University of Washington Reports Development of Compact Tunable Clock Laser

The University of Washington group of Warren Nagourney has previously reported on the construction of a tunable continuous-wave ultraviolet (UV) source of coherent radiation based on a frequency-quadrupled solid state laser system. This device emits more than 10 mW of relatively narrow-band (linewidth about 8 MHz) UV radiation, tunable from 232 nm to 242 nm. The entire system is contained in a 19"x25"x8" box and draws a modest amount of power from the mains (about 100 W). A stable reference cavity that fits entirely within the box has now been constructed to reduce the laser linewidth to about 8 kHz (in the UV portion of the frequency spectrum).

The reference cavity, whose free spectral range is 5 GHz, is temperature stabilized to less than 1 mK and has a linewidth of 160 kHz. The cavity is constructed from a thermally stable invar spacer and is isolated from vibrations. The laser is locked to this cavity using the Pound-Drever method. Analysis of the error signal (using a digital spectrum analyzer) yields an rms laser linewidth (relative to the cavity) of 2 kHz. This width becomes about 8 kHz after frequency multiplication into the UV. The locking is very reliable and robust, giving many hours of trouble-free operation.

In summary, the group has developed a very narrow (8 kHz, or 1 part in 1011), medium-power (10 mW) tunable UV laser source entirely contained (excluding control electronics) in a relatively small enclosure and having modest power requirements. This source will become the "clock laser" for the indium ion frequency standard with the addition of further frequency stabilization (using an external Zerodur cavity) to reduce the linewidth to about 1 Hz.