Ultra-Narrow Laser for Optical Frequency Reference

We present ultra-narrow line width, tunable diode laser system which will be used as a local oscillator in an optical atomic clock and for precision spectroscopy of Sr near 689 nm. Design of the high finesse optical cavity used as short-term frequency reference is optimized with respect to insensitivity to vibrations. We achieved laser line width of about 8 Hz, measured by comparison of two identical systems. The relative phase lock of two lasers is better than 150 mHz. Laser tunability and usefulness for precison spectroscopy were demonstrated through line shape measurement of a 20 kHz wide resonance of the optical cavity.