[Fujimoto, Tanaka, Hirose, Takano]
Summary
We tried locking SRMI. After tuning the filter gain, we easily succeeded in locking it, but further investigation is necessary for good stability.
Detail
For locking DRMI, we tried the SRMI configuration. The idea is to know a hint for a good servo gain of SRCL and MICH in DRMI.
We chose the locking condition as follows:
- The Carrier is at the dark fringe in MICH.
- The f1 sidebands are transmitting MICH by 85 %
- The f1 sidebands are on resonance in SRC
- The carrier would be on anti-resonance, but before entering SRC most carriers are reflected by MICH
- Error signals are taken from POP17
- SRCL: POP17I
- MICH: POP17Q
- No guarantee that these two are well degenerated
The conditions for the light fields are different from those in DRMI, but the signal conditions are similar. We expect that, from the servo gain for locking SRMI in these conditions, we would have a good estimate of them for locking DRMI, especially for locking SRCL and MICH in DRMI.
Once we finished the initial alignment of PRMI with ADS, we misaligned PRM and aligned SRM. First we tried the manual locking by tuning the servo gain of MICH and SRCL. After several trials and errors, we found good numbers:
- SRCL2 gain: 0.5
- MICH2 gain: -40
We could also enable the integrator on each servo. The measured open-loop transfer functions are shown in Fig.1 and Fig. 2. While locking SRMI, we saw many glitches and they disturbed locking (Fig. 3), resulting in lock loss occasionally. We first suspected the reflection back from PRM, so we misaligned it more by requesting MISALIGNED_BF state, but the situation didn't improve at all.
After succeeding in locking, we implemented the guardian state and tested it. However, every time the guardian failed to lock it when the servos were engaged or the integrators were engaged. After other trial and errors, it turned out that the order of engaging servo gains and the integrators was important: without the integrators, we first need to engage the MICH servo first, then SRCL servo. Also, we found that we couldn't engage the integrator in SRCL servo cannot be enabled without breaking locking, nor increase the SRCL servo gain either. It could be that the demodulation phase is not optimised for SRMI and now POP17I and POP17Q are mixed, which makes the control loops unstable. Further investigation is necessary for stable locking.