Ultracompact reference ultralow expansion glass cavity.

We present the first experimental characterization of our ultracompact, ultrastable laser. The heart of the apparatus is an original Fabry-Perot cavity with 25 mm length and pyramidal geometry, equipped with highly reflective crystalline coatings. The cavity, along with its vacuum chamber and optical setup, fits inside a 30 L volume. We have measured the cavity's thermal and vibration sensitivities and present our first estimation of the cavity fractional frequency instability at σy(1  s)=7.5×10-15.