During the cool down phase of the Ring Heater (see logentry 35687)a modulation of the West Arm power has been observed, which can be attributed to the etalon effect.

Indeed, due to the temperature variation of the mirror, which cools down together with the Ring Heater itself, a variation of the OPL in the bulk is induced, which in turn produces a change of the mirror reflectivity and, consequently a variation of the cavity gain and of the circulating power.

To perform a complete fringe, a ΔOPL of λ/2 is needed. Since the ΔOPL = integral{(dn/dT*ΔT)dx}, where the integral is carried over the mirror thickness, a complete fringe is spanned for a temperature variation of:

ΔT =  λ/2/(dn/dT * th) ~ 0.3 K

where dn/dT = 8.7e-6 K^-1 has been used, and t = 0.2 m is the mirror thickness.

In figure 1, subplot 1 the data corresponding to the power  transmitted by the West Arm has been fitted with a sinusoidal function whose frequency decays exponentially.

From the fit, a time constant of about 8 hours has been found for the mirror cool down process.

In the figure, 14 fringes are visible, which correspond to an overall temperature variation of 4.2 K.

Over the same period the temperature variation measured by the Ring heater thermometer is about 5 degrees. This is shown in the second subplot of figure 1.

The cavity Finesse variation due to the etalon is ΔF/F~2.5%, which is compatible with what we expect from the calculations.