Optical cavity for ultra-narrow linewidth laser system

The precise optical resonators represent a key tool for ultra-narrow linewidth laser frequency stabilisation. They can serve as references for lasers of optical clock systems, optical frequency combs locking, dissemination of precise time and frequencies over optical links, or laser interferometry. The critical properties of these resonators are determined by their design, used materials and environmental conditions where they are operated. This contribution describes the design and performance of the “kHz” linewidth optical cavity intended for spectral-narrowing of a 1541 nm fibre laser. The cylindrical-shaped Fabry-Perot resonator with optical mirrors and a spacer part made of ceramics with ultra-low thermal expansion coefficient is placed inside the multi-shielded vacuum and thermostated chamber. The optical setup employs the Pound-Drewer-Hall locking technique for laser locking, and the resonator’s performance is measured by the ring-down measurement.