Justin Paul and Dr. Dallin Durfee, Physics
During the course of the last year I have been working on several components that will be used in a next-generation atomic frequency standard, or atomic clock. Some of the projects that I have been working on include building a temperature controller for a laser diode head, aligning a single mode optical fiber to transfer resonant laser light across our lab, designing and implementing a scheme to bake out our vacuum chamber, testing the coatings of our precision optics, and aligning and characterizing a thermal atomic beam source.
Last fall I built a temperature controller for a laser diode head. I was able to achieve a high level of stability by utilizing a wheatstone bridge to sense small deviations in temperature along with proportional, integral, and differential gain feedback to make precise temperature corrections based on those small deviations. Completing the construction of the circuit and working through its bugs gave me a good understanding of how each component works. Using an oscilloscope I characterized the voltage response of my controller as a function of temperature. I concluded that a temperature fluctuation of 0.1° C corresponded to a one-volt signal in my circuit, which quickly brought the temperature back to normal. In the end my temperature controller was a complete success. It stabilizes the temperature of a laser diode head to within three thousandths of a degree Celsius. If the laser diode head receives a temperature shock of four degrees Celsius, my controller will bring it back to within 0.01° C of the original temperature in less than thirty seconds. This project was extremely useful to me in that I will be able to use this knowledge in the future to design my own specialized circuit that will run under the specific conditions for temperature stabilization of a thermal atomic beam source for use in our atomic frequency standard.
The next project that I undertook was to align a single-mode fiber optic cable to capture resonant laser light from one end of our lab so we could use it at the other end of the lab. I am sorry to report that this project did not meet with the same success as my temperature controller. The fiber that we ordered was a very new technology in fiber optics because the wavelength we needed it for was near the ultraviolet range. I spent nearly 50 hours total trying couple the light into the fiber without success and then concluded that something had to be wrong with the fiber. The company informed us that the fiber was suffering from color-center generation, a mechanism that darkens the fiber over time if the wavelength of light passing through the fiber is on the order of ultraviolet light. We plan to overcome this setback by building our own laser source for the near-ultraviolet light in the future.
Another problem that I volunteered to work on was the dilemma of baking out our vacuum chamber. To attain the sensitivity we need in our experiment, we require a low pressure (about 10-6 Torr) inside the chamber. We needed to heat the chamber, which would force impurities out of the walls and components in the vacuum and into the vacuum pump. My problem was that the chamber was very large and is shaped very irregularly. The rectangular chamber contains ribs which are spaced every 8 inches around the chamber. To solve the problem I ordered some custom heating blankets that could wrap around the chamber, and built some mounts that would push wooden blocks against the blankets to keep them in thermal contact with the outside of the chamber. We insulated the outside by wrapping foil around it. By slowly adjusting the heat of the blankets we were able to raise the temperature of the vacuum chamber. Our preliminary tests are a success, but we have yet to progress far enough in our experiment to require an actual bake-out of the chamber.
A major component of our atomic frequency standard is the precision optics we will be using. As part of my work I helped characterize deflection angles to the micro-radian level [1]. We recently sent them out to be coated so that they would be highly reflective, maximizing the output intensity of the laser beams sent through them. I measured the reflectance of several types of prisms, including beam splitters, penta prisms, right angle prisms, anti-reflective windows, and edge mirrors. The reflectance for all of the prisms except for one group came back according to our specifications. The beam splitters, however, posed a serious problem. We required that they split an incoming beam into two output beams of equal intensity. Instead of having equal intensities, however, the two beams were mismatched by about 30 percent. This result was universal for all of our beam splitters, which leads us to believe that they were improperly coated and that we should be refunded for the error. In addition to measuring reflectance of the various prisms, I measured the relative angles of their deflection to ensure that they were still precise enough to be used in our experiment after they had been coated. The result was not good. The right angle prisms were still within our specifications, but the penta prisms in some cases showed relative angles of about 100 micro-radians compared to their pre-coated relative angles of less then 10 micro-radians. Although the actual re-measuring and characterizing of our precision prisms was a success, we may suffer serious setbacks in creating the atomic frequency standard due to inconsistencies in the reflective coatings of our optics.
The latest project I have been spending my time on is aligning and characterizing a thermal atomic beam source. I have been trying to detect the presence of a calcium beam in our vacuum chamber by measuring the absorption of resonant laser light by the calcium. So far I have not seen any absorption, but I am currently working to ensure that the aperture for the calcium beam is aligned with the transverse beam of laser light.
Working on these projects over the last year has given me great opportunities to learn how to do experimental research and enhanced my laboratory skills. I have had the opportunity to present my results in both formal and informal settings [2,3] and to work closely with a professor that I respect and have learned a great deal from. I know that having this experience has benefited me and will continue to be of benefit in the future.
Sources:
- R. Olson, J. Paul, S.D. Bergeson, and D.S. Durfee, “Self-referenced prism deflection measurement schemes with microradian precision,” Appl. Opt., 44, 4639-4647 (Aug 2005).
- C. Erickson, B. Neyenhuis, J. Paul, G. Doermann, S. Bergeson, and D. Durfee, “Design and construction of a Ca/Sr atom interferometer.” DAMOP 2005.