193-mHz beat linewidth of frequency-stabilized laser-diode-pumped Nd:YAG ring lasers

We describe frequency-stabilized laser-diode-pumped Nd:YAG ring lasers locked to a high-transmission and high-finesse reference Fabry-Perot cavity. The cavity has a resonant linewidth of 38.0 kHz, a finesse of 19,400, and a transmission efficiency of 79.4% for the TEM(00) mode. We measure the frequency stability by locking two lasers one free spectral range apart and observe the heterodyne beat signal. As a result, the spectral densities of frequency noise are reduced to 10 mHz/ radicalHz, and a beat linewidth of 193 mHz is obtained. The linewidth is 0.73 times the Schawlow-Townes limit.