Studies on Coherent Population Trapping Resonance in Rubidium for Application in Atomic Clocks and Timekeeping Methodology for Satellite Navigation

Abstract

Atomic clocks with superior frequency accuracy and stability are essential for timekeeping applications. The application of clocks in satellite navigation demands the accurate predictability of clock behaviour and their utility in space necessitates miniaturization. Coherent population trapping (CPT) is a promising technique for developing miniaturized atomic clocks. Two main objectives are pursued in this thesis. The first objective is to investigate the characteristics of CPT resonance and derive optimum operating parameters as well as system configuration for improved frequency stability of CPT based atomic clock. The second one is the real-time characterization of on-board clock behaviour and development of optimum timekeeping methodology to ensure uninterrupted navigation service to the user. An experimental investigation is carried out to study and optimize CPT resonance characteristics (quality figure and frequency shift) with respect to critical parameters such as dimension of Rubidium vapor cell, laser intensity, cell temperature, Radio Frequency (RF) power, buffer gas species and pressure inside the vapor cell. A theoretical model based on four level atomic system is developed to understand the laser-atom dynamics that governs the CPT phenomena in alkali atoms. A new empirical parameter is introduced in the model to account for the influence of cell size on resonance characteristics. Designed and implemented a methodology to solve the differential equation governing four level atomic system through the utilization of Runge-Kutta 4th order numerical integration method. Theoretical computation of quality figure is carried out for cells with different dimensions, buffer gas pressures, temperatures and compared with the experimental results. The outcome of this study helped in deriving optimum operating parameters and system configuration in order to achieve enhanced frequency stability of CPT based atomic clocks. The performance of atomic clocks used in navigation satellites plays a crucial role in providing the desired position accuracy for navigation users. Continuous monitoring and real time characterization of these clocks are necessary to provide uninterrupted navigation service to the users. In this regard, designed and developed a scheme for characterization and continuous monitoring of on-board clocks using one-way carrier phase measurements to sensitively detect the on-board clock anomalies. Devised and developed a single algorithm that detects anomalies present in the clock data. The characterization scheme developed in this study enables the timely generation and up-linking of clock correction parameters to ensure the user position accuracy within the specification. i In each navigation satellite, satellite clock offset is allowed to accumulate only up to the pre-allocated broadcast limit beyond which the range measurements are unreliable owing to significant error in user position. Hence, the clock offset needs to be maintained within the given broadcast limit, which is referred to as satellite timekeeping. A timekeeping methodology is established to maintain the satellite time optimally within the allocated broadcast limit. New mathematical model is derived to compute optimal frequency offset correction that provides maximum time interval between the two successive corrections to maintain the satellite clock offset always within the broadcast limit. The developed methodology facilitates the seamless navigation service to the user.