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ADF4107 build in an optical phase locked loop

Question asked by a.lambrecht on Dec 10, 2010
Latest reply on Dec 13, 2010 by icollins

Hello everyone,

 

This is my first post, but I will try my very best, since I am hoping that somebody can help me.

 

In my lab we are trying to phase-stabilize two diode lasers witht the help of the ADF4107 chip. We get the beat signal of the both lasers (6.7 GHz) onto a fast photodiode and the amplified signal is the RF signal we want to stabilize. The reference signal (210 MHz) comes from an highly stable frequency synthesizer. We use the buildup in the appendix to amplify and filter the CP-out so that the chip can control one laser.

The problem is now that the beat signal (we can observe that directly via a directional coupler and a spectrum analyzer) has a linewidth of about 6 MHz, which means that both lasers are not phase locked to each other, but the chip says that they are in lock.

Our observation tools for that statement are :

-Error signal (X4 on the schema)

-MUX-Out N-Divider has a very narrow peak at the divided frequency (70MHz).

 

So my question is, how can that be, and is this a known effect due to some "bad" frequencies or something.

 

p.S. sry for my bad english, its not my native language

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