Laser frequency characterization for new generation atomic clocks

Precise timekeeping plays a key role not only in science but also in many technological sectors, including aerospace, global navigation satellite systems, defense, and telecommunication. Very often, only atomic-based frequency references provide the required stability and accuracy. At the same time, the clocks used in technological applications should satisfy precise specifications in terms of size, weight, and power consumption (SWaP). In this regard, vapor-cell clocks based on alkali-metal atoms pumped by a laser have recently received considerable attention, providing low SWaP joined to interesting frequency stability performances. The spectral characteristics of these lasers directly affect the standard stability performance, both in the short and in the medium-long term. Indeed, laser noise is transferred to the atoms, through several physical phenomena like light shift, non-homogeneities inside the clock cell, and so on. It is then of paramount importance to investigate the laser spectral behavior, also because laser noise measurements are rarely found in the literature. This thesis presents the frequency-noise characterization of the laser diode currently employed in a high-performing Rb vapor-cell frequency standard developed at INRIM. The latter employs the pulsed optically pumping scheme (POP) and nowadays represents the state of the art for vapor-cell clocks. The laser noise is measured in the range from 1 Hz to 100 kHz, which is of interest for the atomic clock operation. The assessment of the frequency noise level will allow completing the clock stability budget. The influence of the current driver on the laser-diode frequency noise is also investigated. Moreover, the behavior of the laser stabilization system is analyzed by comparing two independent setups. An upper limit for the frequency stability is measured to be 30 kHz at 7 x 10^4 s, which corresponds to a frequency instability contribution of 10^-15 for the POP clock.