Laser Popcorn

First, please note that Lightsaber pt. 1 is illustrated for easier understanding.

So I'm hoping to soon get my 170 mW laser diode in the mail. (It was pulled from a 20X DVD burner, and it's visible red). This is sufficient to make electrical tape melt and blue balloons pop and carbon paper smoke.   So after ordering, I sat back and thought, "Now what?"

What sort of application should my diode apply itself to?  The first idea I had brought a smile to my face: a popcorn popper.

Popcorn poppers have been on the 'Web lately: see http://www.everythingusb.com/diy-usb-popcorn-popper-15007.html for a USB popper, or http://www.youtube.com/watch?v=jgjx4JROjR4 for a video of cellphones popping corn (what a pity it's a hoax, and furthermore by a corporate wickdeed (that means a person who does a wicked deed, of course)). My thought? Bring a high-powered laser to a tight focus, and drop a stream of unpopped kernels (Fig. 1a)  through that point.  As each kernel hits the hot spot (Fig. 1b), it explodes into fluffy goodness (Fig. 1c) and continues into a waiting bowl (Fig. 1d). 

Sadly, I was not the first to think of it (http://www.metacafe.com/watch/847876/laser_popcorn/).  It's clear from this video that my laser diode is NOT going to instantaneously turn kernels into clouds.  I was reassured, however, that it was not necessary to paint the unpopped corn black (they burn instead of popping).  I do wonder if naturally blue corn might pop more effectively.  

So how to get the popcorn to pop fast? Well, one could try a more powerful laser, of course. A ruby laser pumped by a flashlamp would be ideal.  The kernel would interrupt a light beam as it falls (maybe even a weaker laser), and an appropriately-timed trigger circuit would fire the flashlamp at just the right instant, blowing endosperm into a starchy Styrofoam.  Might even work for puffing wheat. The flashlamp circuit will require some time to recharge, so the time between kernels will have to be moderated, maybe by a synchronized solenoid that releases the popcorn from its chute (where the kernels line up in single file as they come out of the hopper).

A continuous-mode laser would pop corn at a much higher rate; however, other visible colors wouldn't do any better.  Popcorn pops because of steam buildup, so I'm thinking the water absorption spectrum (seen here  ) is the most relevant-- although the color of the outside husk might have a lot to do with it.  Apparently, the redder the light, the better the H2O absorption.  I'm sure a lot of you will correct me, but I can't think of any high-intensity red lasers; well, I suppose argon ions lase in red, and maybe mercury or something. 

The water absorption spectrum makes it clear that visible light is a lousy medium for exciting water.  The deeper the infrared, the better-- in fact the peak is at a wavelength of ~ 3 micrometers (3000 nanometers, as compared to 700-400 nm as a generous estimate of visible light). (Water apparently also absorbs well in the X-ray regime, but X-ray lasers are inconvenient.) So what to use?

A nice option is the carbon dioxide laser, which is fairly efficient and somewhat close to the absorption peak.  Perhaps an even better option would be the water laser.  Yes, water vapor can be made to lase, but it doesn't really coincide with the absorption peak of liquid water. So it looks like CO2 is it.  Any dissenting opinions?

Other options? The microwaves in a microwave oven are gigantic, with a wavelength of about 12 centimeters.  They are effective at heating water only because they rotate the molecules, which then "rub" against each other, making heat essentially by friction. This size, however, won't come to a nice tight focus, which is why your microwave scatters the radiation with a stirrer fan. Looks like a poor choice.  Masers are microwave equivalents of lasers, but would not really improve matters.  

But microwaves give me an idea-- perhaps a high-voltage arc? The electrodes could have just the right spacing and voltage so that they would not arc until the kernel fell between them-- like a bug snapping in a bug zapper.  I can't help but think that the odor of ozone might be unpleasant in your popcorn.  But it's got potential.

Now another problem.  Presuming that the popcorn popper pops popcorn, the fluffy product will suddenly decrease its terminal velocity drastically, going from a projectile to a parachute.  This could cause the nascent popped-corn kernel to hang around the hot zone too long, possibly burning.  Of course, the albedo of the corn also goes up drastically, but this might not be true in the infrared.

 

My thought? If the unpopped corn is grain, the popped corn is chaff.  Place a fan at right angles to both the laser and the drop path (Fig. 1e.), and the suddenly wind-catching popped stuff will immediately waft to the side (Fig. 1f.), no doubt into a prepared melted-butter bath.  This minimizes time in the hot spot. (See illustration). This is the principle behind a hot-air popper, where the popped popcorn gets blown out the top while unpopped seeds swirl around the bottom.

Now that we've figured it out, who's going to build it?