Construction of a Laser Frequency Stabilization System for a Magneto Optical Trap Talia Martin and Dr. Edward Deveney Department of Physics, Bridgewater State University, Bridgewater MA Abstract The purpose of my work was to construct a laser frequency stabilization system for Bridgewater’s (BSU) proposed Magneto Optical Trap (MOT). The BSU MOT under construction is designed to cool and trap 85 Rb atoms using a 120 mWatt 780 nm tunable external cavity diode laser (ECDL). Figure 1 is the electronic energy level structure of Rb showing the trapping and repump energy levels (1). Works Cited 1-Smith, Dale, and Chris Cepero. The Hyperfine Structure of the Rubidium Atom. Web. 5 Nov – Barry, John M. Laser Cooling and Slowing of a Diatomic Molecule. Thesis. Yale University, "Mot Lab Home." Mot Lab Home. Web. 05 Nov Introduction Our ECDL has a frequency linewidth of < 1 MHz which is ideal for selecting a trapping frequency within the < 10 MHz line width typical of atoms and Rb. However, because of ambient conditions that are hard to maintain, the selected ECDL trapping frequency can drift by tens of MHz or more per hour resulting in the loss of trapping and a drift off of the atomic transition altogether. To hold an ECDL to a desired frequency a feedback system is required to stabilize, or lock, the laser to the set frequency. Several feedbacks systems have been developed and being used successfully each offering its own advantages and disadvantages. Because the BSU MOT is being designed and constructed under guidance of colleagues at Yale (Dave DeMille and his students), we are following the scanning transfer cavity lock stabilization system they have perfected(2). Here, the ECDL laser is combined in a scanning Fabry Perot interferometer (FPI) with a drift stabilized ( < 0.5 MHz per hour) HeNe laser. Figure 3 below is the drift stabilized HeNe laser output alone: The output of the FPI is adjusted to show the HeNe peaks separated by the FSR of the FPI (1.5 GHz). Conclusion and Future Directions In my work, I was able to complete all aspects of the stabilization system up to and including the generation of the feedback error signals. I am currently working on calibrating those signals for the final step of feeding them back into the ECDL which I hope to complete this semester so that we can measure the long term drift stability of our system which we hope to be comparable to the Yale groups. Figure 2: The interior of a Scanning Fabry Perot Interferometer Acknowledgements I would like to thank David DeMille and his students; John Barry, Toshihiko Shimasaki and Jeffrey Ammon for the development of the idea we used and for their support answering my never ending questions. I would also like to thank Dr. Edward Deveney, my mentor throughout this project for giving me this opportunity and guiding me every step of the way as well as Stephen Lind, for assisting with many aspects of this project. Finally, I would like to thank the Adrien Tinsley Program at Bridgewater State University for providing funding for this work. Equation. 1. The Airy function is the theoretical irradiance output from a FPI 2. Methodology and Setup Figure 5: The setup of the laser Frequency Stabilization System. The Stable HeNe is in the left front and the ECDL is in the black box in the back. Both lasers are directed through the FPI in the middle of the set up Figure 6: A schematic of our laser frequency stabilization feedback system. A MOT cell (3) is shown where the stabilized ECDL will be directed in the future. Figure 9 Actual data: This is our output using the LabView software developed at Yale of both beams, the stable HeNe peaks (white peaks) used for calibration and frequency reference (vertical dashed white line) decoupled from the trapping ECDL peak (red). Also depicted is the ‘set’ frequency (red vertical dashed line) ultimately this will be our trapping frequency. Figure 10 shows a drift off of the ‘set’ (we did this manually by adjusting the DC offset of the ECDL). The LabView program uses the calibration from the FSR of the HeNe peaks and therefore measured frequency displacement of the ECDL (red) peak from the ‘set’ to send an error correction signal back to the ECDL bringing it back to ‘set’. FPI Figure 2 depicts the confocal FPI we are using and Equation 1 is an expression for the analytic irradiance output from a FPI. In Figure 4 below the trapping ECDL peak (simulation using equation 1) is combined with the HeNe signal. A data acquisition system and Yale software uses the FPI output to first take the FSR of the FPI and the two HeNe (blue) peaks to calibrate an absolute frequency scale and then uses the 1 st HeNe peak as an absolute frequency reference (orange) from which the drift of the ECDL (red) can be computed. In practice, the desired ECDL trapping frequency is defined as the ‘set’ frequency from which the drift is measured (purple). Using the HeNe for calibration and reference, a correction signal proportional to the drift is generated that is fed back to the DC offset of the ECDL to bring the frequency back. With this system, Yale researchers have found that they can typically hold and stabilize a selected trapping frequency for more than 12 hours. HeNe FPI FSR for calibration Results Figure 7 below is of our Rb gas cell and our ability to tune the ECDL within the Doppler broadened 5s – 5p transition level (on the left -off of this resonance and on the right within the Doppler broadened resonance) In practice we tune and stabilize our trapping frequency for the MOT to the hyperfine levels within. Figure 8 below are actual FPI oscilloscope outputs with both the stable HeNe and trapping ECDL combined. On the left is both signals seen with a single photodiode (PD) on a single scope channel. In order, however, for the Yale LabView program to generate the ECDL laser drift correction signal with respect to the reference HeNe, the two signals had to decoupled before entering the data acquisition system. This is accomplished using a 780 nm high-pass optical filter as shown in figure 6 with both signals separated into two scope channels Working Stabilization System Figure 1: Rb 5s to 5p hyperfine transitions.