[Daniel, Sander, Torrey]

A new PD was installed in transmission and reflection for the 775 filter cavity path. These are both high BW 775 optimized PDs. We can then redo the cavity ringdown measurements. 

New ringdown measurement shows a few interesting things. 

1) The initial measurement on the 1811's extra decay term does not show up on these PDs. I simply fit a single exponential to the data and take the decay constant from this.

2) We see this blip - that we now think is associated with the decay of the AOM light, discussed later - only in the REFL PD and not on the trans PD.

3) The REFL and TRANS curves agree to about 1%. Yielding a finesse of 130 and 132 respectively if we believe Matt Evans. Or 260 and 264 if you believe Steck - 7.2.5. I think this is sufficiently accurate measurement of the finesse. Hopefully there is this factor of two and we are much closer to the expected finesse (i.e. not super lossy).

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An interesting aside that came along with this measurement. We believe we know what this blip is on the REFL side of this measurement. Note the approximate time it takes the light coming from the AOM to turn off - AOMturnofftime. This measurement is made in the same way with no cavity locking, just using the first cavity mirror as an HR. This time frame roughly corresponds to the blip being seen on the cavity ringdown measurements - new_ringdown_measurement_zoomed.png. The explanation is as follows: (Image to refer to - rn_image_picker_lib_temp_be29ac5f-b54e-4fc9-849e-b251590347bc(1).jpg). The beginning of the blip is the incoming E field (E_in) beginning to rapidly decay. The REFL pd is reading out the quantity |E_in + Ecav|^2, where E_cav is a negative value because the two signals are 180 degrees out of phase as defined by our PDH lock. Thus, as E_in decays this total quantity decreases. The minimum point of the blip is the point in the E_in decay where E_in = E_cav, maximally destructive (I suspect if this was done at a higher resolution the blip minimum would be 0). After the minimum, the blip rises back up because the light built up in the cavity starts leaking out, increasing E_cav. At the top of the blip E_in = 0 and |E_in + Ecav| is purely from the cavity leaking.