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Entry  Thu Feb 24 17:42:00 2011, Aidan, Update, Green Locking, 15% of end laser sideband power transmitted through cavity 
    Reply  Sat Feb 26 02:33:16 2011, kiwamu, Update, Green Locking, sidebands on beatnote beat_note.png
       Reply  Sun Feb 27 09:43:59 2011, Aidan, Update, Green Locking, sidebands on beatnote 
       Reply  Sun Feb 27 13:09:56 2011, rana, Update, Green Locking, sidebands on beatnote 
Message ID: 4351     Entry time: Thu Feb 24 17:42:00 2011     Reply to this: 4361
Author: Aidan 
Type: Update 
Category: Green Locking 
Subject: 15% of end laser sideband power transmitted through cavity 

I did a quick calculation to determine the amount of sideband transmission through the FP cavity.

The modulation frequency of the end PDH is 216kHz. The FSR of the cavity is about 3.9MHz. So the sidebands pick up about 0.17 radians extra phase on one round trip in the cavity compared to the carrier.

The ITM reflectance is r_ITM^2 = 98.5% of power, the ETM reflection is r_ETM^2 = 95%.

So the percentage of sideband power reflected from the cavity is R_SB = r_ITM*r_ETM*(exp(i*0.17) - 1)^2 / (1 - r_ETM*r_ITM exp(i*0.17) )^2 = 0.85 = 85%

So about 15% of the sideband power is transmitted through the cavity. The ratio of the sideband and carrier amplitudes at the ETM is 0.05

So, on the vertex PD, the power of the 80MHz +/-200kHz sidebands should be around sqrt(0.15)*0.05 = 0.02 = 2% of the 80MHz beatnote.

Once we get the green and IR locked to the arm again, we're going to look for the sidebands around the beatnote.

 

 

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