today for the first time, we were able to make a long term lock of all the elements (Laser on FP cavity and FP-cavity on RF reference) during 20 minutes @50kW without any single loss of lock.

the success of this long term lock was coming from the possibility to drive the Smaract motor in piezo-scan mode and to apply a heating on one vessel of the FP-cavity to let its frequency slowly drift always in the same direction.
then, the FP-cavity motor can follow this drift without any loss of the lock.

even when we lost the lock, it was quite easy to get it back because of the frequency drift, always in the same direction due to the FP-cavity heating with the heating cable.

a good strategy to make this double lock :
1) adjust the laser frequency to the RF reference frequency in using the Smaract motor in "closed loop" mode until reaching a beating < 10Hz.
2) search for FP-cavity resonance in using the ISP motor of the FP-cavity
3) once the laser is locked on the FP-cavity, the FP-cavity length changes => work on the FP-cavity motor to follow the drift until one reachs a thermal equilibrium.
4) once the thermal equilibrium is reached, one can switch on the heating of the heating cable to continue the slow "thermal" drift in the same direction 5V of total voltage should be enough for 1h of work.
5) in the same time, one need to cancel the frequency offset between the RF reference and the FP-cavity using the FP-cavity motor and one need to follow the laser cavity fluctuations using the Smaract motor in "closed loop" mode.
6) once the FP-cavity is at the same length than the RF-reference, one can close the second loop (when both PZT are in the middle of their range).
7) at that point, one need to swap the Smaract motor control to "piezo scan" mode => one follows only the laser fluctuations ~ 1µm peak-peak
8) one always follows the FP-cavity drift using the IcePap controller on the FP-cavity motor, 10 steps by 10 steps => it should work during 1h.
9) when one reachs the thermal equilibrium again, one can add 5V to the heating cable voltage.