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Message ID: 348     Entry time: Thu Sep 9 02:20:32 2010
Author: tara 
Type: Notes 
Subject: problem about PBS/QWP  

We have used an instant PBS/QWP which is a PBS optically contacted to an aligned QWP in front of a reference cavity

(for PDH locking and optical isolation.) It cannot properly block the reflected beam, so certain amount of power got reflected back to

the PMC and back to the laser. This causes higher PMC's RIN. (When the beam path behind the PMC is blocked, no back reflection, RIN decreases.)

The PBS/QWP is removed and replaced by a regular PBS and a QWP.

Now the reflected power is less than 0.15 mW, which is 1.11 mW from the cavity and 0.985 mW when the beam is blocked in front of the periscope.

( it's more than 1 mW before with a PBS/QWP), and I think it can be less. 


How we test it:


There are QWP and HWP(half wave plate) in front of the PBS/QWP in our RCAV path.

They are adjusted(rotated and tilted) and the PBS is rotated so that the beam split from the PBS is minimized.

This is done to make sure that we have a perfect linearly polarized wave going to the cavity.

 The reflected beam power is measured as it reflected off at PMC's outport.

With the instant coupled PBS/QWP set, there is considerable amount of power coming back to the laser,

about half of the power picked up at the power meter coming from the cavity(see yesterday entry here, how we measure power reflected at

each optic).

We tried to correct the polarization by adding another QWP behind the PBS/QWP, but this does not work.

After we change to a regular single PBS and QWP, we can reduce the power by at least a factor of 10.


Note: to replace the PBS/QPS, I have to move the PBS back a bit so there is enough space for a QWP.

When I re adjust the split power, the PBS is rotated so much and it's very disturbing. This did not happen

when I tried it at the first time.





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