stefan.ballmer@LIGO.ORG - posted 02:41, Thursday 23 July 2015 - last comment - 14:53, Monday 24 August 2015(19856)
Coherent broadband noise in OMC_DC_SUM
We observed broadband coherence of OMC_DC_SUM with ASC_AS_C_LF_SUM and ASC_A_RF36_PIT. We made some numbers and plots, using the 64kHz version of the channels.
First the measurements we made on OCXO oscillator:
- ASC_AS_C sees a RIN of about 5e-7/rtHz above 100Hz (either from H1:ASC-AS_C_SUM_OUT_DQ or from H1:IOP-ASC0_MADC6_TP_CH11). The same is true for its segment 1.
- The calculated shot noise RIN at 20mA (quantum efficiency 0.87) detected is 4.0e-9/rtHz.
- The 4.0e-9/rtHz agrees with DCPD_NULL_OUT_DQ's prediction (8.0e-8 mA/rtHz/20mA).
- DCPD_SUM_OUT_DQ sees a slightly elevated RIN of 4.6e-9/rtHz (9.2e-8 mA/rtHz/20mA).
- The RIN in DCPDA (H1:IOP-LSC0_MADC0_TP_CH12, corrected for the whitening) is about 5.9e-8 mA/rtHz, or RIN = 5.9e-9/rtHz at 20mA/2diodes (~15pm DARM offset)...
- ...or about 3.3e-8 mA/rtHz or 1.2e-8/rtHz at 5.7mA/2diodes (~8pm DARM offset).
- ASC-AS_C_SEG1 (H1:IOP-ASC0_MADC6_TP_CH11) and OMC-DCPD_A (H1:IOP-LSC0_MADC0_TP_CH12) shows a coherence of 0.053 at 20mA, suggesting a white noise floor a factor of 0.23 below shot noise.
- At 5.7mA the same coherence is about 0.13, i.e. the white noise floor is a factor of 0.39 below shot noise.
- These two measurements are in plot 1.
- Taking the last two statements together, we predict a coherent noise of
- 5.9e-8 mA/rtHz *0.23 = 1.4e-8 mA/rtHz at 20mA/2diodes (~15pm DARM offset) (RIN of coherent noise = 1.4e-9/rtHz) - The pure shot noise part is thus 5.7e-8 mA/rtHz
- 3.3e-8 mA/rtHz *0.39 = 1.3e-8 mA/rtHz at 5.7mA/2diodes (~8pm DARM offset) (RIN of coherent noise = 4.5e-9/rtHz) - The pure shot noise part is thus 3.0e-8 mA/rtHz.
- AS_C calibration:
- 200V/W (see alog 15431)
- quantum efficiency 0.8 (see alog 15431)
- 0.25% of the HAM 6 light (see alog 15431)
- We have 39200cts in the AS_C_SUM. Thus we have
- 39200cts / (1638.4cts/V) * 10^(-36/40) (whitening) / (200V/W) = 1.89mW and AS_C. (shot noi
- 1.89mW/0.025 = 76mW entering HAM6. I.e. we have slightly more sideband power than carrier power (Carrier: 27mW in OMC transmission).
- Shot noise level on AS_C_SUM is at 2.0e-8 mA/rtHz, corresponding to a RIN of 1.6e-8/rtHz. I.e. the coherent noise seen at 5e-7/rtHz is high above the shot noise. Dark noise TBD.
- The light entering HAM 6 has a white noise of 5e-7/rtHz*76mW = 3.8e-5 mW/rtHz
Bottom line:
-We have ~1.4e-8mA/rtHz, or 1.9e-8mW/rtHz of coherent white noise on each DCPD.
-It corresponds to 3.8e-5mW/rtHz before the OMC, i.e. the the OMC seems to attenuate this component by 2000.
-This noise stays at the same level (in mW/rtHz) for different DCPD offsets.
Next, we switched back to the IFR for testing. plot 2 shows the same coherences (all at 5.7mA / 8pm DARM offset), but on the IFR. Interestingly now AS_C and AS_A_RF36 start seeing different noise below 2kHz. We convinced our selfs that the higher excess noise seen in AS_A_RF36 is indeed oscillator phase noise from the IFR - so that is clearly out of the picture once of the OCXO. (Evan will shortly log the oscillator phase noise predictions.)
64k Channel list:
H1:IOP-LSC0_MADC0_TP_CH12: OMC-DCPD_A (used in plot)
H1:IOP-LSC0_MADC0_TP_CH13: OMC-DCPD_B
H1:IOP-LSC0_MADC1_TP_CH20: REFLAIR_A_RF9_Q
H1:IOP-LSC0_MADC1_TP_CH21: REFLAIR_A_RF9_I
H1:IOP-LSC0_MADC1_TP_CH22: REFLAIR_A_RF45_Q
H1:IOP-LSC0_MADC1_TP_CH23: REFLAIR_A_RF45_I
H1:IOP-LSC0_MADC1_TP_CH28: REFL_A_RF9_Q
H1:IOP-LSC0_MADC1_TP_CH29: REFL_A_RF9_I
H1:IOP-LSC0_MADC1_TP_CH30: REFL_A_RF45_Q
H1:IOP-LSC0_MADC1_TP_CH31: REFL_A_RF45_I
H1:IOP-ASC0_MADC4_TP_CH8: ASC-AS_A_RF36_I1
H1:IOP-ASC0_MADC4_TP_CH9: ASC-AS_A_RF36_Q1
H1:IOP-ASC0_MADC4_TP_CH10: ASC-AS_A_RF36_I2
H1:IOP-ASC0_MADC4_TP_CH11: ASC-AS_A_RF36_Q2
H1:IOP-ASC0_MADC4_TP_CH12: ASC-AS_A_RF36_I3
H1:IOP-ASC0_MADC4_TP_CH13: ASC-AS_A_RF36_Q3 (used in plot)
H1:IOP-ASC0_MADC4_TP_CH14: ASC-AS_A_RF36_I4
H1:IOP-ASC0_MADC4_TP_CH15: ASC-AS_A_RF36_Q4
H1:IOP-ASC0_MADC6_TP_CH11: ASC-AS_C_SEG1 (used in plot)
H1:IOP-ASC0_MADC6_TP_CH10: ASC-AS_C_SEG2
H1:IOP-ASC0_MADC6_TP_CH9: ASC-AS_C_SEG3
H1:IOP-ASC0_MADC6_TP_CH8: ASC-AS_C_SEG4
Images attached to this report
Comments related to this report
Some more estimation - this time for frequency noise: - Shot noise on the refl diodes is given by Pshot=sqrt(2*h*nu*Pr_lock) - The cavity sensing function is P_9_pk = 4*Gam9*P0 * dNu(f)/(f_p + i*f), where P0 would be the carrier power incident on the PD without the IFO. - from this we can estimate a frequency (phase) noise of about 8e-11 rad/rtHz. Gam9=0.219; %alog15874 PSL_low=2; %W Pr_nolock_low=13.7e-3; %W PSL_lock=24; Pr_lock=3.5e-3; %W IMCt=0.88; att=Pr_nolock_low/(PSL_low*IMCt); P0=PSL_lock*IMCt*att; inlockdrop=Pr_lock/(P0); Pshot=sqrt(2*h*nu*Pr_lock); dphi=Pshot/P0/4/pi/Gam9;
For reference, I ran the numbers on where we would expect the sidebands to show a resonance feature. I used the following values: RITM=1939.3m RETM=2241.54m L=3994.485m Checking accidental sideband resonances in the arm cavities: Resonance condition: fres = FSR * (q + (l+m+1)*fTM/FSR) Free Spectral Range (FSR) : 37.5258 kHz Transverse Mode Spacing (fTM): 32.4297 kHz Checking f1 sideband: q=242 l+m=0 Freq. diff. = 18.2284 kHz q=242 l+m=0 Freq. from antiresonant = 0.534516 kHz q=242 l+m=1 Freq. diff. = 14.2013 kHz q=241 l+m=1 Freq. from antiresonant = 4.56162 kHz q=241 l+m=2 Freq. diff. = 9.10514 kHz q=-242 l+m=0 Freq. diff. = 18.2284 kHz q=-243 l+m=0 Freq. from antiresonant = 0.534516 kHz q=-243 l+m=1 Freq. diff. = 13.1322 kHz q=-244 l+m=1 Freq. from antiresonant = 5.63065 kHz q=-244 l+m=2 Freq. diff. = 8.0361 kHz Checking f2 sideband: q=1212 l+m=0 Freq. diff. = 16.0903 kHz q=1212 l+m=0 Freq. from antiresonant = 2.67258 kHz q=1212 l+m=1 Freq. diff. = 16.3393 kHz q=1211 l+m=1 Freq. from antiresonant = 2.42356 kHz q=1211 l+m=2 Freq. diff. = 11.2432 kHz q=-1212 l+m=0 Freq. diff. = 16.0903 kHz q=-1213 l+m=0 Freq. from antiresonant = 2.67258 kHz q=-1213 l+m=1 Freq. diff. = 10.9942 kHz q=-1214 l+m=1 Freq. from antiresonant = 7.76872 kHz q=-1214 l+m=2 Freq. diff. = 5.89804 kHz
Evan, Matt, Lisa We did one more test for the broadband coherence noise: Common mode gain +3dB vs -3dB We see no chnge in the broadband level of the noise below 10000Hz. However, we do see an FSS gain oscillation at 7320Hz showing up in the OMC_DCPD_SUM - but not in AS_C_LF or AS_A_RF36 - in fact that coherence has adip where we get the frequency noise oscillation. This strongly suggests that our broadband noise is NOT frequency noise. Evan also took the frequency noise transfer function - a preliminary analysis here also confirms: the frequency noise should be significantly below the O(1e-8mA/rtHz) noise level we see.
Images attached to this comment
Note that the higher order mode estimates above were made using a slightly wrong modulation frequency. Updated estimates for the correct modulation frequency are attached to alog 20147
- ASC-AS_C GETS 2.5% of the HAM 6 light (see alog 15431) (NOT 0.25%)
Actually AS_C gets 400ppm of the light entering HAM6 -- the OM1 mirror was swapped from 5% transmission to 800ppm transmission in early April. See alog:17738.