Photosensitive tubes

[elcraigo]

I can try some experiments in the Laser Lab at work. Before anyone asks, I will wear my laser goggles.
It will be UV laser diodes (351 nm). We might have other wave lengths in stock.
I can change or modulate frequency and pulse width. Also, adjust the power (low mW range).
I can try different glass filters also, along with different focusing optics.
 
I'll try it using SEX and Quickie amps, because they are easier to hook up.  Think the tubes in these are good candidates?

I could try an old foreplay 1 - just not as easy to hear in the lab. Probably would hook an o-scope up instead.
I do have lots of 12AU7. I'm thinking RCA clear tops are a good choice, easier to get the light to the cathode?

1.  Does the sound vary with light intensity?  Try holding the light closer and farther away.
2.  Is there a "color" below which there suddenly is no sound?  LEDs are pretty monochromatic but glass filters and such sometimes let weird frequencies through.
3.  If you create a narrow beam of light, does the effect change if you shine it on different parts of the tube?  Shining it on the anode should do nothing at all, shining it on the grid shouldn't have much effect, but shining on the cathode should have the greatest effect.
4.  (Advanced)  Does the effect change with heater current?

Probably won't do #4; but any other experiments to try while I have the stuff hooked up?

[R.Mackey]

If you've got actual lasers and know they're spectrally pure, don't waste time with color filters.

With a nice, tight beam I'd be particularly interested in the difference between illuminating the cathode versus the anode.  You're likely to get some reflection either way so I doubt either will give you a zero reading, but you should see a significant difference.  RCA clear tops sound like a good choice.  However, the tube glass might attenuate or even block UV depending on exactly what sort of glass it is, so be ready for it.

Can you vary the laser peak power, or just total power (i.e., pulse width)?

I'd probably start with a 1 kHz square wave, and monitor the control signal and the output simultaneously using a storage oscilloscope.  Should answer the basic question of cause very quickly.

If the effect is definitely there, the question would be what the efficiency of the system is -- how many volts out versus how much power in.  Should be totally linear with respect to pulse width, more-or-less linear with peak intensity.  If you have a strong enough laser you might even hit a saturation point where the cathode is fully illuminated and can't spit out any more electrons, but I'm guessing this would take a LOT of light.

Using a Foreplay I'm guessing the volume control will have no effect on the light-induced output signal, but you should probably check this too.

[elcraigo]

Yes, real lasers. I won't bother with the filters.
I can vary both the peak power and total power.
I can set the current into the laser diode from 0 A to 4 A with an adjustable current source (based on a OPA549), and adjust the pulse width from 0 to 100%, continuous wave (CW). I have no plans to go above mW range. I do have 2 & 4 W laser diodes (but don't plan on using those $$$)
I'm guessing I can produce enough light to find the saturation point if I did turn it up.
With the optics I have, I can get a tight beam, but not a pin-point dot.

[porcupunctis]

Craig, if you don't mind me asking, where do you work that you get access to a lab full of lasers?  I love the fact that you are interested in experimenting until you get to the bottom of this issue.  I have the hardest time getting my students to adopt that sort of attitude. 

Keep us posted on what you learn.

[2wo]

Put the kite higher into the storm Egor!

They called me mad at the university, mad!

[elcraigo]

Randall,
I work at a small company that designs instrumentation for measuring high speed train track and train wheels.
We use UV lasers and high speed cameras to digitize the image of the track while the train is moving at speeds up to 400 KPH, 300 KPH is more typical, not in the US of course.
This experiment is fun for me because I've been out of college for 32 years and it helps keep the rust off.
I'm an EE and generally just keep my head down doing circuit design and writing embedded code.
I do understand you wanting to get your students to just dig in and figure out. Just dare them to be wrong and learn from it.
Heck use a pencil and paper if have you to.

We should thank Bolivar for starting the experiments. If I could mail Bolivar the laser lab, I would.

[porcupunctis]

Craig,
Thanks, stay curious.

[jrebman]

Now this is what I call fun! :-)

Craig, that time since college puts us within a yer or two of each other, and I too used to do a lot of embedded design back in the day.

Looking forward to what you find out.

-- Jim