Polarisation

[Matt Wilkinson]

We need to talk about polarisation!

At Herstmonceux, we are still emitting linear polarised laser light after an attempt to make it circular with a 1/4 wave plate failed. The circular polarisation was lost through the coudé mirrors before the beam left the emitter.

Would everyone please describe here the polarisation characteristics of their SLR systems.

• What is polarisation state of the laser light emitted by your SLR system?
• How are you achieving this?
• Have you tested this and know that you are sending what you think you are at all telescope azimuths and elevations?
• Can you control the emitted polarisation?
• What are the polarising properties of your receive path [At Hx, we found our receive path to be highly selective to polarisation and we had to replace our dichroic mirror]
  

In the future, we plan to look further at circular polarisation and would be grateful for any advice from those that are able to send circular polarised light.  We also hope to return to looking at controlling the emitted linear polarisation orientation using a 1/2 wave plate.

Thanks
Matt

[jsteinborn]

Hi Matt,

polarization effects were never thought as a problem for the original 10Hz design of our station and after
switching to kHz there was also no apparent reason to start an investigation.

Nevertheless we played around with a l/2 wave plate several month ago to check if the polarization plane of
out refurbished HighQ Laser has changed. We noticed variation in our calibration return rate if we rotate the
wave plate.

So the next step would be to try to make the polarization circular with a l/4 wave plate. We may run into the
same problem as you, but we should try at least.

How did you check that the circular polarization was lost?

Best
Jens

[Matt Wilkinson]

Hi Jens

The full story of our battle with polarisation in our system is here https://cddis.nasa.gov/lw17/docs/papers/session10/07-Wilkinson_ILRS17_polarisation.pdf.

Bottom line is that we were losing laser energy through our coudé path depending on the mirror position and we were losing > 50% of our return signal through a polarisation selective dichroic mirror.

The first thing you should do is establish if it is caused on the transmit or receive side. My favourite test was to point the telescope at the zenith and rotate in azimuth and record the intensity of the laser light backscatter at night. The polarisation state is preserved in the reflection by the small spherical water droplets in the atmosphere. We saw clear variation in intensity on both the transmission and reflection side of the dichroic but in opposite magnitudes.

If you've got polarisation problems, switching to circular will not solve this, but would take out the variation. We've not been successful in transmitting circular so would be interested to know how you get on.

Matt

[jsteinborn]

Thanks Matt,

I have the suspicion that we have the same problem. It seems that we are losing energy while pointing in specific directions.
We will definitely try the azimuth rotation test.

I will keep you informed.

Best
Jens