Narrow-Line and Frequency Tunable Diode Laser System for S–D Transition of Ca⁺ Ions

Abstract

A new type of diode laser is used for the development of an extremely narrow linewidth and frequency-tunable clock laser system for a Ca+ ion optical frequency standard. The required reduction of linewidth was achieved by locking the laser to an ultrahigh-finesse ultralow-expansion glass (ULE) reference cavity. The long-term frequency drift is reduced by stabilizing a cavity temperature at the point that the thermal expansion coefficient of the ULE is zero. As a result, the laser linewidth is decreased to lower than 25 Hz and the long-term drift is less than 0.025 Hz/s. The other laser offset locked to the stabilized laser is continuously frequency tunable in the entire free spectral range of the reference cavity without degradation of stability and linewidth. This laser system is actually applied to optical frequency standard development and used for the observation of the 2S1/2–2D5/2 transition of Ca+ ions.