Products mu beta or the first order hyperpolarisabilities beta of both isomers of dithienyl ethene derivatives by their ground state dipole moments mu have been determined experimentally by the EFISHG technique, then calculated by a semi-empirical method. Both calculations and measurements put into evidence a large increase in mu beta in going from the open isomer (OF) to the largely pi conjugated closed isomer (CF). This photoinduced change in second order NLO coefficient is used to modulate the Second Harmonic Generation (SHG) signal generated by a PMMA film doped with one diarylethene under alternate irradiation with UV (325 nm) and visible (514 nm) light.