Atom coherence and quantum interference have led to many interesting optical phenomena such as coherent population trapping (CPT), electromagnetic-induced transparency, laser without inversion and control of spontaneous emission.

The atomtic coherence effects has one of the important forward subjects in laser physics and quantum optics, recent studies has shown that the quantum coherence and interference has led to a lot of new effects such as electromag-netically induced transparency, coherent population trapping, switching from subluminal propagation to superluminal propagation, etc.

This is a coherent population trapping phenomenon which cannot be accounted for with the convential models.

It is realized that coherent population trapping (CPT) and electromagnetically induced transparency (EIT) are modulated by coherent phase modulation in a V type atomic system excited by delay time laser pulses; the modulation period can reach femto second order if the proper energy levels are selected.