A Question about PDH locking a cavity

[kelly0303]

Hello! I am building a bow-tie cavity and I am trying to lock it using the PDH method. From reading some stuff online I understand the concept but the figures they show in the tutorials online are different from what I see on my oscilloscope. I am attaching below two pictures (the second one is the zoomed-in version of the first, in the central region). The blue is the light measured in transmission, while the pink is the PDH signal. While the overall shape looks like what I see online, there is a lot of noise on top and also clear oscillations. I am not sure what to do. The lock should happen when the pink curve crosses zero, but because of the oscillations, this happens many times. How would it even know to what zero to lock to? What should I do? Thank you!

 

[tech99]

Is the modulation index set to a suitable level?
Do you have an isolator after the laser?

 

[kelly0303]

Is the modulation index set to a suitable level?
Do you have an isolator after the laser?

The index is such that the sidebands are about half the main component.

I have a bow tie cavity, so the light reflected from the cavity is at an angle and doesn't go back towards the laser, so the isolator would probably not help, but the laser supplier claims the laser has an isolator built in already.