Thermal-noise-limited ultra-stable laser operated at 698 nm

Abstract

Ultra-stable lasers are indispensable in high precision measurements. Here we demonstrate an ultra-stable laser system that reaches the thermal noise limit of the optical cavity for the 698 nm clock transition laser of strontium atomic optical clock. This is achieved by locking the frequency of the external-cavity diode laser (ECDL) to a full ULE optical cavity with a length of 10 cm. By suppressing noise such as temperature disturbance, vibration, residual amplitude modulation (RAM), and fiber phase noise, the measured laser linewidth is less than 0.9 Hz. After subtracting the linear drift of 60 mHz/s, the frequency instability of the beat signal reaches 1.2 × 10-15 at 1 s averaging time, which is close to the thermal noise limit of the optical cavity.