Travis, Jenne, Terra, Kiwamu,

A brief report on a rare fault mode associated with FSS and ALS.

At one point tonight, we noticed that both end station green lasers did not lock to the arms. It turned out that both the end stations lost the beatnote against the PSL light. Looking at various channels, we found that it was actually the reference cavity which locked to a next fringe, resulting in a large shift in the NPRO frequency. This corresponded to a temperate crystal feedback of -0.25 K (compare this to nominal of 0 K). A strange thing is that the temperature sweep for the FSS auto-locker had been set to [-0.1 K 0.1 K] which means that theoretically the chance of hopping to the next fringe is very small. Anyhow, relocking the FSS back to the nominal fringe by disabling/enabling the auto-locker fixed the issue. Both green lasers re-acquired the PLL without an issue.