I took measurements at the green beat setup on the PSL table, and found that our power / mode overlap situation is still consistent with what Koji and Manasa measured last September [ELOG 10492]. I also measured the powers at the BBPDs with the Ophir power meter.
Both mode overlaps are around 50%, which is fine.
The beatnote amplitudes at the BBPD outputs at a frequency of about 50MHz are -20.0 and -27.5 dBm for the X and Y beats, respectively. This is consistent with the measured optical power levels and a PD response of ~0.25 A/W at 532nm. The main reason for the disparity is that there is much more X green light than Y green light on the table (factor of ~20), and the greater amount of green PSL light on the Y BBPD (factor of ~3) does not quite make up for it.
One way to punch up the Y beat a little might be to adjust the pickoff optics. Of 25uW of Y arm transmitted green light incident on the polarizing beamsplitter that seperates the X and Y beams, only 13uW makes it to the Y BBPD, but this would only win us a couple dBms at most.
In any case, with the beat setup as it exists, it looks like we should design the next beatbox iteration to accept RF inputs of around -20 to -30 dBm.
In the style of the referenced ELOG, here are today's numbers.
XARM YARM
o BBPD DC output (mV)
V_DARK: + 1.0 + 2.2
V_PSL: + 7.1 +21.3
V_ARM: +165.0 + 8.2
o BBPD DC photocurrent (uA)
I_DC = V_DC / R_DC ... R_DC: DC transimpedance (2kOhm)
I_PSL: 3.6 10.7
I_ARM: 82.5 4.1
o Expected beat note amplitude
I_beat_full = I1 + I2 + 2 sqrt(e I1 I2) cos(w t) ... e: mode overwrap (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: -13.2 -21.5
o Measured beat
P_RF: -20.0 -27.5 [dBm] (53.0MHz and 46.5MHz)
e: 45.7 50.1 [%]
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