Question about PDH locking a cavity

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    Cavity Oscilloscope
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SUMMARY

The discussion centers on the challenges of locking a bow-tie cavity using the Pound-Drever-Hall (PDH) method. The user reports discrepancies between expected and observed signals on their oscilloscope, particularly regarding noise and oscillations in the PDH signal. Key factors include the modulation index, which is set to a level where sidebands are approximately half the main component, and the presence of an isolator, which may not be beneficial due to the cavity's design. The user seeks guidance on achieving a stable lock despite these issues.

PREREQUISITES
  • Understanding of Pound-Drever-Hall (PDH) locking techniques
  • Familiarity with oscilloscope signal interpretation
  • Knowledge of modulation index and its impact on cavity locking
  • Experience with optical isolators and their applications in laser setups
NEXT STEPS
  • Research techniques for reducing noise in PDH signals
  • Learn about optimizing modulation index for effective cavity locking
  • Investigate the role of optical isolators in laser systems
  • Explore troubleshooting methods for PDH locking discrepancies
USEFUL FOR

Optical engineers, physicists working with laser systems, and researchers involved in cavity locking techniques will benefit from this discussion.

kelly0303
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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!

20240410_181748.jpg

20240410_181806.jpg
 
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Is the modulation index set to a suitable level?
Do you have an isolator after the laser?
 
tech99 said:
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.
 

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