optical question

If you don't like the razor blade thing, we use to use aquadag to coat the inside of big pumping tubes for low temperature stuff. It's black graphite stuff and should absorb in the IR.

George H.

sample

come

piece

avoid

approach

beam,

which

ribbed

If you can get it, black chrome plating on copper ought to do pretty well. Black chrome is a bunch of vertical chrome needles on the surface (classic light trap).

?-)

The scatter is slower than you might think, unless the sand is a good absorber. It could easily be nanoseconds.

Make sure you test it in the IR, and at your power level.

Cheers

Phil Hobbs

Yikes. Yes, a couple of inches of sand in a tube could stretch a light pulse for nanoseconds. That would be fun to try some day.

I'd better not use sand.

At a couple of watts, I probably can't get into big trouble.

We have a grit blaster; I could scruff the inside of the tube first, then use the blackening stuff. Maybe curve the tube a little, and poke in the sampling fiber from the other end.

Small integrating spheres are good for matching femtosecond pulses to the capacities of photodiodes. A typical 1-inch sphere will stretch any short pulse to roughly 3 ns.

Cheers

Phil Hobbs

In our business, the speed of light is a real nuisance.

Hmm would you like it to be faster or slower?

(One of my favorite* questions on my qualifying exam was what would it be like if planks constant was an order of magnitude bigger and then smaller.)

George H.

• it's the only question I can remember.. the rest were boring math things mostly.

Turns out water absorbs at this wavelength pretty well. Construct a hollow block filled with water and two fiber ports, one for the light coming in and one for sampling the light. Put the sample port on an adjacent side to the incoming beam and recess it into a hole to minimize the chance of any light reaching the sample port in one bounce. With a couple of Watts coming in continuously it would only need to be a couple of inches on a side to dissipate the heat and not be overly warm. A cell phone dissipates that much power.

Or you don't need to construct much at all...

One classic approach is a curved piece of ordinary drawn copper tubing. The curve comes for free, as such tubing is sold at plumbing stores as a roll of tubing.

Fire the beam straight into one open end, and the beam is forced to bounce around as it tries to follow the curve. Make the inside a shiny (not flat) black surface, so there will be little backscatter. Arrange things so the beam hits the walls at a slight angle - the specular reflections will be forward, and will hit the wall time after time.

One can put a photodiode at the far end of the tube to detect what little gets through the beam dump, so you can tell that the beam is in fact entering the tube correctly.

The scientific equivalent that I read in a pre-laser optics book is a curved horn (tapered tube with closed small end) made of hand-blown glass, coated on the outside with carbon black paint.

Joe Gwinn

That's usually right, but not at 1-2 W. The paint will get toasted off where the first bounce occurs.

The original Wood's horn was coated on the outside with lampblack (i.e. candle soot). Works great, but it's a bit messy. (R. W. Wood is another of my technical heroes--I forgot to put him in the last list I posted.)

Cheers

Phil Hobbs

I assume we are talking about the copper tube. I'll grant that paint could be overwhelmed if the beam is too small, but black chrome plating or the like should handle it. Or, just corrode the copper with sulfur fumes.

My other approach is a bit of black iron gas pipe with both ends capped and an eccentric hole drilled in the wall. The beam passes through the offset hole, hits the inside of the far wall at about 30 degrees, and keeps on bouncing around the inside of the pipe.

This is the blackbody source used in reverse.

I couldn't recall the name. Thanks. The books I read had dispensed with the soot and changed over to a soot-based black paint.

Joe Gwinn

Yup. My fave is Krylon #1602, as noted upthread. Works amazingly well on fused quartz.

Cheers

Phil Hobbs

I have always used 1602 too. One odd use - the top of the shelf below the windshield and above the dashboard, to cut down on reflections interfering with vision.

Joe Gwinn

On Fri, 14 Nov 2014 16:00:58 -0500, Joe Gwinn Gave us:

You must be a football player. :-)

What about mil paints actually meant to be baked on and have high temp service applications?

Those guys had decades to get to know what they were doing (the chem engineers making mil paints way back when).

I think the intent is warpaint, not optics.

The dashboard would melt.

But it's a good point. There are high-temperature black paints intended for woodburning iron stoves (graphite and water-glass based?). I bet these paints wouldn't be bothered by a laser that small. There are also shiny black paints intended for gas grills and exhaust systems.

Joe Gwinn

Good point. I've never tried those.

Cheers

Phil Hobbs

On Fri, 14 Nov 2014 16:39:30 -0500, Joe Gwinn Gave us:

I have always contended such and joke about it all the time.

It is near impossible to 'catch glare' from those angles of incidence, and if you are that much like Tiger Woods, you probably are not cut out for the game.

Yes... War Paint indeed. I laugh when someone denies it. Cough...

On Fri, 14 Nov 2014 16:39:30 -0500, Joe Gwinn Gave us:

We mixed powdered silica into our potting to conduct heat better in HV applications.

I'd bet that beads, and silica powder and an absorptive epoxy like stycast as a binder inside a sealed outer skin, might trap pretty good. Make the receiving "cup" just that. A (deep) cup shape with a parabolic end, of course... Or I guess a single pinpoint on a narrow stalk of a super-reflective media, and then those reflections get fed to the absorbing media. Instead of just a little added, like we were doing, one would flood the cup with beads and silica powder and barley have enough epoxy to bind it all together. Stycast can almost be peered though in thin enough cross section. Of course, I refer to human eye observations.

Kind of like a bunch of hunting arrowheads together. Instead of razor blades, the points style would reduce any incidental reflection count to a very low number. I guess particulars of the beam which will be entering would weigh in.

Actually you want to minimize the sand if possible. Adding any large amoun t of epoxy causes far less scattering as it more or less index matches th e glass.

I first saw the beads during a visit to the left coast as a Service Enginee r.

The graduate student who invented the hollow bead technique had up to 5 Jou le pulses attenuated by stacked "Tic-Tac" boxes in front of his photodiode detector. This was because his professor would not give him further budget for ND filters. He would add or remove boxes to change OD as needed. The sa me craft store had a wide variety of plastic boxes.

Necessity is often the mother of invention.

While I would not suggest his technique for day to day work, when you need a quick improvisation it works.

I left him a spare beam sampling wedge from my tool kit. That should have m ade his life a bit easier.

Steve