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Message ID: 1167     Entry time: Tue Nov 25 16:06:55 2014
Author: Dmass 
Type: Things to Buy 
Category: PDH 
Subject: EOM response 

I replicated Koji's impedance measurement setup for those-who-cannot-afford-an-impedance-mesaurement-kit (a la this elog), but did not see any large dips in the impedance from mechanical features. I guess this is not so surprising, as the coupling of acoustic modes into impedance isn't necessarily large.

After discussing with Chris, we decided to misalign the polarisation into the EOM and measure the transfer function of Volts -> RIN.

Attached are plots of [RIN/V] through the EOM - the magnitude fluctuates with the polarization misalignment, but the phase remained consistent.

Using this as a proxy for [rad / V], it is unclear which one would be easier to use as a high frequency path - The West EOM has a much higher Q single resonance, and larger phase excursion, but we might be able to correct for it electronically, whereas the East EOM has larger more frequent phase excursions at lower frequency.

REGARDLESS, both of these EOMs are in use for generating sidebands, and if we want to have a high frequency phase correction path, we need to purchase two more EOMs**

 

 

** we explored using a bias T at the laser to switch to laser current mod produced sidebands to free up the EOMs and save $$, but this made the laser noise go up and we were unable to track down the culprit over a couple days. We also began to explore making a circuit which had resonant gain and DC coupling so that we could use one EOM for both paths, but we found little enough evidence that this was efficient (but it is interesting), and thus abandoned it.

Attachment 1: EOM_V2RINcompare_full.png  40 kB  | Hide | Hide all
EOM_V2RINcompare_full.png
Attachment 2: EOM_V2RINcompare_zoom.png  25 kB  | Hide | Hide all
EOM_V2RINcompare_zoom.png
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