In ordinary time delayed four wave mixing with incoherent light

the forward

backward pump and probe beams are from one source. In phase conjugation configuration

signal

I

s

(τ) is from two dynamic gratings formed by interference of probe beam with forward or backward pump beams respectively. The complicity of the relation betwean

I

s

(τ) and dephasing and depopulation times makes fitting error large.We propose in this paper a two source method

where backward pump beam is from a source independent of the one for forward pump and probe beams. In this case only one dynamic grating may be formed in the medium.