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. The relation between the delaytime laser pulse and the population of each energy level are investigated by the numerical and analytical solutions. The characteristic and regularity of population modulated by laser fields phase are obtained as well. Utilizing this mechanism, one can get the quantum switch,of which the modulation period can get to femtosecond order.