Sheila, Daniel, Nutsinee
Late alog, but here it is!
Quick conclusion: 25.3% loss calculated from measured 3MHz transimpedance and single sideband transmission during full IFO lock. I forgot to set the oscillascope to 50Ohms impedance when I took the measurement of RF in Vpk so that's where most of the uncertainty would come from (after correcting it by a factor of 2).
Details: To measured the transimpedance we bounce the beam of the ITMY (PR2, SR2 misaligned). We first scanned CLF single sideband (crystal temperature moved away from nlg region but optimized for co-resonance) with 24dB whitening on the OMC DCPD on. Then we locked the OMC and took note of the current read by the DCPD SUM (without 24dB whitening). Using the transimpedance at DC (400Ohms), Vrms measured off RF in (the one that goes into the common mode board, this is to avoid 23dB attenuation) and the 3MHz transmission of 0.1 (using the OMC cavity pole of 0.3MHz) we calculated the transimpedance at 3MHz to be 659.5Ohms.
Z_rf = V_rf_rms/sqrt(I_clf*I_cr)
Then during the full lock, using 20mA for DCPD sum current and Vrms of the 3MHz RF in measured on the floor we calculated CLF current:
Iclf = (1/Icr)*(V_ifo_rms/Z_rf)^2
Using the DCPD responsivity of 0.858 A/W we calculated the loss to be 23.5%. The factor of 10 to correct for OMC transmission has been included. So this is the amount of loss we have between OPO -> SRM -> OMC input.
The code used for this calculation is attached.
Nutsinee, Sheila
We had another look at the transimpedance and loss from these single sideband measurements.
To summarize the measurement:
In single bounce off ITMY we measured 13mA of carrier (Ic) through the OMC after running both AS and OMC alignment loops and offloading them. With the OPO temperature detuned so that we have only a single sideband of CLF, we adjusted ZM1/2 to align the squeezer to the OMC, turned up the OMC whitening to 24dB, and measured 1.21uA of CLF power. We then locked the OMC on the carrier, injected the single sideband at 3MHz from the squeezer, and measured -18.6dBm of 3MHz on the demod .
Finding Transimpedance:
- Using an OMC Finesse of 390 and FSR of 261MHz, the transmission of the OMC for the 00 mode at 3.125 MHz is 1/(1+(2*Fin*sin(pi*3e6/FSR)/pi)^2) = 1.1% of the transmission on resonance (the factor of 10 above was an error). So the peak photo current we expect at 3 MHz is sqrt(Ic*Iclf*0.011) = 13.4uA.
- After the OMC transimpedance amp, there is a split off chassis that amplifies the 3MHz signal from the DCPDs, D1700376. The test report that we have found is E1700363, which reports a gain of 19.7dB for 3 MHz, so the -18.6dBm measured at the demod is 3.8mV peak of 3MHz signal out of the transimpedance amp.
- The transimpedance at 3MHz is then 287 Ohms.
- Koji's measurement is roughly consistent with this.
Estimating loss:
- With the OPO temperature detuned so we have no nonlinear gain and only a single 3 MHz sideband, Nutsinee measured 0.757uW of 3MHz on SQZT6. With the interferometer locked, Nutsinee measured at the demod 117.6mV pp with the scope in high impedance mode, which means the peak voltage out of the transimpedance amp was 3 uV once we take into account the 19.7dB gain of the 3MHz pick off chassis. Using the transimpedance from the first measurement, we have a peak photo current at 3 MHz of 10.6uA.
- Using the DC photo current of 20mA, and the photodiode responsivity of 0.858A/W, this implies that there was 7.1 nW of 3MHz single sideband reaching the OMC DCPDs during the measurement.
- Using the 1.1% transmission of the OMC for the 00 mode at 3MHz, we get an efficiency of 76.5% for the propagation of the 3MHz sideband from the beam diverter to the 00 mode arriving at the OMC.
Loss budget:
This 76.5% efficiency sounds similar to the efficiency calculated above, but that is because two errors are roughly canceling (overestimating the transmission of the OMC at 3MHz and leaving out the gain of the 3MHz pick off chassis).