I did mode scan of PR2-flipped half-PRC to see if it behaves as we expect.
Measured finesse was 107 +/- 5 and g-factor is 0.98997 +/- 0.00006.
g-factor is 0.9800 +/- 0.0001. (Edited by YM; see elog #8056)
Finesse tells you that we didn't get large loss from flipped PR2.
Since we have convex TM in front of BS, PRC will be more stable than this half-PRC.
Method:
1. Aligned half-PRC using input TT1 and TT2 by maximizing POP DC during lock. It was not so easy because POP DC fluctuates much at ~ 3 Hz with amplitude of ~ 30 % of the maximum value because of the beam motion (movie on elog #8039).
2. Unlocked half-PRC and took POP DC and PRC error signal data;
> /opt/rtcds/caltech/c1/scripts/general/getdata -d 1 -o /users/yuta/scripts/PRCmodescan C1:LSC-POPDC_OUT C1:LSC-REFL11_I_ERR
Ran again and again until I get sufficiently linear swing through upper/lower sidebands.
3. Ran modescan analyzing scripts (elog #8012).
Result:
Below is the plot of POP DC and PRCL error signal (REFL11_I).
By averaging 5 sets of peaks around TEM00;
Time between TEM00 and sideband 0.0347989 pm 0.00292257322372 sec
Calibration factor is 317.995971137 pm 26.7067783894 MHz/sec
FSR is 34.5383016129 MHz
FWHM is 0.323979022488 pm 0.0145486106353 MHz
TMS is 1.55827297374 pm 0.00439737672808 MHz
Finesse is 106.606598624 pm 4.78727876459
Cavity g-factor is 0.989971692098 pm 5.65040851566e-05
Cavity g-factor is 0.980043951156 pm 0.000111874889586
Discussion:
Measured finesse is similar to measured PRM-PR2 cavity finesse(108 +/- 3, see elog #8012). This means loss from flipped PR2 and beam path from PR2 to TM is small.
I'm a little suspicious about measured g-factor because it is hard to tell which peak is which from the mode scan data. Since half-PRC was not aligned well, high HOMs may contribute to POP DC. Astigmatism also splits HOM peaks.
PRC 3 Hz beam motion was there for long time (see, for example, elog #6954). BS is unlikely to be the cause because we see this motion in half-PRC, too.
Also, beam spot motion was not obvious in the PRM-PR2 cavity. My hypothesis is; stack resonance at 3 Hz makes PR2/PR3 angular motion and folding by PR2/PR3 makes the beam spot motion.
Next things to do:
* PRC g-factor
- Calculate expected half-PRC g-factor with real measured curvatures, with error bar obtained from RoC error and length error (JAMIE)
- Calculate expected PRC g-factor using measured half-PRC g-factor (JAMIE)
* PRC 3 Hz beam motion
- Do we have space to put oplevs for PR2/PR3?
- Can we fix PR2/PR3 temporarily?
* PRMI
- Align incident beam, BS, REFL, AS, and MI using arms as reference
- lock PRMI
- PRC mode scan |