Graduate Unexpected irregular reflection signal from a high-finesse cavity

[teen_physicist]

I am observing an irregular, aperiodic noise pattern in the reflection signal of a high-finesse optical cavity (finesse ≈ 20,000). The cavity is normally operated using a standard Pound–Drever–Hall (PDH) locking configuration, where an EOM provides phase modulation.

The signals shown in the attached figures were recorded with the modulation turned off. Under these conditions, when scanning the laser frequency across a cavity resonance, I expected to observe a simple reflection dip. Instead, the reflected signal shows a noisy, irregular structure that does not resemble a clean resonance feature.

I have briefly surveyed the literature and found discussions of possible contributors such as:

• coupling to higher-order transverse modes,
• free-running frequency jitter of the laser,
• coherence or interference effects involving multiple cavity modes.

However, the example signals presented in those works look qualitatively different from what I observe here, so it is unclear whether the same mechanisms are responsible.


Has anyone encountered similar behavior in a high-finesse cavity when the PDH modulation is disabled?
What physical mechanisms could lead to such an irregular reflection signal, and what experimental steps would you recommend to diagnose or mitigate this noise?