Laser Frequency Control for Ion Trapping

Undergraduate #346
Discipline: Physics
Subcategory: Physics (not Nanoscience)

Lauren Weiss - Amherst College
Co-Author(s): David Hanneke, Amherst College, Amherst, MA

In atomic, molecular, and optical physics, we use lasers to control the quantum states of atoms and ions for applications such as quantum infoprocessing, precision measurement, and sympathetic cooling of other ions. Because the laser frequency must be in resonance with the atomic frequency, it is important to be able to tune the laser in a stable manner. We use a diode laser and provide tunability and stability with feedback from a diffraction grating. This external-cavity diode laser (ECDL) experiences changes in frequency dependent on temperature, laser diode current, and diffraction grating angle, which we change using a piezoelectric transducer (PZT) behind the grating. Over the course of the day, the laser frequency tends to drift if it is not stabilized. Also, variations in temperature, diffraction grating angle, and laser diode current can cause mode hops, in which the frequency jumps discontinuously. The goal was to be able to finely adjust and stabilize the frequency of the ECDL without mode hops. Two methods of controlling the laser frequency were tested: a circuit that varies the laser diode current in proportion with the piezo voltage (diffraction grating angle) and a confocal Fabry-Perot cavity. The circuit prevents mode hops by scanning the length of the internal cavity (by adjusting the laser diode current) and the extended cavity (by adjusting the PZT voltage) in proportion to smoothly change the frequency. A Fabry-Perot cavity is a pair of highly reflective concave mirrors facing one another that serves as a length reference for monitoring our laser wavelength. One mirror has a piezo behind it that pushes on it to change the length of the cavity. By moving the mirror over small distances, we can detect integer numbers of wavelengths. The scannable length serves as a reference for the laser frequency (and wavelength). The Fabry-Perot cavity can be used to lock the ECDL to a HeNe laser, since HeNe lasers are highly stable in their frequencies. Using the circuit, it was possible to tune the ECDL (without mode hops) over a 7 GHz range. The Fabry-Perot cavity allowed us to effectively view the spectrum of the laser and detect mode hops. These two methods make it possible to control the frequency of a laser for atomic physics applications. In the future, this ECDL, along with the mode-hop-free tuning circuit and the Fabry-Perot cavity, will be used in an experiment to try to determine changes in fundamental constants. The circuit design was adapted from C. Petridis et al, ‘Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating.’ The Fabry-Perot cavity design was adapted from Cheyenne Teng’s thesis, ‘Frequency Control and Stabilization of a Laser System.’

Funder Acknowledgement(s): This work was supported by the Clare Boothe Luce Foundation, the Amherst College Dean of the Faculty, and the US National Science Foundation.

Faculty Advisor: David Hanneke, dhanneke@amherst.edu

Role: I constructed and tested the Fabry-Perot cavity. I also adapted the circuit design, built a prototype and tested it. Then, I built the final version and tested that.