TJIIRRS: Number 10A of an Ongoing Series;
First Steps: Observing Beam Patterns Without and With a Mirror

(2006 April 14 ff)

Note: All of the following images were taken using dye extracted from fluorescent “highlighter” markers. To see an enlargement of any of these images, click the small one.

This is the pattern from a cuvette (filled with dye extracted from a yellow-green marker) that is tilted at an angle to the beam that it is producing. The bright spot is caused by reflections from the walls (despite the tilt), and is ordinary lasing; the larger and more diffuse glow may partly be caused by scattering from particles of junk in the dye, but is mostly superfluorescent lasing (lasing without any mirrors), which is often incorrectly called “superradiant” lasing. (If I correctly understand the Wikipedia definition, superfluorescence is also referred to as ASE, Amplified Spontaneous Emission. True superradiance, on the other hand, is something quite special and unusual, and does not occur in ordinary everyday lasers.)

The next photo shows the cuvette and pattern both, so you can see the tilt. The cylindrical lens is in front of the cuvette, and the nitrogen laser is behind it. The top of the cuvette is tilted over to the right, and the beam is focused to a more-or-less-horizontal line across the front face of the cuvette, which is why the bright spot is above the cloudy spot on the target, off to the left. The curved object at the left edge of the photo is the concave mirror that is doing the initial focusing.

The bright spot at the top is very bright, which indicates that parasitic lasing off the cuvette walls is stealing energy from the superfluorescent beam. Here is what happens when I add a single mirror to a pattern of this sort:

Notice that there is still a very bright spot at the top of the pattern. The corresponding beam emerging from the other side of the cuvette is now reflected by the mirror, and forms the bright spot at the bottom of the pattern. The bright spot slightly above the middle is 2-pass lasing — basically part of the superfluorescent output reflected off the mirror and back through the dye, a process that considerably narrows the beam.

Here is a much better pattern, to which, again, I have added one mirror:

Now there is only one bright spot, in the middle of the diffuse superfluorescence region. I created the improved pattern by focusing the nitrogen laser beam more carefully onto the front window of the cuvette. Unfortunately, it’s very hard to photograph the beam on the cuvette because the fluorescence is so bright (see the second picture, above), so I don’t have a picture of that.

Also unfortunately, the gain of the dye is so high that it proves extremely difficult to tune it with the cylindrical lens in place. Here is what happened when I tried:

(This pattern is reflected from the grating, which is why it is reversed left-right from the one in the previous picture.) The row of dots extending to left and right from the central bright spot may indicate that I have the grating correctly adjusted; it was about the only evidence I had, with the cylindrical lens in position, that I was even trying to tune the output. (Tuning is covered much more thoroughly on the next page.)

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This work was supported by
the Joss Research Institute

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Last modified: Wed May 10 14:59:49 EDT 2017