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Manasa, can you please estimate what kind of mode matching we have on the PSL table between the arm greens and the PSL green? We *do not* want to touch any optics at this point. Just stick in a power meter to see how much power we're getting from each beam, and then think about the peak height we see, and what that might tell us about our mode overlap. If we determine it is total crap, we can think about measuring the beams that go either toward the camera, or the DC PDs, since neither of those paths require careful alignment, and they are already picked off from the main beatnote path. But first, what is our current efficiency? Yarm is first, then Xarm, since Yarm seems worse (peak height is larger for non-00 modes!)
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Estimate loss along the Y arm beat path:
1. Measured the beam powers (before the beam combiner):
Y Arm green = 35 uW
Y PSL green = 90 uW
==> Pbeat ~ 2 * sqrt (35 uW * 90 uW) ~ 112 uW
2. Expected power of RF signal
Assuming the PD to have transimpedance ~ 2kV/A and responsivity ~ 0.3A/W,
the expected power of the RF signal = (Pbeat * Transimpledance* Responsivity)^2 / (2 * 50ohm) ~ 45uW = -13.5 dBm
3. Measured power of Y arm beat signal
Turned OFF the beat PDs and rerouted the RF cables such that the spectrum analyzer was reading the RF signal from the Y beat PD itself (without any amplifiers or the beat box itself in the path).
Turned ON the beat PDs and the Y arm beat signal power on the spectrum analyzer measured -58dBm
Even if we consider for losses along the length of the cables, we are still at a very bad state.
4. Bad mode matching??
I don't think mode matching is our main problem here.
Toggling the shutter several times, even with the non-00 modes, the maximum beat power we can see is -50dBm which is still very far from the actual expected value. |