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User:jonsinger
Date:2013-05-15 10:48
Subject:A More or Less Edible Laser
Security:Public

I recently found Stephen Wilk’s article “Edible Lasers: What’s the Next Course?” (which appeared in Optics & Photonics Magazine) and got into email correspondence with him about it, during the course of which I realized that I couldn’t actually recall having lased a dye that was dissolved in glycerol. (I probably did lase Rhodamine B in glycerol a long time ago, in a galaxy far, far away; but that was then and this is now, and that effort, if it did indeed occur, has long since faded from my memory.)

Accordingly, on Tuesday evening I set up a homebrew cuvette, and lased Rhodamine B in glycerol with longitudinal pumping from a nitrogen laser, something I think I’ve only done once or twice before. Almost nobody ever bothers to pump a dye laser longitudinally with the output of a nitrogen laser. There are several reasons for this, one of which is that half of the output from the dye goes back into the nitrogen laser and is lost unless you take steps to prevent that from happening, which is itself a nontrivial exercise. I didn’t take photos on Tuesday, but what I saw was interesting enough that I returned on Wednesday afternoon and did it again; see below.

I used Rhodamine B because it has an intriguing character: the efficiency of its fluorescence emission is directly proportional to the viscosity of the solvent that it is dissolved in. If you put RhB in methanol, the quantum efficiency is only about 0.43; by the time you get to glycerol it is more like 0.96, which is excellent — that’s comparable to the efficiency of Rhodamine 6G or Fluorescein, both of which are extremely good laser dyes. RhB absorbs very little at the wavelength that the nitrogen laser provides, so the increased efficiency is extremely helpful. I could have added a second dye to absorb the UV and convert it to a wavelength range that RhB absorbs better (I have done this in the past to assist several dyes that don’t absorb much at 337 nm, as have various other folks), but that would involve extra tweaking, and I wanted to do this expeditiously. In addition, most laser dyes are toxic, and that’s not what I’m trying for here.

After the longitudinal pumping test succeeded I added more RhB, and lased the solution with transverse pumping. Here are two photos. The first shows the setup, and the second shows the output on a piece of paper. The solution was probably inadequately mixed; in addition, the glycerol has been heated unevenly by waste energy from previous pumping pulses, and it has almost certainly developed Schlieren, which interfere with the optical path and cause the beam to spread out. (Schlieren are irregularities in refractive index; think about the “wiggles” you see on the pavement when the sun shines down through the exhaust from a bus or truck, or what happens when you mix two liquids that have significantly different refractive indices, for example water and isopropyl alcohol or even isopropyl alcohol and ethanol.)

           


[There is a PRA LN-1000 TEA Nitrogen laser just outside of the picture, at right. It puts out pulses of ultraviolet light with wavelength of about 337.1 nm; the pulses last a little less than 1 nanosecond. The beam from the nitrogen laser goes through a cylindrical lens (visible at the right edge of the “setup” photo), and then through an ordinary achromat (which fluoresces in the “lasing” photo, clearly indicating that it is absorbing some of the UV from the LN-1000, grump). These two lenses focus the pump beam to a narrow line across the front of the dye solution in the cuvette, which you can’t see because the fluorescence of the dye is so bright. This is the usual way of pumping a dye laser with a nitrogen laser, though people typically use only a single cylindrical lens; I added the second lens to get a more compact setup.]

I like Rhodamine B, and it is less toxic than Rhodamine 6G; but you still wouldn’t want to drink it, so I redid the demonstration with Fluorescein:

           


I’m sure this solution would taste nasty, but you could certainly drink a small amount of it without hurting yourself, so it qualifies as edible. I didn’t even have to add any dish detergent to it. (The original edible laser was a slight cheat — they were obliged to add a few drops of detergent to their material. In fact, I have heard Ted Hänsch say, concerning that particular issue, “At that point, Art stopped insisting on eating the experimental subjects.” Ahem. ;o)

[T. A. Hänsch, M. Pernier, and A. L. Schawlow, “Laser Action of Dyes in Gelatin”, IEEE Journal of Quantum Electronics, Volume QE-7 (January 1971), page 47. Also see http://histsoc.stanford.edu/pdfmem/Schawlow_Arthur.pdf , though this and other texts I find on the Web contain an apparent inaccuracy: AFAIK, they did not put Fluorescein into Jell-O™. They put it into unflavored gelatin and, as I mention above, it did not immediately work; like RhB, Fluorescein does not absorb well at 337 nm, on top of which gelatin is not as transparent as glycerol. They had to put so much dye into the gelatin that they got concentration quenching, which interfered with the fluorescence enough to prevent lasing. The detergent helps prevent this problem.]


Serendipity Strikes Again




As I mention above, I returned this afternoon to see whether I could get a photo of the results with longitudinal pumping. I removed the cylindrical lens, leaving the ordinary lens, and this is what I saw on the paper viewing screen with the cuvette of dye moved to the side:



This is a fine image of the inside of the laser head, showing the electrodes and the discharge. When I put the cuvette back in position, I was surprised to discover that I was getting lasing on both axes at once:



(Lasing on more than one axis at a time is far from unheard-of; but it’s not common, and this is certainly the first time I’ve done it or seen it with nitrogen-pumped dye — I just happen to have chosen a dye concentration that supports it under my conditions, and a lens with appropriate focal length.)

Here are separate photos of the two outputs, for a bit more detail:

           


What I find particularly spiffy and interesting is the fact that the longitudinal lasing output from the dye cuvette is still an image of the inside of the nitrogen laser head (!). Notice that the L output is yellowish, while the T output is green. My belief is that this is because the L output has to travel through a lot more unexcited dye solution before it gets out of the cuvette, and the dye absorbs the short-wavelength tail of the emission. (This is a well-known and well-understood issue, very common with fluorescent dyes.)


Note: I took these photos with my iPhone. If anyone reading this has a strong need for higher resolution images, please contact me via email.

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User:jonsinger
Date:2013-05-07 23:49
Subject:Baby-Steps in Welding: My First Actual Project
Security:Public

Every week, Chas. Colburn (3D Metal Parts) runs an open house that he calls “Maker-Tech Tuesday”. Chas. has been doing welding for at least 35 years, and it is something he is happy to teach people. I’ve been wanting to learn to weld for even longer than that, so even though I didn’t have any applications in mind I jumped at the chance. (The fact that he is a superb teacher didn’t hurt.)

I have now had three lessons, and I intend to make my first three sets of baby-steps into a sculpture at some point, but that hasn’t happened yet. What did happen is that I realized that I do have a project: I like to run my potter’s wheel as a standup wheel (to help avoid back trouble), and once I get to the point where I’m at least half-decent at welding I will be able to make a stand for it instead of just putting it up on cinderblocks again.

Something else, however, has taken precedence: the storm door at the house I’m moving to was not in great shape, and after it took additional damage from the wind a few weeks ago I removed it. My landlord, who is a friend and who is as good as they get, was kind enough to give me another storm door, which I managed to put in place by changing out the left edge of the frame and doing a bit of minor surgery so it would fit in the available space. (The tall brown stripe at the left edge of the new door is the frame piece that came attached to it; it was originally about half an inch too tall.)



Unfortunately, when I tried to close the storm door after I attached it, I discovered that there was a problem:



The handle is at just about the same height as the doorknob, and it protrudes far enough out that the storm door can’t close all the way if the house door is shut. (The dent in the doorknob, partly visible behind the handle, is a pre-existing issue.)

I thought about various ways to deal with this problem, and on Monday afternoon I ended up buying a nice matched bottle opener and drink sifter at a thrift store. My initial attempt at attaching the bottle opener to the shaft of the handle mechanism, last night, was unsuccessful, and this afternoon I took the various pieces to Maker-Tech Tuesday, figuring that I would cut some pieces of square steel bar stock and bolt them to the opener so I could attach it securely to the shaft. Chas., however, took one look and suggested that I just turn the opener around and weld the butt end of its handle to the side of the shaft. (The outer handle is held on with a setscrew and is easy to remove, so attaching the inner handle permanently would not prevent me from putting the door back together.) For a few moments I was reluctant, but it was clearly so much easier and less involved than what I’d previously had in mind that I gave in very quickly. Beside, it would constitute an actual Welding Project.

These particular bar implements are made of stainless steel, which generally has a reputation for being difficult to work with, but Chas. has some welding rods that have nickel in them and can be used with stainless, and he thought I’d be able to deal.

The original shaft from the door looked like it might possibly be aluminum, but that didn't really make sense to us, as ordinary aluminum is not all that strong or durable, and is not well suited to this type of application, so we figured it was more likely to be stainless. We were, however, giving too much credit to the manufacturers of the door: it was, indeed, aluminum, and when I attempted to weld the bottle opener to it the results were pretty silly. (I may show a photo here at some point.)

Chas. and I looked for an appropriate piece of steel bar stock so I could make a replacement shaft, but we didn’t find any, so I told him I’d go to the hardware store and try to buy some. He suggested that I try Fasteners, Inc. [4817 Lydell Rd, Hyattsville,  MD  20781], one of his regular suppliers. Sure enough, they had 5/16" steel key stock in 1-foot lengths. I bought two pieces plated with zinc and two plain ones, to be sure that Chas. would have some if anybody needed it for a project. (In general, we are expected to provide our own materials; but extra is always appreciated, the more so because some people may not always be able to bring what they need.)

The butt end of the bottle opener being somewhat funky at this point, I removed the paddle from the drink-sifter, chopped an appropriate length off one of the zinc-plated key stock pieces, and welded the butt end of the sifter to it, angling the handle slightly outward in case it might otherwise be too close to the door for easy operation. Here’s what the assembly looks like, with the outside handle loosely in place:



When I got home this evening I reinstalled it:



It now clears the knob of the front door, though it’s hard to tell from this photo:



(It originally cleared by only a millimeter or so, which is how it was when I took the photo, but my landlord found the rest of the frame that came with the new door, and brought it over yesterday morning. The new door is very slightly narrower than the old one was, and I had to shim the frame about half an inch so the latches would engage, but that was easy — I had appropriate pieces of wood on hand. The new frame seems to hold the storm door slightly farther out than the old frame did, and the new handle now clears the knob of the regular door by about a centimeter. Still not even remotely enough room for the original handle, though.)

The weld isn’t exactly pretty, but it is certainly GEFWIF:



[GEFWIF is something I got from Chas., who is more fiendish than I am: I would use it to mean “Good Enough For What It’s For”; he uses it to mean “Good Enough For Who It’s For”. Maybe not grammatically correct, but clear enough.]

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