Attachment #1 - comparison of phase tracker servo angle fluctuations for the green beat vs IR beat.
- To convert to Hz, I used the PT servo calibration detailed here.
- This is only a function of the delay line length and not the signal strength, so shouldn't be affected by the difference in signal strength between the IR and green beats.
- For the green beat - I divided the measured spectra by 2 to convert the green beat frequency fluctuations into equivalent IR frequency fluctuations.
- There is no whitening before digitization. I believe the measured spectra are dominated by ADC noise above ~50 Hz. See this elog for the frequency discriminant as a funtion of signal strength, so 5uV/rtHz ADC noise would be ~2 Hz/rtHz for a -5dBm signal, which is what I expect for the Y beat, and ~0.5 Hz/rtHz for a +5dBm signal, which is what I expect for the X beat. Hence the brown (Green beat, XARM) being lower than the green trace (IR beat, XARM) isn't real, it is just because of my division of 2. So I guess that calibration factor I applied is misleading.
- I did not yet check the noise in the other configuration - arm lengths controlled using ALS, and POX/POY as the OOL sensors. To be tried tonight.
Attachment #2 - RIN of the DCPDs.
- I noticed that over 10s of seconds, the GTRY level was fluctuating by ~5%.
- This was much more than any drift seen in the GTRX level.
- Measuring the RIN on the DCPDs (Thorlabs PDA36A) supports this observation (spectra were divided by DC value to convert into RIN units).
- There is ~120uW (1.6 VDC, compatible with 30dB gain setting) incident on the GTRX PD, and ~6uW (170 mVDC, compatible with 40dB gain setting) incident on the GTRY PD.
- Not sure what is driving this drift - I don't see any coherence with the IR TRY signal, so doesn't seem like it's the cavity.
Characterization of the green beat setup [past numbers]:
- With some patient alignment effort (usual near-field/far-field matching), I was able to recover the green beat signals.
- Overall, the numbers I measured today are consistent with what was seen in the past when we had the ability to lock using green ALS.
- The mode-matching between the PSL and AUX green beams are still pretty abysmal, ~40-50%. The mode shapes are clearly different, but for now, I don't worry about this.
- I saw some strong AM of the beat signal (for both EX and EY beats) while I was looking at it on a scope, see Attachment #3. This AM is not visible in the IR beat, not sure what to make of it. The frequency of the AM is ~1 MHz, but it's hard to nail this down because the scope doesn't have a very long buffer, and I didn't look at the frequency content on the Agilent (yet).
o BBPD DC output (mV), all measured with Fluke DMM
XARM YARM
V_DARK: +1.0 +2.0
V_PSL: +8.0 +13.0
V_ARM: +157.0 +8.0
o BBPD DC photocurrent (uA)
I_DC = V_DC / R_DC ... R_DC: DC transimpedance (2kOhm)
I_PSL: 3.5 5.5
I_ARM: 78.0 3.0
o Expected beat note amplitude
I_beat_full = I1 + I2 + 2 sqrt(e I1 I2) cos(w t) ... e: mode overlap (in power)
I_beat_RF = 2 sqrt(e I1 I2)
V_RF = 2 R sqrt(e I1 I2) ... R: RF transimpedance (2kOhm)
P_RF = V_RF^2/2/50 [Watt]
= 10 log10(V_RF^2/2/50*1000) [dBm]
= 10 log10(e I1 I2) + 82.0412 [dBm]
= 10 log10(e) +10 log10(I1 I2) + 82.0412 [dBm]
for e=1, the expected RF power at the PDs [dBm]
P_RF: -13.6 -25.8
o Measured beat
P_RF: -17.95dBm (80 mVpp) -28.4dBm (24mVpp) (40MHz and 42MHz)
e: 37% 55 [%]
I also measured the various green powers with the Ophir power meter (filter off):
o Green light power (uW) [measured just before PD, does not consider reflection off the PD]
P_PSL: 18 24
P_ARM: 400 13
The IR beat is not being made at the moment because I blocked the PSL beam entering the fiber. |