Dallin S. Durfee, Physics and Astronomy
Description of the Laser
In order to get a dye to lase, you have to pump it with energy. You need to get more of the lasing molecules into the higher energy excited state than the lower energy ground state. The way that I pump the dye is with a flash tube.
The flash tube is a glass tube with an electrode on either end. Almost all of the air is sucked out of the tube. The remaining molecules of air can then move easily through the tube. When a high voltage is placed across the electrodes, the molecules ionize. The ions flow towards the negative electrode, and the electrons flow towards the positive electrode. As they flow, they bump into each other and convert some of their kinetic energy into internal energy. This internal energy is then emitted as light. The light from the flash tube needs to be extremely bright in order to effectively pump the laser.
An elliptical reflector is used to focus the light from the flash tube onto the dye. The reflector was made out of an aluminum pipe. The inside of the pipe was polished to a mirror finish, and then the pipe was smashed into an ellipse. The flash tube and the lasing tube were then placed at the two foci of the ellipse.
A high quality mirror is placed at each end of the lasing tube. These mirrors were carefully aligned to produce the stable optical cavity necessary for lasing. One of the mirrors was placed on a piezoelectric disk. The piezoelectric can be used to adjust the length of the optical cavity with a precision on the order of nanometers.
Design Phase
I began design work for the dye laser in the winter of 1992. At the time I was considering funding the laser on my own. I thought that such a project would give me valuable experience with laser physics. I was also interested in learning more about precision metal machining.
Nearly a thousand of hours were dedicated to the design of the laser. A large part of this time was spent devising a way to align the laser cavity mirrors and numerically testing the stability of each design. A large portion of time was also devoted to the design of the flash tube and the reflector to focus the light from the flash tube onto the lasing tube.
Much of my early design work was spent trying to come up with a design which could be built on a very small budget. But certain necessary components could not be obtained for less than several hundred dollars. When I found out about the undergraduate grants offered by the Office of Research and Creative Work, I realized that this would be my only chance to make this project work.
Upon obtaining the grant, I was able to finalize most of my designs. Before receiving word that my grant was approved, I had already found suppliers for most of the parts that I needed. So when the money was available, I was able to get parts into the mail immediately. But I had a very difficult time finding a set of laser quality mirrors for a price that I could afford.
Construction Phase
The construction of the laser was done in the physics underground lab. The physics department machinist, Wes Lifferth, gave me advice on how to operate the machinery that I used.
Since I was trying to complete a complicated project on a relatively small budget, I was forced to make many parts that other’s would have simply purchased. But this allowed me to customize parts and gave me more experience in technical work.
As construction progressed I was forced to make several design changes. As the project progressed, I became more capable of determining what types of designs would be the simplest build. Construction of this project began in December of 1993, and continued until the beginning of May of 1994.
Performance of Laser
The finished laser didn’t work as well as I had hoped. The pumping mechanism performed extremely well, and the light coming out of the laser was quite bright. But the light didn’t come out in a well collimated beam. And it didn’t seem to be extremely coherent. I am almost certain that this is due to misaligned mirrors. I made two attempts at aligning the mirrors, a time consuming and tedious process, and both time the results were the same.
There are two reasons why the mirrors might not be well enough aligned. First of all, I was forced to use a two flat mirrors instead of a combination of a flat and curved mirror. This type of cavity is much more sensitive to misalignment. When I tried to purchase a spherical mirror, I found that most of the companies I spoke to didn’t make a spherical mirror with a curvature anywhere near the one I needed. They all stocked a whole slew of mirrors with curvatures greater or lesser than the one I needed. But for some unknown reason there was a large gap in the radius of mirrors available.
The 20 em mirror that I wanted fell right in the middle of the gap. I finally found two companies that offered to make a 20 em spherical mirror for me. But one of them had a price greater than I could afford, and theyboth-sent-me-their quotes toolateto_do me any good. SoJ’I\’asJQrs;esl_t()\lS~ all111ch less robust design.
The second reason for poor alignment is due to the alignment mechanism. It originally employed a set of differential screws. These screws would have allowed me to position the mirrors to within less than one thousandth of an inch. But after making these screws, I found that I hadn’t allowed any room for the screws to tilt. So as I adjusted the mirror, the screws would bind. In the end I had to use ordinary machine screws, with a precision of about one hundredth of an inch.
I have presently found a supplier for a spherical mirror, and have found a way to make differential screws that won’t bind. But these changes would require additional money which I can’t afford, and another month or two which I don’t have. I am disappointed that I didn’t get a better beam after all my work. But perhaps some day I will return to Provo and be able to make these corrections.
Performance of Power Supply and Dye Cooling Loop
In addition to the laser itself, I made a control box. This box contains the power supply, the dye cooling loop, the piezoelectric controller, the flash tube vacuum system, and the controls and meters for these systems.
The power supply turned out extremely well. I designed the power supply to operate in two modes. In the “pulse” mode, a partially rectified 3000 volt line charges up a large laser discharge capacitor. When the capacitor is fully charged, the 180 joules of energy it has stored is released in just a few nanoseconds. This allows the laser to produce high powered pulsed. In the “continuous wave” mode, the power supply supplies a continuous 6000 volt AC output.
High voltages required extreme safety precautions. Heavy insulation was used, and the power supply was enclosed to keep wandering fingers away. I constructed an acrylic shield which encloses the laser itself. This prevents unwary users from getting shocked, and also acts as a shield if the flash tube were to explode while discharging.
The dye cooling loop also worked extremely well. It seems to provide adequate cooling to keep the dye from breaking down. The fan on the cooling loop also serves a secondary purpose of cooling the electronics. The only snag in the cooling loop was a pump that I had to replace. The replacement pump broke down as well. I believe that ethanol has corroded the pumps and made them lock up. A higher quality pump is needed to permanently fix this problem.
Conclusion
As I stated before, I am very disappointed that the laser didn’t perform as well as I expected it to. But I am extremely grateful for the experience and enjoyment that I gained while working on this project. Since obtaining the grant from the Office of Research and Creative Work I have invested several hundred hours into this project. Before applying for the grant, I estimate that I spent nearly a thousand hours in planning and ground work. This work has given me experience in computational methods, precision machining, electronics, lab technique, and laser physics, among other things. This project allowed me to apply many things I have studied at BYU and has made course work much more exciting. It also helped me get accepted into a quality graduate school and to qualify for fellowships.
I would like to extend my deepest thanks to the Office of Research and Creative Work for making my dream project possible. I would also like to thank Michael Lines, Wes Lifferth, and Ross Robinson, and Nan Ah You for their help and advice. I would like to thank my advisor, Dr. Larry Knight, and I would especially like to thank my wife Memorie for her support and help.