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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: 2300     Entry time: Thu Feb 28 16:00:59 2019     In reply to: 2296     Reply to this: 2303
Author: awade 
Type: DailyProgress 
Category: WOPO 
Subject: Pol launch into PM fiber 

I've set up a rotating PBS and half-wave plate to provide polarization adjustment into the 532 nm fiber without misalignment the spatial alignment.  Here I've used a PRM1 rotation mount with a SM1PM10 lens tube mount for beam cube prisms.  The lens tube mount is supposed to be for pre-mounted cubes but I've inserted some shims to hold it in place and it seems to work well like that.  It means I can get a nice clean linear polarization at all rotations.

After spatially aligning the input beam I stepped the rotation of the PBS (and accordingly the L/2 wave plate) and pulsed the temperature of the fiber using a heat gun.  After some walking I found that for the current fiber rotation (0 deg) the linear polarization was aligned with the fiber axis at 88 deg PBS rotation (here 0 deg PBS rotation is aligned for p-pol transmission, well almost). I made some adjustments to the alignment of the fiber collimator in the fiber launch, I aligned the slow axis key with the vertical so that the fast axis of the fiber is p-pol.

Keying on PM fibers

As a side note the keying of PM fiber patches is typically with the slow axis aligned with the key notch. The WOPO's PM fibers are keyed so that the alignment key is along the slow axis of the fiber (i.e. aligned with the stress rods). Figure below illustrates the configuration.

Replacing the 532 nm patch with fresh PM fiber

I was getting a large jitter in the power levels as measured at the output of the old SM and PM fibers (on the order of 10%).  These power fluctuations were not present on the input side.  I thought this was an alignment jitter or a polarization effect.  However, I was unable to minimize it by improving the input polarization at the launch.  When I tapped various mounts there didn't seem to be a corresponding correlation with output power jitter of the fiber.  When I checked the end of the PM fiber (P3-1064PM-FC-2), I saw that there was damage about the core (see pictured below).  It seems like maybe I had some kind of etalon effect from this burn mark and the launch.  After replacing the 532 nm PM fiber with a fresh one that arrived last week the power is much more stable and I was able to easily​ find the pol alignment going in. 

Damage to PM fiber end.  No amount of aggressive cleaning will remove the mark in the middle.
The fiber will need to be cut and a new connector spliced on.

 

Next job is to replace SM fiber for the 1064 nm delivery with PM fiber so there is a well defined polarization for launching into the homodyne detector.

 

 

Quote:

Alignment of the pumping 532 nm polarization into the WOPO is important to getting the correct phase matching condition.  For the periodically polled Lithium Niobate (LN) waveguide the phase matching is type-0: and pumping and fundamental wavelengths are in the same polarization.  The AdvR non-linear device is coupled with polarization maintaining fibers (Panda style), which are keyed at their FC/APC ends.  This means that with the correct launch polarization we should be correctly aligned with the proper crystal axis for degenerate down conversion (at the right chip temperature). 

Replacing Broadband PBS

Till now I was using non-pol maintaining patches to coupling into the WOPO fiber ends.  This should have been ok, but it is hard to figure out exactly which polarization is optimal so I switched to a pol-maintaining patch because it can be aligned separately and then the keyed connectors give you automatic alignment.  I had some issues trying to find the optimal polarization going into the fiber and I've now traced this back to the polarizing beam cubes.  I've been using Thorlabs PBS101 which is a 10x10x10 mm^3 beam cube that is supposed to be broad-band (420-680 nm).  When I checked the extinction ratio I saw Pmax=150 mW, Pmin=0.413 mW on transmission between extremes.  This is an extinction ratio of Tp:Ts = 393:1 which is much less than the spec of >1000:1.  Not sure what's going on here, the light going into the BS is coming directly from a Faraday isolator and a half-wave plate.  With some adjustment to the angle of the wave plate I can do a little better but it should be nicely linearly polarized to start with.

I've switched out the PSB101 for the laser line PBS12-1064 I remeasured extinction ratio (Pmax=150 mW, Pmin=27.6 µW) Tp:Ts = 5471:1 (better than the quoted 3000:1 spec).  This is good, at least now I know what is going on. I am also putting in an order for a 532 nm zero order quarter-wave plate, so that we can be absolutely sure we are launching in linear light always.  

Aligning light into pol-maintaining fiber

I previously thought I might be able to use the frequency modulation technique to align the light through the polarization maintaining fiber.  There is a birefringence in PM460-HP fiber of  3.5 x 10-4.  The phase between ordinary and extraordinary axes over the whole fiber length is

\Delta\phi = \frac{2 \pi \Delta n L}{c}f

Where L is fiber length, \Delta n is the birefringence and f is the laser frequency.  The idea is to launch linearly polarized light into the fiber and then at the readout place a polarizer rotated to be 90°: ramping frequency will produce an amplitude modulation on the dark fringe.  However, even with 1 GHz of frequency ramp this is only a 15 mrad effect for a 2 m fiber, its likely to be too small to see over other effects.  This is not enough to be able to fine align polarization.  

Instead I'll use the heat gun method.  I'll fire linearly polarized light into the fiber and measure the output with a crossed polarizer.  If the input polarization is correct there should be no power changes on the output as the fiber is thermally cycled. Its only two meters long so hopefully this effect is easy to see.

 

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