Testing power laser, and why is paper not affected...

For a lower power laser I use a photodiode and the ammeter on my DMM. (You have to know the laser wavelength and the spectral sensitivity of the photodiode.) and then calibrate some attenuators and go as high as you like... (Assuming the attenuators don't melt.)

Say I watched you a bit trying to tweak lens focus.. I was thinking if you went to lower power you could tweak the lens while it's running. (Drive the laser with your own current source.)

George H.

On a sunny day (Mon, 29 Feb 2016 12:38:00 -0800 (PST)) it happened George Herold wrote in :

The beam is quite wide actually, is that the big photo diode you were recently talking about?

Neutral density filters?

Well, for me it is safety first. The goggles that came with it are very strong, already start filtering out most green, so the world looks red, really difficult to see things, could not even read the green LCD on my power supply to read the current, had to use a big LCD multimeter to see the voltage.

It is easy to accidently point in the wrong direction and hit some reflecting object and get eye damage in spite of these goggles. Not worth it.

As to the focus, what you see is that the spot on paper hardly changes, it lights up a lot of the surroundings where the beam lands.

Anyways, safety first.

Hmm then a solar cell perhaps?

Sure or any number of other things... It's one wavelength so you don't really care what the attenuation is at other wavelengths. (And you are going to measure the attenuation at this wavelength.) So you can do something as simple as the reflection from a piece of glass.. I did FIR spectrocsopy with a laser and we used sheets of paper as attenuators... (calibrated at each laser wavlength.)

Yeah goggles good! You don't want the beam going out your window either.

George H.

Put pencil marks on paper.

Or, use 10,600nM laser, CO2.

Paper looks black at 10,600nM. Well, it's actually hot and incandescing with 300K blackbody infrared.

Spatial coherence = speckle

I've seen "laser speckle" from white sources: sunlight bounced from a curved windshield a block away, landing on a white wall in the shade. Colored speckle, like confetti. The shadows of my hands were sharp enough to display Fresnel diffraction fringes.

Any source will display diffraction speckle if you can manage to send it through a micron-size pinhole. But for any source but laser, th makes them too weak.

I had a similar experience some decades ago. Don't have the holos or the book any more, too many moves.

You're very welcome. Nonlinear optics has some other applications that might interest you.

Not that complicated after you familiarize yourself with what the terminology means, and yes, one or more lifetimes.

Powdered? I wouldn't because I'm not set up to grow crystals from melts using nasty toxic fluxes like potassium fluoride Czochralski-wise...

Those can be grown like rock candy, in water!

Sure, I can see the punks explaining to the cops how they got second-degree burns by bouncing their laser pointer off your window...

No, that's ordinary Fresnel reflection- the beam will bounce into the sky. You want to reflect it back to the source!

I've been entertained and informed by your posts, and thought you'd find it at least entertaining, if not usable. :P

Mark L. Fergerson

On a sunny day (Mon, 29 Feb 2016 13:52:35 -0800 (PST)) it happened George Herold wrote in :

I like that idea of the Peltier, the one I have is big enough, and looks like it will not evaporate on the spot.

With the helium neon laser back then, I pointed it everywhere once I got it working, it reflected on a teaspoon in a cup, and was painful in the eye, no damage. That thought me something.

As far as outside goes, I remember they were looking for an alien spacecraft or something I heard in the news. ;-) I am not kidding.

Very few had lasers back then.

On a sunny day (Mon, 29 Feb 2016 15:25:57 -0800 (PST)) it happened Bill Beaty wrote in :

Yea, problem with IR lasers is that you cannot see where those point (see the beam). Well, once they hit you can...

On a sunny day (Mon, 29 Feb 2016 19:42:22 -0800 (PST)) it happened " snipped-for-privacy@bid.nes" wrote in :

OK

Now we have something! Are those dangerous to handle?

Yea, retroreflector is the right thing, old car rearlight perhaps.

On Tue, 01 Mar 2016 07:35:16 GMT, Jan Panteltje Gave us:

You are wearing the wrong glasses.

Years ago I had a laser pointer out at night. I have car dealership with cars lined up, about 100 to 125 yards away. I was amazed when I hit the taillights at the amount of return I got.

Can some explain what the crystals are doing? Do they just grow at angles suitable for retroreflection? I got lost on this.

Thanks, Mikek

somewhat related question - how fine can you focus this thing?

There's technology solution (or two, if you count IR videocams). It uses phosphors that are charged from UV or daylight, which decay in IR (so it glows visibly when IR impinges).

... and also what is the unfocused spot diameter at say 3 meters?

On a sunny day (Tue, 1 Mar 2016 12:53:37 -0800 (PST)) it happened whit3rd wrote in :

Cool, there exist very powerful IR lasers, there is a video on youtube of a guy who build a CO2 laser gun. You could use a small laser pointer for aiming too.

If you had a _cheap_ (not like gold foil) IR reflective paint, then you could defeat those new navy IR guns that can burn holes in small vessels.

Yes my Sony starlight video cams also are great in the IR. One night I did see the neighbor and his black dog come home with that cam. But large parts of that dog were : _red_. I realized that is where the muscles were, large part of the body, dog had been running. Lit up brightly :-)

Since no one offered an explanation, I'll offer up one reason I ask. I have 2' lead zirconate?titanate discs in several thickness's. Probably a totally different material and not useful for the experiment. Jan, if they have any use in the experiment, I'd be happy to part with one or two. PZT 8 ceramics Mikek

On a sunny day (Thu, 3 Mar 2016 08:12:14 -0600) it happened amdx wrote in :

Thank you for the nice offer. But I already have those big piezo disks in my ultrasound transducers.

Crystals are fun however, I was just watching this:

It melts at ~1100C IIRC.

Safe to handle barehanded. Don't eat the crystals, don't drink the solutions or use them for eyewash.

Some absorption of the green light by the red plastic, and the beam will still spread, but those are Good Things- they'll get a diffused eyeful of their own beams.

Mark L. Fergerson

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Briefly, the reflection occurs *inside* the crystal, not at its surfaces. The electric field peaks of the pump light change the refractive index of the crystal over very short distances- the two pump beams interfere to crea te a moving diffraction grating inside the crystal that the signal beam the n interferes with, creating the output beam. The output beam's amplitude is the complex conjugate of the signal beam so it propagates in the exact opp osite direction.

The holography part comes in when you look carefully at the moving diffra ction pattern- it's a hologram. Also, either or both of the pump beams can be spatially modulated as in by being sent through a transmission hologram before entering the crystal. The output beam will also be so modulated.

Since in some cases the pump beams "give" their energy to the output beam , you can have a hologram *amplifier*.

Don't feel bad, it's fairly esoteric physics. The Wikipedia article I lin ked to above goes into more detail.

AFAIR PZT has no applicable nonlinear optical properties but as I said it 's been a while.

Mark L. Fergerson