[Rana, Alberto]
This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.
This plot shows the spectrum noise from the RF output of the photodetector.
(here you should be able to see an attached figure, if not it's probably becasue imagemagic has having problems with displaying png files)
The tall peak at 21.5 MHz is the AM modulation introduced by the EOM. It seems to be caused by a misalignment of the EOM which might be somehow modulating the polarization.
The mount in which the EOM sits is not very solid. We should change it with something similar to that of the other two EOMs in the Mach Zehnder.
By tightening down the plastic screws of the mount Rana reduced the amplitude of the AM modulation by 20dB.
The bump in both the dark and shot noise are in corrispondence of the resonance of the PD's electronics. As it appears, the electronics is not well tuned: the bump should coincide with the AM peak.
In the case of the dark noise spectrum, the bump is due to the thermal noise of the electronics. It's a good sign: it means that the electronics is good enough to be sensitive to it.
Transimpedance Estimate
As a "sanity check" we made an approximate estimate of the transimpedance to make sure that the PD is dominated by shot noise rather than other noises, ie electronic's noise.
- Supposing that the laser beam hitting the PD was shot noise limited, we measured 1.1V at the DC output. That let us estimate the photocurrent at DC of 20mA, for a 50Ohm output impedance.
- The shot noise for 20mA is 80 pA/rtHz
- From the nosie spectrum, we measured 3e-7 v/rtHz at 21.5MHz
- The impedance at RF is then Z_rf = 3e-7 V/rtHz / 80e-12 pA ~ 4000 Ohm
- Since the RF path inside the PD has a gain of 10, the transimpedance is ~400Ohm, which is about as we (ie Rana) remembered it to be.
- The PD seems to be working fine.
|