RICE UNIVERSITY A Narrow Linewidth Diode Laser System for Strontium Laser Cooling Applications by Sarah B. Nagel A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE Approved, Thesis Committee: Thomas C. Killian , Chair Assistant Professor of Physics and Astronomy Randall G. Hulet Fayez Saro¯m Professor of Physics and Astronomy Jason H. Hafner Assistant Professor of Physics and Astronomy Houston, Texas April, 2004 ABSTRACT A Narrow Linewidth Diode Laser System for Strontium Laser Cooling Applications by Sarah B. Nagel 1 3 The diode laser system for laser cooling on the S0 ! P1 intercombination line of strontium discussed in this thesis allows us to cool and trap an atomic strontium sample to 15 ¹K. Samples in this temperature range are useful for the development of the next generation of atomic frequency standards, cold collision studies, and as a step towards quantum degeneracy. This laser system consists of a Littrow con¯guration external-cavity diode laser, which is frequency locked to a high ¯nesse cavity. The cavity is subsequently locked to an absorption feature to produce a few mW of tunable, stabilized, single mode power at the target wavelength. We present the design and characterization of this system, including a linewidth measurement of 70 kHz, as well as preliminary results from cooling and trapping. Acknowledgments While the text of this thesis requires only a few short pages, the list of those I wish to acknowledge has grown to amazing proportions! Without their help, this project would not have been completed, and I will undoubtedly call on them again in the days to come. I can only hope that they will not be o®ended by the humble acknowledgements given here { wholly inadequate and certainly incommensurate with their contributions. First, I would like to thank my committee members for their time and energy. Special thanks go to my advisor, Dr. Tom Killian, for his patience, guidance, and encouragement. They say that ¯rst-generation graduate students are the best trained { the fault for any deviation from this trend falls squarely on my shoulders. The work for this thesis could not have progressed without the support of my labmates, friends and family. For countless reminders not to worry, but to "just do whatcha gotta do," special thanks go to Clayton. For myriad late-night conversations and being my Barnabas, I thank Kerry. I could not have done it without you. For phone calls, care packages, and their uncanny ability to joke me out of my blackest funk, I thank my family. Their love and support have carried me through many scrapes and stumbles. I can only love them the way they have loved me: extravagantly. And while they may not know much about lasers and ultracold atoms, they have taught me those things that are most important: to strive for excellence and to live with compassion. I hope to honor them with this work. Contents Abstract ii Acknowledgments iii List of Figures vi 1 Introduction 1 1.1 Laser Cooling and Trapping of Neutral Atoms . 1 1.2 Atomic Structure of Strontium . 4 1.3 Doppler cooling on a narrow optical transition . 6 1.4 Linewidth Concerns . 8 1.5 Diode Lasers . 9 2 689 nm Diode Laser System 12 2.1 ECDL . 12 2.2 ECDL and a High Finesse Cavity . 16 2.2.1 PDH Error Signal . 16 2.2.2 Cavity Servo Electronics . 19 2.2.3 Linewidth Measurement . 22 2.3 ECDL+Cavity and Atomic Sample . 23 2.3.1 Spectroscopy . 24 2.3.2 Servo Electronics . 24 2.4 The Entire System . 27 v 3 Results 30 3.1 Blue MOT Operation and Diagnostics . 30 3.2 Red MOT Operation and Diagnostics . 33 3.3 Outlook . 35 A Troubleshooting 37 A.1 Replacing a Laser Diode . 37 A.1.1 Initial Alignment . 38 A.1.2 Threshold Current . 39 A.1.3 Target Wavelength . 40 A.2 Cavity Error Signal Acquisition and Locking . 42 A.2.1 Cavity Error Signal Acquisition . 42 A.2.2 Locking to the Cavity . 43 A.3 Atomic Error Signal Acquisition and Locking . 44 A.3.1 Atomic Error Signal Acquisition . 44 A.3.2 Locking to the Atomic Feature . 45 References 47 List of Figures 1.1 Partial Strontium Energy Level Diagram . 5 1.2 Extended Cavity Diode Con¯gurations . 10 2.1 ECDL schematic . 13 2.2 Electronic Protection Network . 15 2.3 ECDL with Cavity . 18 2.4 Cavity Error Signal . 19 2.5 Cavity Servo Schematic. 20 2.6 Fast Path Locked Signal . 21 2.7 Locked Cavity Signal . 22 2.8 Fourier Transform of Locked Error Signal . 23 2.9 Saturated Absorption Schematic . 25 2.10 Saturated Absorption Error Signal . 26 2.11 Saturated Absorption Servo Schematic . 26 2.12 Saturated Absorption Locked Signal . 27 2.13 System Schematic . 29 3.1 Absorption imaging . 31 3.2 Blue MOT Absorption Image . 32 3.3 Blue MOT Temperature Determination . 32 3.4 Red MOT Absorption Image . 34 vii 3.5 Red MOT Temperature Determination . 35 Chapter 1 Introduction This introductory chapter includes discussions of laser cooling and trapping, the atomic structure of strontium, and an introduction to diode lasers, all of which are necessary to motivate the work documented in this thesis: the fabrication, characterization, and use of a diode laser system suitable for laser cooling on the 1 3 S0 ! P1 line of strontium. 1.1 Laser Cooling and Trapping of Neutral Atoms Over the last 30 years, laser cooling and trapping techniques have revolutionized experimental atomic physics. Because these techniques allow a signi¯cant reduction in translational energy, these systems approach the ideal ensemble of stationary atoms, allowing us to probe their interactions among themselves as well as interactions with the environment. Laser cooling has become a standard laboratory tool for producing cold (< 1 mK), dense (1010-1011 cm¡3) samples of atoms. Magneto-Optical traps [1] are now commonplace and provide a starting point for branching into numerous directions of atomic and quantum physics. The basic mechanics of laser cooling and trapping are discussed in Metcalf and van der Straten's book Laser Cooling and Trapping [2]. Using resonant radiation pressure to cool atoms was proposed independently by Hansch and Schawlow [4] and by Wineland and Dehmelt [5] in 1975. The basic 2 idea, commonly known as Doppler cooling, is that atoms travelling towards an opposing laser ¯eld will preferentially absorb light that is detuned below (to the red of) the center of the atomic resonance. This preferential absorption is due to the Doppler e®ect which causes the light to be shifted into resonance with the atoms. On average, fluorescence is symmetrically distributed in space, which leads to a net atomic momentum loss. Momentum transfer from red-detuned light causes a damping force that opposes the motion of the atom. Wineland et al. [6]were the ¯rst to observe radiation pressure cooling in their experiments with trapped magnesium ions in 1978. Balykin et al. [7] were the ¯rst to experimentally observe this e®ect in 1-D cooling of a sodium atomic beam in 1979. Shortly after, Phillips et al. [8, 9] added a magnetic gradient ¯eld to compensate for the changing Doppler shift and to keep the atoms in resonance with the cooling beam as they slowed down. This cooling scheme is often called Zeeman slowing. In 1984 Ertmer et. al [10] used a swept-frequency laser chirp to track the Na atomic resonance as the atoms were slowed from an initial beam velocity to 600 m/s to a ¯nal gas cloud velocity of about 6 m/s (50 mK). By using three pairs of intersecting, orthogonal, counterpropogating red-detuned laser beams Chu et al. [11] extended Doppler cooling into three dimensions in 1985. Atoms with speeds below a certain capture velocity were rapidly cooled to a remarkable temperature of 240 ¹K. Although there was no position-dependent force, and thus the sample was not trapped, the overlap region 3 of the laser beams con¯ned the atoms for 100 ms. This con¯guration is known as optical molasses. Without any position-dependent restoring force, the atoms eventually random walk out of the cooling beams. The temperature limit of this molasses is found by balancing the cooling due to the damping force and the heating from the statistical fluctuations of the force caused by random absorption and emission of photons. Using a Fokker-Planck semiclassical model, several theoretical treatments [12, 13, 14, 15, 16] yield the well-known Doppler limit for laser cooling, given by h¹¡ k T = cool ; (1.1) B d 2 where Td is the Doppler-limited temperature, and ¡cool is the linewidth of the atomic transition used for cooling. This limit says that the minimum kinetic energy of the atoms is equal to the energy width of the cooling transition. It is interesting to compare this limit to the recoil-limited temperature, which occurs when the kinetic energy of the atom is equal to the recoil energy imparted to the atom when it absorbs a single photon: h¹2k2 k T = : (1.2) b r M Here, k is the wavenumber of the light and M is the mass of the atom. For most experiments, this is the hard limit for laser cooling since it involves the minimum interaction with the laser ¯eld. For typical cooling transitions, the Doppler-limited temperature is 100 ¡ 1000 times greater than the recoil-limited temperature. By adding a magnetic quadrupole gradient ¯eld whose zero coincides with the 4 optical molasses center, Raab et al. [1] utilized the internal structure of the atom to produce a 3-D restoring force that trapped the atoms for long periods of time.