I tried PDH lock with a servo borrowed from TAMA, but could not lock.
At that time I found the laser power was larger or smaller than usual.
I monitored output voltage from the servo, and it was about 14V which saturated PZT frequency tunig range.
I played with gain, but it did not affect the output.
Actually, I did not adjust the offset before turnig on the integrators.
So I will re-try the lock as Matteo suggested; tune the offset first, then turn on the integrators with smaller gain around the resonance.
I re-tried lock, but could not.
At first, I adjusted the offset of servo to set the monitored error signal to zero.
Also I minimize the gain of servo.
I tuned the laser frequency around the resonance, and turned on the integrator.
I tried several times with different conditions (inverted or not, and so on..), however, could not lock...
There may be some problems such as high finess, large gain, etc...
Actually, the efficiency of laser PZT is 50MHz/V, and the slope of error signal is 4.2e-7V/Hz.
Therefore, it needs to optimize the gain of the servo.