(Aidan B, David H, Thomas V, Matt H....Alastair H (remotely))

We have been trying to align the table the last few days, but ran into an issue with trying to get a good extinction of the beam reflected off the two "wavelength" branded polarisers that are after the Beckhoff controlled HWP.

First for a little bit of history. For the two table builds at LLO, the reflection off the 2nd of the polarisers after the Beckhoff controlled HWP could be minimised to a couple mW as measured by a power meter.  (in fact I think I even heard could get down to 0.2mW). However, the two tables at LLO were built up only in "phase 1" build. Thus they do not have things like the AOM installed (which will be used for intensity stabilisation).

Here at LHO we are trying to do the phase 1 and phase 2 builds at the same time to work out as many issues as we can before trying to build up the other table at LHO, and do the phase 2 builds at LHO.

When doing the alignment the first time around, we found that for ~10W (about 20%) CO2 power we couldnt get the power reflected off the polariser to be anything less than ~10mW. We tried various times to fine tune the rotational angle of the polarisers as the power reflected is very sensitive to angle. However no success. We decided to simplify the setup to have a look at what was going on.

Firstly we put another HWP right after the ouput of the laser and then we looked at the performance of the very first polariser (II-VI brand) that is right after the laser. At an angle of 60 degrees on the HWP we got maximum transmission through this first polariser, and by fine tuning the angle of the polariser, we can get an extinction ratio of around 200:1 (p to s polarisation) which is what we roughly expected for the laser output (and double checked this was the case by removing the HWP and seeing that the results for what is transmitted and refected from the polariser essentially stays the same). So we are getting what we expect from the laser.

We then experimented with what we see when we placed wavelength type polariser after the AOM (so setup is laser, HWP, polariser, AOM, polariser). In this simple setup we couldnt get the reflection off the polariser after the AOM to be at a low power level like expected (we got around 0.43W  at only 20% power level). Removing the AOM decreased the reflected power off the second polariser by around half this amount. We even tried moving the HWP to between the two polarisers and playing with it and looking at what we see, but minimum reflected power could get was still around 200mW (at 20% power setting of laser on PWM mode).

So decision was to make a real simple experiment and mimic what essentially LLO has (except for a bunch of steering mirrors between first polariser and Beckhoff controlled HWP). So we had a setup of laser, polariser, HWP (mimicking beckhoff controlled HWP), polairiser, polariser. With this setup we were able to get roughly the results see at LLO (ie a few mW of power reflected off the last of the polarisers when laser running in CW mode at full power). 

So now went back and went for broke and put everything back as should before phase 2 layout (ie AOM back in, polarisers after beckhoff controlled HWP), and after realigning/checking alignment of everything, the minimum power we could get off the reflection of the 2nd of the polarisers after the Beckhoff controlled HWP when laser is in CW mode is ~95mW AAAAArrrrrggggghhhhhhh. At a hunch I ripped the AOM out (it only transmits the beam so should not affect the alignment pulling it out), and immediately the power reflected off the second of the polarisers after the HWP went down to a few mW. 

Thus it seems the AOM is affecting the polarisation of the beam. We are going to look further into it (may be due to some kind of stress being induced to the AOM in someway), we may also look at changing the layout (move the first of the polarisers to after the AOM) to mimimise the affects of what the AOM is doing to the polarisation of the beam. The hunt goes on..................