sheila.dwyer@LIGO.ORG - posted 21:21, Wednesday 01 April 2015 - last comment - 08:48, Thursday 02 April 2015(17624)
ASL and tidal rearrangement
Sheila, Daniel, Kiwamu
Since we moved the ALS fiber AOM from a fixed frequency drive to the IMC VCO on saturday, we have had some annoying ALS locklosses. This morning we thought through what we are using as a frequency reference now, and made some changes to the way that we lock and engage tidal.
- We request the arms to unlock after each lock loss, so that the tidal can bleed off, and don't ask them to lock again until the IMC is locked.
- We are no longer using the slow servo that moved the IMC VCO to beign the COMM PLL in range. This is no longer needed, and doesn't make sense anymore.
- We start with each arm running local high bandwidth tidal on each arm, (which means that they are also following the IMC VCO through the fiber)
- We first lock COMM, and turn off the local tidal on the X arm. Now the IMC length is following the X arm, and the X arm VCO stays in range because the fiber light also follows the IMC VCO
- Then we lock DIFF, as soon as DIFF starts to ramp on the gain, we turn off the Y arm local tidal. The local tidal is needed to bring the DIFF PLL in range, but would cause a verry slow instability in the system if it stays on when DIFF is locked.
- The common tidal servo now only has to compensate for the slow motion that really is tidal, (not micorseism). We have been leaving it off until we get off of ALS, and this seems fine for now, although we might need to engage it earlier with a very low bandwidth if we want to keep ALS locked for long time periods.
So the final configuration is that the Y arm is the reference, the X arm length follows the Y arm length, and the IMC follows the X arm. This is working much better I think, we haven't had the frustrating locklosses this afternoon and evening.
For a historical perspective: Using the IMC VCO to drive the fiber AOM was our original configuration. During the HIFO tests we switched to a fixed frequency oscillator for the fiber AOM to de couple the arm locking from the IMC. As a consequence of the many improvements in the seismic isolation systems we became more sensitive to wind induced motions at very low frequencies. In turn this required more range in the VCOs used to lock the green light to the arm cavities. So much so that we frequently run out during elevated wind activity. Most of this motion turns out to be common. So, by locking the IMC to an arm cavity and by letting the fiber AOM follow the IMC we effectively reduce the required range for the end station VCOs. The IMC VCO has four times the range because the reference cavity AOM is double passed and because of the different wavelength. The penalty we pay is that the IMC now needs to be locked stably for enabling green arm cavity locking. However, during the current commissioning period and during observational runs this is not much of a disadvantage.