Realization and characterization of optical frequency standards

During the Ph.D. Course I worked on the realization and the characterization of an ytterbium optical frequency standard. Since year 2000, it is possible using optical frequency comb to directly and reliably scale a frequency measurement in the optical domain to a measurement in the microwave domain. This possibility allows the realization of high accuracy and high stability optical frequency standards, whose atomic quality factors are several orders of magnitude higher than the best microwave ones. Among others, the alkaline earth atoms are very promising and, once trapped in an optical lattice, are capable of a short term stability approaching 10−15 at 1 s. A ytterbium optical clock is currently being developed in the laboratories of the Optics Division of Istituto Nazionale di Ricerca Metrologica (INRIM) The experiment aims to cool and trap ytterbium atoms in a two stage magneto-optical trap (MOT) (at 399 nm and 556 nm) and to probe them in an optical lattice with a ultrastable laser at 578 nm. This thesis presents the realization of the required laser sources, the stabilization of the clock laser, the development of the cooling and trapping stages and the design of a new experimental setup. The blue and green radiations for the two-stage MOT at 399 nm and 556 nm are obtained by second harmonic generation in non-linear crystals. The yellow clock laser at 578 nm is generated by sum of frequency in non-linear crystal. The clock laser is stabilized with the Pound-Drever-Hall technique on a high-finesse Fabry-Pérot cavity. The temperature stabilization of the cavity is implemented with a novel Active Disturbance Rejection Control scheme. The frequency noise of the laser is characterized with a stability 3 × 10−15 at 1 s. Atoms are trapped in the blue magneto-optical trap at 399 nm and transferred in the green trap at 556 nm. A new experimental setup is designed, studying the vacuum chamber, the MOT coils and the atomic source. I have been guest researcher at National Institute of Standards and Technology (NIST) for six months in 2011. I will describe development of NIST ytterbium optical clocks during my visit.