  In a Lambda-type system employing a two-photon pump field, a four-wave mixing
field can be generated simultaneously and hence a closed-loop system forms. We
study theoretically on the effect of the relative phase between the two
incident fields on the generated four-wave mixing field and the
electromagnetically induced transparency. It is found that the phase of the
generated four-wave mixing field is a sum of the incident relative phase and a
fixed phase which is irrelative to the incident relative phase. Hence the total
phase of the closed-loop system is independent of the incident relative phase.
As a result, the incident relative phase has no effect on electromagnetically
induced transparency, which is different from the case of a Lambda-type loop
system closed by a third incident field.
