I'm currently struggling with a problem in North ISS which I think needs to be documented here. Here's the synopsis:

The problem:

• After running for the weekend with high gains and gain peaking at ~45 kHz, I found this week that the North ISS was overloading.
• The power level reaching the cavities change over the course of the day by +- 20-30% usually (still a mystery to me), so I assumed this could just be because of increased laser power saturating the SR560.
• But what I have found is that except for some random instances, I can't keep the gain high enough in this ISS path.
• At the start, I thought this problem is at least reset-able by unlocking the North FSS and locking the cavity back. But later, I've found that it is not working either now.
• The defining feature is, as I slowly increase the gain (giving ample time for the loop to settle), at the first point where I see any sign of overload, it happens at a rate of ~300 mHz, that's once every 3  seconds!
• I checked at the photodiode output at this gain setting and I see the loop gets unstable and for about half a second every 3 seconds or so.
• On increasing the gain further, this overload just becomes a constant condition.
• My first guess is that somehow the lower unity gain frequency is becoming unstable. But the mystery is:
  • Why did this not happen before?
  • Why is this not seen in the South Path at all? It is healthy as ever.
  • What really changed during the weekend?
  • And why once in a while, it was actually working?
• The only difference in the SR560 settings for the two loops is that the South path has 'invert' ON while North path does not. At first, I was perplexed by this too. South needed 'invert' to be stable and the North did not. Maybe this is the reason?
• I have ruled out any failure in SR560 by switching it with another one and seeing the exact same phenomenon.
• Remaining suspects are a damaged BNC cable/connector, damaged photodiode or EOAM (I wish not). However, some bad control loop configuration is my primary suspect.

I would like it if anyone has any comments on any of the points above or suggestions to tackle this problem. I can make some measurements and post them on requests.