We tried to fit with various function the LC retardation as a function of applied voltage.

Up to now we were using R = a + b/(1+(V/c)**d)**e (1pole in attached figure) but it seemed not optimal as seen from the residual shape.

We now tried the 2 poles function : R = a + b/(1+(V/c)**d)**e + b/(1+(V/f)**g)**h and 2 different poles :  R = a + b/(1+(V/c)**d)**e + i/(1+(V/f)**g)**h.

These 2 functions improved a lot both residual rms and peak-to-peak values. We are now able to reconstruct the LC retardance better than for polarimetry.

New step is to add the temperature dependence.