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Entry  Wed Feb 20 10:38:46 2019, awade, DailyProgress, WOPO, Fixing 532 nm polarization linearity issues 
    Reply  Thu Feb 28 16:00:59 2019, awade, DailyProgress, WOPO, Pol launch into PM fiber 19-02-26_19-46-51_5122.jpgPMFiber.pdf19-02-28_17-52-41_5131.jpg
       Reply  Tue Mar 12 16:35:43 2019, awade, DailyProgress, WOPO, Pol launch into PM fiber 1064 nm 
          Reply  Wed Mar 13 12:44:41 2019, awade, anchal, DailyProgress, WOPO, Pol launch into PM fiber 1064 nm IMG_5149.JPG
Message ID: 2305     Entry time: Wed Mar 13 12:44:41 2019     In reply to: 2303
Author: awade, anchal 
Type: DailyProgress 
Category: WOPO 
Subject: Pol launch into PM fiber 1064 nm 

[awade, anchal ]

After a bit of reading I've realized that the standard use of these PM fibers is to launch along the slow axis (see for example Thorlabs and OzOptics info on fiber beam splitters).  It should be much of the sameness for patch cables, but polarization sensitive elements like beam splitters are mostly tested and specified for slow axis launch unless they are custom made to order. 

We are switching the polarization alignment to slow axis in the 1064 nm and 532 nm fiber coupling.  Anchal is re-optimizing​ the 1064 nm launch to get the PM fiber extinction ratio back to a good place.  We've also changed input launch to use a laser line PBS mounted in a rotation mount for clean linear polarization.  With the optimized setup the for the 1064 nm fiber path the output polarization signal goes from 3700 mV to 39.3 mV which is an extinction​ ratio of -19.7 dB.

Here the max theoretical extinction​ ratio is 

ER = -10 \log_{10}[\tan^2(\theta)]

which would place our goodness of alignment to with 0.61 deg.

Updated 1064 nm launch. Uses rotation mounted PBS for guaranteed linear
polarization, half wave plate is to maximize power.




I've replaced the SM fiber in the 1064 nm launch with a PM fiber (P3-1064PM-FC-5). I also moved the fiber collimator (F240APC-1064) back 2.54 cm back to give more space for a PBS cube (to check linearly of the light).  

For the 1064 nm launch it seemed to be a lot harder to find the initial alignment of the collimator using the alignment of the back propagated 650 nm fiber laser source. Here I aligned a pair of irises in the forward propagating direction and then back propagated through the PM fiber using 650 nm to get the initial​ pointing of collimator. I don't know why this is so much harder than the 532 nm case.  I suspect one of the steering mirrors is not really reflecting off the front dielectric surface.  In the end I did a bunch of systematic walking of the fiber launch mount and eventually fount the alignment.  

From 4.44 mW of input light I get 2.74 mW of light out the other end of the fiber.  This is an efficiency of 62 % which is more than enough for my needs.  I expect the HD will only need 1 mW (2 mW max), so this is fine. Getting this in coupling higher will require a bit of lens walking, not really worth it at this stage.

I had already carefully aligned the collimator orientation to put the fast axis on aligned to p-pol (wrt the table), by eye.  It seems like the launch pretty much hit the correct launch polarization on the first go.  I see little variation in the polarization when I pulse the heat on the fiber.  This is now good to go for optimizing the homodyne visible and polarization overlap output from the SQZ.


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