With the filter cavity aligned, I checked the newly modified TAMA PD signal to noise ratio.

According to elog 2414, the modified TAMA PD should have SNR 5 times better than the old design.

I measured PD noise when there was no light incident on these two PDs. The noise is mainly electronic noise at this moment. I also measured the signal with 40mW green power and a 50:50 BS pick-off mirror. To compare SNR easily, I multiply newly modified PD both signal and noise by a factor of 140. This makes both PDs electronic noise overlap.

As shown in the attached figure, the signal from the modified PD is larger than the old PD. SNR is roughly a factor of 2 larger (low frequency ~1.7, high frequency ~2.3). This measurement result is not consistent with 5 times SNR increase.