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Message ID: 1316     Entry time: Wed Aug 28 11:13:07 2013
Author: Evan 
Type: DailyProgress 
Category: BEAT 
Subject: Intensity-to-frequency transfer function 

[Tara, Evan]

Last week we tried measuring a transfer function which takes intensity fluctuation induced at the south EOAM and returns frequency fluctuation as read out by Tara's beat setup. This is therefore meant to be the measurement corresponding to Tara's code farsi.m (on the SVN at CTNLab/simulations/misc).

We used the SR785 in swept-sine mode. As measured previously, the EOAM response is 3×10−5 W/V (although I think this number should be rechecked, since we've fooled around with the EOAM in the meantime). The beat PLL readout is 7.1 kHz/V (when the Marconi is set to its 10 kHz setting). [Edit, 2013–10–18: by comparing with the newer measurement in PSL:1368, I think the Marconi must have actually been at the 1 kHz setting, so the conversion factor is 710 Hz/V and the transfer function measurement below is a factor of 10 too high.] These two numbers give the conversion factor necessary to convert from V/V into Hz/W.

The attached plot shows the measurement and the expectation from Tara's code. Here I'm using the version of the code as checked out last night, and in my local copy of the code I changed the cavity length to 1.45″ and the input power to 1 mW. Below 200 Hz, the agreement in magnitude is good: the overall shapes agree the values are within a factor of 3 of each other. The phase also appears to be good below 40 Hz or so. Above 200 Hz the transfer function is apparently dominated by some other effect.

Attachment 1: intensity_to_frequency.pdf  82 kB  | Hide | Hide all
intensity_to_frequency.pdf
Attachment 2: intensity_to_frequency_code.zip  17 kB
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