So far I've made my webpage (which you're now looking at) and started getting to know the people with whom I will be working over the next several weeks. At this point I still have no real idea what I want to do for a project. Hopefully soon I'll be able to talk with Proffessor Noe and figure out something.
Well I got a project a few days ago: messing with the modes of a He-Ne tube while I wait for parts for an open-cavity He-Ne laser that will be a comination demo piece and experimental device. It will be a good demonstration about the basics of how lasers work to the high school students who will be showing up soon and many others in the future. The open-cavity tube will also act as a device for studying the production of various modes of lasing because the output coupler mirror will be adjustable - allowing the more exotic TEM modes to lase.
So far I've gotten a basic experimental setup done for a tradtional "baseball bat" style He-Ne laser to experiment with the polarizations of different modes inside the lasing cavity and how they interact. These modes should be only TEM00 modes but a study of them will prepare me to deal with the more exotic modes that we are hoping to produce with the open-cavity laser.
I've found that the theory and experimental differences in the mode spacings are accurate to within 5% for the "stick" laser. The most likely source of error is that the length of the laser is not known exacly and therefore the theoretical mode spacing is affected by an unknown amount of error.
I am begining to examine the polarization charictaristics of the various types of lasers here. The "white" laser seems to slowly increase in amplitude and then suddenly drop back to some base intensity before begining its incrase again. This pattern is, of course, reversed if the polarizer is rotated by 90 degrees in either direction so that is increases suddenly to some value and then slowly decreases from that before suddenly jumping again. The O-scopes cannor resolve a change on the order of seconds , which this is, and therefore a different source of measurements is needed. If the computer-connectable mutlimeter can be made to take data at a given interval then the phenominon can be studied more rigorously. In the meantime I will attempt to examine this in the "small" and "black" lasers and to take as much data as possible without computational aids.
I've got an Agilent multimeter that has an RS-232 port. The user's guide contains a bsic program in C that could be adapted to my needs but apparently there is a .h file missing from the computer that has windoze and a compiler on it. The agilent website has an interface program that should work fairly well but its ~9 Meg and the windoze machine's NIC doesn't seem to work. If I can get the file to the win machine I should be able to start collecting data but right now that seems like it may be difficult.
The laser pointers do not seem to suffer from "mode-hopping" despite the fact that, due to the small size of their cavities, only one or two modes can "fit" under the gain curve at any one time so one would expect that as the lasing cavity expanded because of thermal effects the laser output power would vary with time as has been seen with the metrologic laser. The reason for this is not known.
The laser pointer also seems to have an even number, most probably 2, modes lasing at a given time because a quick experiment showed little to no sensitivity to rotation of the laser reletive to the polarizer.
The lack of polarization sensitivity is perfectly understandable if an even number of modes are lasing because, since each mode is polarized orthogonally to the two modes next to it, if there is an even number of modes then there will be a roughly even exchange of power between the two of them as the angle changes. However the lack of "mode-hopping" due to thermal effects is less easily understood unless the diode is sufficiently efficient and cooled that the tempature stabilizes very quickly from room temp.
The parts for the open-cavity laser have come in! Yesterday I got all of the pieces put into the frame save the mirror, which we still have to decide if we're going to salvage from a broken (0.2mW of lasing output only) tube that we also purchised or from somewhere else. Since the tube we're testing with is fairly large-bore we should be able to get it lasing with a fairly normal, non-dielectric mirror - albiet at only a fraction of the normal power. However upon attemping this with a flat, first-surface mirror placed about 28cm away from the 60cm focal length HR mirror I was unable to get lasing. I believe that the divergence of the light leaving the bore is sufficient to require a concave mirror of some sort in order to achieve lasing because the divergence of the beam after leaving the internal brewster window is simply too high for a flat, non-dielectric mirror to achieve the low losses needed for lasing. A flat dielectric mirror may be sufficient and a concave mirror with a reasonable focal length almost certainly will be but these guesses have not yet been tested.
I have observed that the divergence from the beam is 8.6+/-0.4mR Which agrees quite well with the theoretical prediction of 8.7mR for a tube of 2mm diameter and 23cm length. However this agreement only occurs if the small spot at the center of the beam is measured. There is also a "halo" of light surrounding it caused by the emission of light from closer to the B-window than the back. The actual shape will be measured and compared with theoretical predictions sometime soon.
Using a planar dielectric mirror and using the technique described on page 16 of Lasers and Modern Optics in Undergraduate Physics by Brandenberger I was able to get the open-cavity He-Ne tube to lase. So far I have achieved lasing in the TEM 2,1; 2,0; 1,1; 1,0,and 0,0 (Gaussian) modes by both adjusting the mirror and by placing a strand of hair inside the cavity. Pictures are available here if you are interested.
Last Friday I was able to get a number of additional modes to lase and I first began experimenting with the effect of distance between mirrors of what modes will lase. I found that the closer the two mirrors are the more exotic (high-order, superpositioned) the modes will be. This was confirmed when I placed the mirror external to the casing at a distance of about 55cm (out of a maximum of 60 due to stability issues) and was only able to get the TEM0,0 mode to lase. I also got the following modes to lase:
TEM0,1 + TEM1,0
TEM0,2 + TEM2,0
And obtained pictures of several of these which are now posted on my website. During the past few days I have gained experience re-aligning the mirror although it is still a slow and arduous task. To reduce the number of times that this task must be performed I have screwed in 3 screws into the table and placed a spacer between them and the laser casing. This allows me to easily and quickly move the laser case while allowing the laser to continue operating, although it does "mode-hop" somewhat as it does so. Today I attended a lecture on computational neurobiology which would have been much more interesting had I had a somewhat better understanding of the topic beforehand. I also wrote a small program to calculate the divergence and intensity profile of the non-lasing light as it emerges from the bore as mentioed in the first paragraph of my previous journal. Hopefully this will allow me to compare theoretical and experimental results if I can obtain a sufficiently high-quality picture of the light.
Today Sam of Sam's Laser FAQ is visiting us. He gave a talk on using a visible laser as a "piggyback" for a microwave signal which was very interesting. Afterwards in the lab he showed us how to get a small amount of orange (~610nm) and a couple other lines of red (~650nm) by using a long focal length mirror and a CD, which was used as a diffraction grating with a spacing of 1.6 micrometers. He also viewed my open-cavity laser and examined the beating modes that I saw in the long "stick" laser. He also talked with the other students here about thier projects.