  We report on a theoretical study and experimental characterization of
coherent population trapping (CPT) resonances in buffer gas-filled vapor cells
with push-pull optical pumping (PPOP) on Cs D1 line. We point out that the
push-pull interaction scheme is identical to the so-called lin per lin
polarization scheme. Expressions of the relevant dark states, as well as of
absorption, are reported. The experimental setup is based on the combination of
a distributed feedback (DFB) diode laser, a pigtailed intensity Mach-Zehnder
electro-optic modulator (MZ EOM) for optical sidebands generation and a
Michelson-like interferometer. A microwave technique to stabilize the transfer
function operating point of the MZ EOM is implemented for proper operation. A
CPT resonance contrast as high as 78% is reported in a cm-scale cell for the
magnetic-field insensitive clock transition. The impact of the laser intensity
on the CPT clock signal key parameters (linewidth - contrast -
linewidth/contrast ratio) is reported for three different cells with various
dimensions and buffer gas contents. The potential of the PPOP technique for the
development of high-performance atomic vapor cell clocks is discussed.
