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This paper describes the application of a novel active disturbance rejection control (ADRC) to the stabilization of the temperature of two ultra-stable Fabry–Perot cavities. The cavities are 10 cm long and entirely made of ultralow- expansion glass. The control is based on a linear extended state observer that estimates and compensates the disturbance in the system in real time. The resulting control is inherently robust and easy to tune. A digital implementation of ADRC gives a temperature instability of 200 μK at one day of integration time. [ABSTRACT FROM PUBLISHER]

We stabilized the frequency of a compact planarwaveguide external cavity laser (ECL) on a Fabry-P?rot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 10?14, comparable to the stability achievable with a fiber laser (FL) locked to an FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 10?19 relative stability, and we show that its performances using the ECL or FL are comparable. Thus ECLs could serve as an excellent replacement for FLs in optical links where cost-effectiveness and robustness are important considerations. [ABSTRACT FROM PUBLISHER]