The goal of the activity is to use the arm cavities as diagnostic FP to measure HOMs of the sidebands which resonate into de CITF.
We locked the DRMI with a positive CARM SET (3000 Hz) and made a scan of the CARM SET toward positive values. See pic1.
We used the information on the position of the SB in the arm cavity spectrum (pic 2, 3) and the spacing of HOM to identify SB HOM during the arm scan with locked CITF. (pic 4)
The expected position of the SB HOM agrees well with the measurement. We could double check the identified HOM by looking at the shape of the transmitted beam on B7 Cam. A video of the B7Cam during the scan can be seen here. (link)
In particular we focused on the HOM2 which can be used to estimate the mismatching of the SBs. As can be seen from the video, for the four SB HOM2 the brightest peak is a mismatching/astigmatic mode (pic 5). (Note that there is a splitting of the resonace frequency of HOM of the same order due to astigmatism of the cavity),
In order to assess the mismatch we cannot measure directly the height of SB fundamental modes, as CITF unlocks when they become resonant in the arms.
We can instead compare the power of the SB mode of order 2 (P_HOM2_CITF) measured on B7/B8 PD during the CITF CARM set scan and the height of the SB TEM00 measured on B7/B8 PD during the simple arm scan (P_TEM00_SC)
We have the following relations:
P_HOM2_CITF = 1/2 * P_in * G_PR * T_ARM * MM
P_TEM00_SC = 1/2 * P_in * T_PR * T_ARM
Where P_in is the input power on the BS, G_PR is the SB recycling gain, T_PR is the transmission of PR, T_arm is the arm cavity transmission and MM is the mismatching.
The mismatch can be then computed as MM = (P_HOM2_CITF/P_TEM00_SC)*T_PR/G_PR
We will do as slower scan around HOM2 to have a more precise estimation of the peak height.
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Other notes
1) We noted that locking DRMI with the opposite offset wrt the usual one brings it to a lower level of 12MHz_mag (roughly 0.08 instead of 0.1). The power could be fully recovered by adding an offset on the PR angular TY angular loop.
2) In order to have a more precise estimation of the peak position during the scan we made available a new CARM SET channel sampled at 10kHz (LSC_CARM_SET_FS).