Since a free-running laser typically has a fast linewidth way too broad for high resolution spectroscopy of optical clock transitions, it is necessary to pre-stabilize its frequency relative to a narrow resonance of a carefully designed optical cavity. An optical Fabry-Perot cavity consists of high reflectivity mirrors separated by a spacer, usually made of a low expansion material such as ULE glass. When the frequency of the laser is tuned such that an integral number of wavelengths fits in the gap between the two mirrors, constructive interference leads to the build-up of an intense standing wave inside the cavity and light is transmitted through the cavity. These cavity resonances are spaced by c/2L (where c is the speed of light and L is the length of the cavity), termed the free spectral range, which has a typical value of around 500 MHz.
Since the stability of an optical atomic clock based on large numbers of trapped neutral atoms can be limited by the thermal noise of the clock laser reference cavity, we continue to strive to advance cavity performance. Presently we are designing new cavities with lower thermal noise limits through optimal choice of the materials used for the cavity spacer and the cavity mirror substrates and coatings. Cryogenically-cooled cavities may provide another path to reduced cavity thermal noise and improved laser performance.