Compact ultra-stable laser

We present an ultra-stable laser based on a compact Fabry-Perot cavity with a thermal noise limit expected at 2 × 10
¹⁵
in trems of fractional frequency stability. The 25 mm long cavity is housed in a cubic vacuum chamber with a volume of about 2.2 L. The measured phase noise is 3 dBrad
²
/Hz at 1 Hz with a simple optical set-up at 1542 nm.