Matteo and Yuhang

The filter cavity detuning was found to be changing when we lock filter cavity with GR (elog2642, elog2640, elog2636). The reason is that the correction signal sent to main laser sensed by GR is not exactly IR required.

However, we found out if we change main laser frequency manually by large amount (500MHz), the detuning almost doesn't change before and after main laser frequency change. This is already observed in elog2636. In elog2636, the first unlock didn't make correction signal change, which tells us that main laser frequency was changed by ~40MHz and the re-lock is fast enough that cavity length didn't change. Therefore, in the re-lock, we saw the detuning changes continously. However, the second unlock of elog2636 had correction signal going back almost to zero. We can see that around this un-lock, there were some oscillations, which makes the re-lock have time to cross ~40 FSR and arrives at the position where correction signal can be close to zero. In the first un-lock, cavity is locked to the same FSR, but different FSR is locked during the second un-lock, which makes the detuning go back to zero. This is because the same frequency change is required for GR and IR when we go to other resonances.

Instead of sending correction signal below 0.1Hz to end mirror, we can send it to main laser temperature. This makes the acquire of resonance easier and faster. The stability of lock is also more stable in this case, comparing to send all signal to main laser. In addition, we know the filter cavity length should change less than few tens of micro-meters per day. But the main laser frequency is expected to have drift more than 100MHz per day (from PLL observation). Even from the laser manual, the stability of laser frequency DC value is described to be changed by less than 1MHz per minute with a constant room temperature. So it seems to be reasonable to take filter cavity length as a reference below 0.1Hz.

With this locking strategy, I monitored filter cavity detuning stability by about 20hours on 23rd July. A screen shot of this monitor is attached in this elog without calibration. Upper: correction signal to main laser temperature. Lower: detuning. In the first 15 hours, the detuning stability is about 5Hz. But similar to elog2636, there is detuning change happend in the last five hours, which seems to come from some other detuning change mechanisms. One example is that the AOM which is currently used will have frequency drift of 220Hz after one year.

Notice: AA and pointing loops are always closed.