Influence of Laser Noise on the Optically Pumped, Atomic-beam Clock

The optically pumped atomic-beam clock offers the potential for orders-of-magnitude improvement over conventional beam clocks. In part, this improvement stems from the use of diode lasers to efficiently prepare the atoms prior to entering the Ramsey cavity region and then to efficiently probe the atoms after they have passed through the cavity. However, while the diode lasers typically used in these beam clocks are single-mode devices, the quantum-noise associated with the single-mode is ofen non-negligible. Here, we describe our efforts to construct a realistic computer model of the clock, taking into account the multilevel nature of the atom along with the pump and probe lasers’ amplitude and frequency fluctuations. Our goal is to develop a numerical means for generating the clock signal’s time series, and in this way to isolate those laser-related processes that may play an important role in the clock’s performance. INTRODUCTION Over the past several years, there has been considerable interest in employing single-mode diode lasers in various types of atomic clocks [l]. In the gas-cell frequency standard, efforts focus on replacing the lamp of the conventional clock with a diode laser. Not only does this replacement provide a greater signal-tonoise ratio [2], but additionally an ability to construct all-optical devices, thereby eliminating the microwave cavity from the gas-cell clock and the concomitant microwave power dependence of the clock’s frequency [3]. In the case of the cesium-beam atomic clock, optical-pumping with diode lasers and optical detection of the clock signal improves the clock’s signal-to-noise ratio and results in more symmetric microwave spectra [4]. Additionally, the use of diode lasers eliminates the conventional “A” and LLB” magnets, thereby reducing the clock’s weight and removing the potential for Majorana transitions. Though single-mode diode lasers produce highly coherent fields, relative to the Rb and Cs atoms’ energy level spacings, the phase noise of these lasers is not necessarily negligible. Since there is little mode partition noise in a single-mode diode laser (i.e., 6I/(I) [SI), the principal source of noise in the laser is associated with a random walk of the laser field’s phase, hence the reference to this type of field as a phase-diffusion-field or PDF [6]. (Note, however, that at low Fourier frequencies the laser’s phase noise is dominated by flicker noise [7].) Typically, the laser’s noise is parametrized by the laser linewidth, YL, which for conventional diode lasers is on the order of 50 MHz; for DBR diode lasers the linewidths are on the order of 2 MHz, and for external-cavity lasers the linewidths are 0.1 MHz. Given the important role that laser fluctuations play in the resonant field-atom interaction [SI, we have initiated a project to study the influence of laser noise, both the laser’s phase noise and its amplitude noise