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Message ID: 1339     Entry time: Wed Sep 18 18:19:52 2013
Author: Evan 
Type: DailyProgress 
Category: RF 
Subject: Measurement of PDH modulation depths 

I took measurements of the carrier and sideband power transmitted through each cavity in order to get the modulation depth Γ of the EOMs. The modulation depth is related to the transmitted powers by Γ2/4 = PSB/Pcar. [Edit: I checked last night in Mathematica, and it seems the approximation Γ ≈ J1(Γ)/J0(Γ) is good to about 1% for Γ < 0.3.]

First I aligned the cavities to get good refl visibility (about 90%). Then I aligned the transmitted beams onto the ISS transmission PDs. Then I unplugged the EOM HV actuation on the TTFSS.

To get the transmitted carrier power, I locked each cavity as usual and then wrote down the voltage on the ISS PD. To get the sideband power, I flipped the sign of the fast actuation on the TTFSS, thereby making the servo lock on the sideband. I then wrote down the voltage on the ISS PD. I also blocked each transmitted beam to get the dark voltage on the ISS PDs.

  South North
Locked on carrier (mV) 639±1 1920±10
Locked on sideband (mV) 21.1±1.5 26.5±0.01
Dark (mV) 8.7±0.2 10.1±0.1
Γ 0.281±0.017 0.185±0.003

For posterity, I also took triangle-wave sweeps of the ISS transmission, the refl DC, and the error signal. The oscilloscope traces are attached. [Edit: from a quick look at the error signal traces, I get slopes of (164±10) kHz/V for the south cavity and (199±12) kHz/V for the north cavity.]

In other news, the south PDH error signal looks a bit asymmetric; I think it might need a phase adjustment.

Attachment 1: pdh_modulation_traces.zip  303 kB  Uploaded Thu Sep 19 14:44:17 2013
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