  The ac Stark shift (or light shift) of the 6$^{2}$S$_{1/2}$ (F $=$ 3
$\leftrightarrow $ 4) transition in $^{133}$Cs, as observed through coherent
population trapping under pulsed excitation, is measured using a $^{133}$Cs gas
cell and the $D_1$-line vertical-cavity surface-emitting laser. This light
shift can be calculated using density-matrix analysis. We derive an expression
for this shift as a function of light intensity, showing that it varies
linearly with respect to light intensity only with intensities higher than
1.0~mW/cm$^2$. For pulsed excitation of high laser intensity, the variation in
light shift is 20 times lower than that when using a continuous wave. The
differences between the results of theory and experiment are discussed, taking
into account the difference in conditions assumed; the results from theoretical
analysis, taking the attenuation of the first-order sideband into account,
approximately agree with the experimental results. The light shift is reduced
by shortening the observation times.
