Résultat de la recherche
All-optical microwave generation using frequency combs
Optical frequency combs from modelocked femtosecond lasers provide a direct, phase-coherent link between the radio frequency and optical domains. This thesis describes the development and realization of an ultra-stable microwave oscillator based on optical-to-microwave frequency transfer using two different optical frequency comb technologies.
An optical reference oscillator is realized based on Pound-Drever-Hall stabilization of a laser to a high finesse Fabry-Perot cavity. A multilayer vacuum enclosure system was designed, built, assembled and characterized, with the aim of achieving a high filtering of any external thermal perturbation that might affect the cavity, leading to a thermal time constant of more than six days.
In this thesis the noise properties of a commercial Er:fiber optical frequency comb and of an Er:Yb:glass laser oscillator frequency comb, referred to as ERGO and developed in ETH Zurich, have been investigated. The benefit of the ERGO diode-pumped solid-state laser (DPSSL) in terms of low noise operation is demonstrated by a 20-fold improvement in the residual fractional frequency stability of the stabilized CEO beat as compared to the Er:fiber comb. The measured CEO integrated phase noise of only 0.72 rad rms is one of the lowest values reported for a self-referenced comb the 1.5 µm spectral region. Furthermore, this thesis gives new insights into the noise in a fiber frequency comb not only from the experimental point of view but also by providing a theoretical model addressing the cross-influence between the CEO and repetition rate stabilization loops.
Finally, generation of microwave signals via optical-to-microwave frequency division using both fiber and DPSSL frequency comb technologies is discussed. The evaluation of the generated microwave signals was made with a transportable ultra-stable cryogenically-cooled sapphire oscillator. The microwave signals generated with both combs showed a similar fractional frequency stability of 5×10 15 at 1 s and several possible present limitations have been investigated. However, the use of the ERGO comb proved to be beneficial compared to the Er:fiber comb in terms of close-to-carrier phase noise of the microwave signal, resulting in a 20 dB improvement in a large Fourier frequency range (100 Hz – 100 kHz) offset from a 10 GHz carrier.