Reports 1-1 of 1 Clear search Modify search
IOO (IMC)
kenta.tanaka - 2:10 Tuesday 24 August 2021 (17989) Print this report
IMC FSR and absolute length measurement

Yuzurihara, Akutsu, YamaT, K. Tanaka

This is a quick report.

We measured the FSR and the round trip length of IMC as in klog613 .

  1.  we actuate MCe with a 0.05 Hz triangle wave using an awggui in the length direction. In this time, we monitored the excitation signal (K1:VIS-MCE_TM_ISCINF_L_EXCMON), the transmitted power (K1:IMC-CAV_TRANS_OUT), the reflected power (K1:IMC-CAV_REFL_OUT) and the error signal (K1:IMC-SERVO_OUT2_IN1)
  2. As a rough estimation, I used the cursor function of ndscope to find the relationship between frequency and time from the position of the sidebands. Since the sideband frequency and FSR are 13.78 MHz and 5.7 MHz, respectively, the sideband position was determined to be the signal located 13.78/5.72 = 2.4 FSR away from the carrier.(Fig. 1). From this, the relation of 13.78MHz/1.846s was obtained.
  3. Then, using the cursor function, the number of seconds between 00 modes was determined to be 0.771s from the transmitted light intensity signal. (Fig. 2)
  4. Therefore, the FSR and the round-trip length can be obtained as follows

FSR : 13.78 MHz / 1.846 s * 0.771 s = 5.76 MHz

Round trip Length = c / FSR = 2.9979e+8 m/s / 5.618 MHz = 52.1 m

Detailed analysis will be done later.

Images attached to this report
Comments to this report:
tomotada.akutsu - 22:00 Tuesday 24 August 2021 (18000) Print this report

YamaT, Ushiba, Akutsu

This evening, we just analyzed things using the same data left in the ndscope by K. Tanaka (last night) to see what will be the results if using different peaks by ourselves by using cursors in the ndscope. Just for our quick check, we adapted 1.848 sec for the carrier and RF sideband (13.78 MHz); we learned that the ndscope could more zoom in, and the more detailed peak searching could be done.

Then, we saw the three carrier peaks are separating like 0.771 sec, 0.808 sec, and 0.744 sec, so these should correspond to 5.749 MHz, 6.025 MHz, 5.547 MHz as free-spectral range, and also correspond to 52.147 m, 49.758 m, and 54.03 m, respectively, while the designed absolute length is 53.3 m. Hmm, so large error!

In this data, it seems swept with MCe. The other options would be

  • Sweep again with MCe but with lower speed.
  • Sweep with thermal actuator to NPRO.
  • Give up and use the method reported in 616.
kenta.tanaka - 1:59 Wednesday 25 August 2021 (18005) Print this report

Yuaurihara, K. Tanaka

We analized yesterday's measurement data and obtained the FSR = 5.62 +/- 0.03 MHz, round-trip length = 53.3 +/- 0.3 m and finesse: 461 +/- 28. FSR and the round-trip length are consistent with the design valu, previous measurement (klog615) and Akutsu-sans' estimation (klog18002). On the other hands, finesse is lower than these results.

 # What we did

  1. The time series data we have used is attached Fig. 1. Of this data, the data from 13 to 18 seconds was used.
  2. Got the interval it takes to sweep between FSR (t_FSR) = 0.7485+/- 0.016 sec from the fitting (Fig. 2).
  3. Got the interval it takes to sweep from TEM00 peak to sideband peak( t_sb00) = 0.337 +/- 0.010 sec from the time stamp from upper and lower sideband
  4. we can do the calibration from the time to the laser frequency by 13.78 MHz/(2*t_FSR+t_sb00). So FSR is calibrated to be 5.62 +/- 0.03 MHz
  5. Which correspond to the roundtrip length of 53.3 +/- 0.3 m.
  6. Got t_FWHM = 0.0016 +/- 0.0001 sec from the Lorentzian fitting of the transmitted beam peak. One example plot of the result of the fitting is attached (Fig. 3).
  7. By t_FSR/t_FWHM, we got IMC finesse to be 461 +/- 28

 

Images attached to this comment
tomotada.akutsu - 3:37 Wednesday 25 August 2021 (18007) Print this report

Thanks for the more precise analysis than ours. About the finesse estimation, your value 461 +/- 28 might be consistent with the value 458 found in wiki (but not klog-ed...) dervied from the cavity pole measurement.

And 461+/-28 would have a certain range overlapping with 540+/-70 reported in 618, so it is not so bad; so far, not necessarily saying "significantly get lowered" than the past. Or, will someone do a Baysian test :) ?

Search Help
×

Warning

×