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Message ID: 344     Entry time: Wed Sep 8 00:17:41 2010
Author: tara 
Type: Notes 
Category: Laser 
Subject: Debugging Laser Sta 

 Today I investigated which optics reflect most power back to the PMC and the laser.

The back reflection seems to limit our sensitivity.

the power meter is placed to measure the reflected light that comes back and reflects off the PMC's mirror.

The beam path to the ACAV is blocked and ignore for now, since AOM in the path will act as an optical isolator.

The reflected power from this path will be smaller than that of Rcav's path.


periscope [M]  1/4&PBS  [L]  PLCX     [K]    Mirror








                                             Blocked           PBS  [F]  PLCX [E]   1/2 [D]  EOM [C] PLCX  [B]   1/2 [A]      \  pmc  / <---- beam

                                                                                                                                                           power meter


A razor blade beam dump was placed at each position (alphabet in square brackets).

The red entries corresponds to second set of data after realignment


B 52.3 uW           49.2 uW 

C 0.184 mW        0.184mW

D 0.748 mW        0.745 mW

E 0.748 mW        0.752 mW

F 0.749 mW         0.754 mW

G 0.831 mW

H 0.831 mW

I 0.831 mW

J  0.831 mW

K 0.831 mW

L 0.831 mW           0.88 mW

M 0.831 mW          0.88 mW

reflection from RCAV     1.53  mW     1.96mW

It seems that the reflection from the cavity is giving us a hard time.

The PBS with 1/4 plate might not well adjusted to minimize the back reflection.

After I minimized it by tilting/rotating the PBS, the PBS was tilted around 5 degree  (judging by my eyes) and it looked bad.

The beam is almost clipped on the edge, and Frank noticed that the beam height was not right.

I realigned the beam again (the efficiency is ~97%).

Unfortunately, I lost the beam so much I had to remove a few optics ( 2 mirrors adjacent to the RCAV) behind RCAV,

and used a CCD camera to see the beam while adjusting the knobs. I placed the optics back, but

I haven't adjusted the beam path for beat measurement yet.

Now the PBS is quite flat on the table.

I  measured the power reflected at each optic again (see red entries in the table above.)

I rotated the PBS to see if the power reflected from RCAV can be reduced. It turns out that

I couldn't do much, only from 1.96 - > 1.93 mW.

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