Ultra-low noise microwave generation using femtosecond lasers and applications

We report on our fiber-based mode-locked laser approach to the generation of ultra-low noise microwave signals, and its use for frequency metrology. We make use of two erbium fiber mode-locked lasers phase locked to a common ultra stable laser source to generate an 11.55 GHz signal with a relative phase noise of −111 dBc/Hz at 1 Hz from the carrier. The corresponding residual frequency instability is below 2.3×10⁻¹⁶ at 1s and about 4×10⁻¹⁹ at 6.5×10⁴ s [1]. For a single fiber system, we demonstrate absolute stability about 3×10⁻¹⁵ between 1 s and 10 s when locked to an ultra-stable optical cavity and after dissemination over amplitude-modulation fiber link for a few hundred meters. The microwave signal from the fiber system is subsequently used to perform Ramsey spectroscopy in a state-of-the-art Cesium fountain [2]. The resulting clock system exhibits a stability of 3.5×10⁻¹⁴ τ- limited by projection noise. These experiments demonstrate the potentiality of ultra-high-performance, robust and long term operations fiber-based frequency combs for operational-level tasks in time and frequency metrology using microwave signals.