The Passive Solar Laser

Using the basics of laser physics, we concluded that a laser could in theory be powered by means other than a flashlamp or another laser. While sunlight might not initially seem to be a source of power that could be focused into a beam equivalent to the concentrated flashes of a flashlamp reflected from one focus of an ellipse to another, or another laser, there is, theoretically, a way to harness enough power to excite the dye enough to get a laser beam. This beam will not be tunable, and in fact will cover several wavelenghts at once, and be the same color (or close to) the dye itself.

Because the sun's rays come from so far away, the ones that reach the earth are practically parallel. Of course they are not truly parallel, coming from a incoherent source, but the angle of divergence is negligible for our purposes. Assuming that the rays are parallel, they can be focused on a single line using a parabolic, reflective trough.

diag. 1

This trough, if it has enough surface area, should concentrate enough light to cause excitation, and stimulate the emission of photons.

The dye tube (optical cavity) will be essentially the same in this project as in the tunable, flashlamp-powered laser. The only difference will be that the dye will not be circulated throught the solar laser, as it probably will not be powerful enough to require constant renewal of the dye.

Trough Construction

1.) Trace out a parabola on a piece of poster-board, using a focus point and a directrix, and plotting all of the points equidistant from both. This parabola should be at least 1-2 feet in width, and should be fairly shallow, to make construction easier. (As long as it is parabolic, the actual equation of the parabola will not matter, it will focus light onto the focus regardless.)

2.) Use the poster-board parabola as a guide to cut out at least two, probably three copies of it out of 3/4" plywood. These will give the trough its shape, and everything else is built around them, so make sure they are as accurate as possible.

3.) Bend 1/4" masonite around the outside of the parabolas and screw or nail it into place. Form a trough, with one parabola at each end. The masonite will naturally want to bow outward in the middle section of the trough, but a center parabolic support, and strips of 1"x2" planking on the outside, nailed or screwed through the masonite and into the parabolas, should suffice to keep the trough parabolic enough, depending upon its length. The length of the trough should be at least 2'.

4.) Drill 1/4" holes at the focus of each parabola to hold the dye tube.

5.) For the reflective surface, use aluminum flashing, polished if possible (glass cleaner should suffice). Attach it with brads nailed through the masonite and into the 1"x2" supports.

6.) Build a base for the trough that will allow it to stand on flat ground, and be tilted towards the sun. There are many ways this base can be cnostructed, but two things are necessary. It must be able to be held in place at any angle from the ground along both the length and width of the trough. (Make it tilt from side to side, and up and down.)

Testing

To ensure that the parabola is reflecting correctly, the laser should be tested at various stages during its construction. Once the flashing has been attached, and the holes drilled at the focus, testing can begin. Using a 1/4" dowel in place of the dye tube, another laser can be used to test specific points. Simply point the laser perpendicular to the face of the trough, and if the laser light is reflected exclusively onto the dowel rod, the laser is working correctly in that area. However, this method is limited, and once the base is constructed, and the laser can be placed perpendicular to the sun's rays, the entire surface can be tested at once. Use the dowel rod again to test here, and if it is working correctly, make your dye tube and fire away! (See Laser Basics for information on dye tube construction.)

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