Compact, thermal-noise-limited optical cavity for diode laser stabilization at 1 x 10-15

We demonstrate phase and frequency stabilization of a diode laser at the thermal noise limit of a passive optical cavity. The system is compact and exploits a cavity design that reduces vibration sensitivity. The sub-Hz laser is characterized by comparison to a second independent system with similar fractional frequency stability (1 x 10-15 at 1 s). The laser is further characterized by resolving a 2 Hz wide, ultranarrow optical clock transition in ultracold strontium.