 
> bandwidth signals from a distant, red (630nm) light source that is too 
> dim for the dark-adapted eye to see.
>
> * * *
>
> A while ago I was playing with my MultiAlkalai PMT and various 
> high-power LEDs at MHz rate modulation and demodulation and was rather 
> disappointed in the PMT's high-frequency response at very low light 
> levels - at first.
>
> As I increased the frequency the overall amplitude on the scope stayed 
> sort of constant, but the signal disappeared into noise. At first I 
> was puzzled, but then I did some quick, back-of-the-envelope math and 
> realized that I simply had too few "photons per cycle" impinging on 
> the PMT.  As the waveform became indistinct, the scope was no longer 
> being triggered reliably on what had been a sine wave at lower 
> frequencies/higher amplitudes.
>
> Once I synced the scope with the original source signal I could see 
> that the original signal was still there, but somewhat "undersampled" 
> and "fuzzed up" by the uncertainty and the number of photons/electrons 
> and still rather noisy.  The upshot is that at some point, sensitivity 
> of the detector becomes irrelevant if you don't have enough photons!
>
> * * *
>
> As for the size of the "spot" of light, refer to this page:
>
> http://www.modulatedlight.com/optical_comms/fresnel_lens_comparison.html
>
> Using a good-quality Fresnel lens of reasonable f/D ratio (around 
> unity) it should be possible to focus the vast majority of light onto 
> a spot that is a few hundred microns diameter - probably much smaller 
> if one employs a "secondary" lens.
>
> A "conventional" lens/mirror system should be able to focus to a 
> smaller spot, but it's worth noting that even if you accept some 
> losses with a "sloppy" Fresnel (e.g. the spot is somewhat bigger than 
> the detector itself) better light-gathering power (related to 
> aperture) would be easier to achieve with that Fresnel than a much 
> heavier and bulkier - but more accurate - conventional lens that was 
> smaller.
>
> One definite advantage of a "conventional" lens - and a 
> correspondingly-small active area on the detector - would be related 
> to Field-of-View (FOV):  If your primary challenge is discrimination 
> of a modulated light amongst a sea of other lights, the comparatively 
> large FOV of a Fresnel and its tendency to scatter light may be a 
> detriment.
>
> 73,
>
> Clint
> KA7OEI
>
>
> Steve Noll said:
>> I'd like to see that discussion too!
>>
>> 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.
>>
>> For the latter, really can't beat a photomultiplier tube. Next would be
>> an avalanche photodiode.
>> 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.
>> Unfortunately, they're really expensive, even as surplus, as they're not
>> easy to make. Also require ~1800V. As the operating point is temperature
>> sensitive they're usually TE cooled.
>> 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.
>>
>> 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. 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?
>> If not then I'd go with a regular silicon photodiode which you can get
>> with much larger active areas. And they're cheap.
>>
>> 73,
>> *Steve J. Noll, WA6EJO
>> http://www.qrz.com/db/WA6EJO
>> http://www.linkedin.com/in/stevejnoll
>
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