yesterday I did 2 tests to try to understand the origin of the 20Hz oscillation which is dominant in the remaining 10-20ps rms jitter between the transmitted pulses and the RF reference generator.

10ps rms jitter is equivalent to phase jitter dphi = 2*pi*f0*dt = 2mrad rms @ 33MHz or 30mrad rms @ 500MHz.

with V0 = 1Vpeak of beating signal amplitude, the equivalent rms beating voltage is dV = V0 * sin(dphi) ~ V0 * dphi = 2mV rms @ 33MHz or 30mV rms @ 500MHz

1) I did a beating between the internal photodiode of the laser with an external 33MHz oscillator (the photodiode is too slow to use higher harmonic).
the difficult part is to see the 2mV rms noise on a 2Vpp oscillating signal, so I locked the external 33MHz reference oscillator with the beating signal => see first plot.
there is no trace of 20Hz oscillation in the beating signal => the lock is too good and removed the oscillation ?

2) I did a beating between the photodiode in reflection of the FP-cavity (so the signal is not coming only from the oscillator but is going also through the Alphanov amplifier) with the 500MHz RF Ring generator.
I cannot the lock the generator anymore, so the measurement is done in open loop. I adjust the laser Frep with the motor to try to cancel the beating frequency => see 2nd plot
there is no trace of 20Hz oscillation in the beating signal => it is in contradiction with the previous post : "conclusion: the 20Hz oscillation is coming from the laser cavity" ?!?

maybe we need a more complex measurement scheme with the possibility to measure in the same time the 10-20ps rms jitter coming from the locked FP-cavity transmitted signal/500MHz Ring generator
AND the beating signal between the laser or amplifier with 500MHz local reference generator... to be done...