Experimental and theoretical comparison of different optical excitation schemes for a compact coherent population trapping Rb vapor clock

We have investigated, theoretically as well as experimentally, the relative merits and demerits of using three different optical configurations for a compact coherent population trapping (CPT) vapor clock using ⁸⁷Rb. These correspond to the following choices of polarizations for the two Raman beams: lin ∥ lin, (σ, σ),, and push-pull optical pumping (PPOP), applied on the D1 manifold. We have used a multi-level atomic model to study the dependence of the CPT spectrum on axial as well as transverse magnetic fields for these three schemes. Corresponding experimental studies have been performed using a laboratory scale CPT clock employing a two cm long, isotopically pure rubidium cell, loaded with a buffer gas. We observed a CPT contrast close to 20% with a sub-kilohertz linewidth by adopting the PPOP scheme. We discuss the strengths and weaknesses of each of the three optical excitation schemes, and present frequency-stability measurement data for the prototype clock.