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Entry  Thu Oct 23 15:27:19 2008, Alberto, Update, General, Abs length TRX_aplot_03_04_05_together.png
    Reply  Thu Oct 23 18:23:47 2008, Alberto, Update, General, Abs length TRX_plot_06_07_08_together.png
    Reply  Thu Oct 23 18:27:04 2008, Alberto, Update, General, Abs length HOM_Frequencies.m
Message ID: 1074     Entry time: Thu Oct 23 18:27:04 2008     In reply to: 1072
Author: Alberto 
Type: Update 
Category: General 
Subject: Abs length 

Here are the measurements I've got yesterday. The plot shows the transmitted power after the X arm while sweeping the frequency of the beat between the two lasers. That frequency is changed by scanning the frequency of the local oscillator of the PLL (that is the Marconi).
The X arm cavity has been locked to the TEM00 of the main beam. I tilted ITMX in order to enhance the higher modes of the secondary beam with the purpose of making them beat with the main beam.
Three traces are shown in the plot correspondent to three different measurements in which I clipped the transmitted beam at the X end with a razor blade from up and from the side of the photodiode.
Both the beats of the TEM00 mode of the main laser with the TEM01 and TEM10 modes of the secondary laser are expected to be at 6.2763 MHz. The plot has a candidate peak at 6.325MHz but it does not appear on both the measurements with the blade. the peaks at 3.897MHz and 7.795MHz are the first and the second longitudinal modes of the X arm cavity.

Here is the Matlab code I use to calculate the HOM frequencies.
Attachment 1: HOM_Frequencies.m  488 Bytes  Uploaded Thu Oct 23 19:28:43 2008  | Hide | Hide all
% FP Cavity HOM Frequencies Estimate
% Alberto Stochino, October 2008

R1 = 7280;      % Mirror1 radius of curvature
R2 = 57.57;     % Mirror2 radius of curvature
L = 38.458;     % Length of the FP Cavity
n = 1;          % X Order of the Mode
m = 0;          % Y Order of the Mode

c = 299792458;  % Speed of Light
... 11 more lines ...
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