I made the first trial of locking a Power-recycled single arm.

 This is NOT a work in the main stream,

but it gives us some prospects towards the full lock and perhaps some useful thoughts.

      Optical Configuration         

• Y arm and PRM aligned. They become a three-mirror coupled optical cavity
  • Power Recycling Cavity (PRC) is kept at anti-resonance for the carrier when the arm length is off from the resonance point
  • Hence bringing the arm length to the resonance point lets the carrier resonate in the coupled cavity
  • BS behaves as a loss term in PRC and hence results a low recycling gain
• Everything else are misaligned, including ITMX, ETMX, SRM and BS
  • Therefore there are neither Michelson, X arm nor Signal Recycling Cavity (SRC)

   Lock Acquisition Steps    

1. Misalign PRM such that there is only Y arm flashing at 1064 nm
2. Do ALS and bring the arm length to the resonance point
3. Record the beat-note frequency such that we can go back to this resonance point later
4. Displace the arm length by 13 nm, corresponding to a frequency shift of 200 kHz in the green beat note
5. Restore the alignment of PRM.
6. Lock PRC to the carrier anti-resonance condition using REFL33I. At this point the arm doesn't disturb the lock because it is off from the resonance anyway
7. Reduce the displacement in the arm and bring it back to the resonance

     Actual Time Series     

Below is a plot of the actual lock acquisition sequence in time series.

• The data starts from the time when the arm length was kept at the resonance point by the  ALS servo.
  • At this point PRM was still misaligned.
• At 120 sec, the arm length started to be displaced off from the resonance point.
• At 250 sec, the alignment of PRM was restored and the normalized DC reflection went to 1.
  • Error signals of PRC showed up in both REFL33 and POOY11
• At 260 sec, PRC was locked to the carrier anti-resonance point using the REFL33_I signal.
  • Both REFL33 and POY11 became quiet.
  • REFLDC started staying at 1, because the carrier doesn't enter to the cavities and directly goes back to the REFL port.
• At 300 sec, the arm length started to be brought to the resonance point.
• At 400 sec, the arm length got back to the resonance point.
  • The intracavity power went to 3.5 or so
  • REFLDC went down a bit because some part of the light started entering in the cavities
  • REFL33 became noisier possibly because the Y arm length error signal leaked to it.