  We study resonance contrast by a two-step pulse observation method to enhance
the frequency stability of coherent population trapping (CPT) atomic clocks.
The proposed method is a two-step Raman--Ramsey scheme with low intensity
during resonance observation and high intensity after the observation. This
method reduces the frequency variation in the light intensity and maintains a
high signal-to-noise ratio. The resonance characteristics were calculated by
density matrix analysis of a $\Lambda$-type three-level system that was modeled
on the $^{133}$Cs D1 line, and the characteristics were also measured using a
vertical-cavity surface-emitting laser and a Cs vapor cell.
