As a signature of thermal effect inside the PMC a scan of the PMC length at low speed (~ 1 FSR at 0.5 and 2 Hz) gives rise to an asymetrical PDH and transmission peak.

Some mirror contamination and the associated intra-cavity absorption and the subsequent heating explain this behavior:

case 1: the PMC resonance is reached while enlarging the PMC length

Enlarging the PMC length using the piezo makes the resonnance closer and closer, which raises the intra-cavity power. The absorbed power increases, so the temperature on the coating, which in turn elongates and reduces the PMC length. This effect hence beats the enlarging due to the piezo. As a result it enlarges the PMC PDH and PMC TRA signals (versus time) wrt to the case where no thermal effect is involved, since the equivalent elongation has been reduced for a cst time slot

case 2: the PMC resonance is reached while shortening the PMC length

Shortening the PMC length using the piezo makes the resonnance closer and closer, which raises the intra-cavity power. The absorbed power increase, so the temperature on the coating which in turn elongate and further reduce the PMC length. This effect hence adds to the enlarging due to the piezo. As a result it sharpens the PMC PDH and PMC TRA signals (versus time) wrt to the case where no thermal effect is involved

plot1 the scan is made at 0.5 Hz

plot2 the scan is made at 2.0 Hz

The effect is more pronounced at low speed because of a larger thermal load inside the cavity.

PSL_PMC_PZT varies from 1.85 V to 2.05 V on a sine waveform, (next time it will be better to use a sawtooth)

# This can be used to measure the amount of pathological elongation and the associated amout of heat and absorption level. But it still requires some simulation

# This can be used to check the evolution of the intra cavity-contamination assuming a weekly scan in a constant configuration. Don't know yet the sensitivity of such a method