User:Anthony Salvagno/Notebook/Research/2010/08/11/Imaging AOM - Task 1 and 2

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After having learned a lot and nothing at the same time about our laser we are going back to complete what we set out to do.

Question

What is this and how can we minimize it?

When we adjust the AOm from 1V to 5V we see the profile above on the QPD. There is some sort of stabilization that is occuring and it causes the beam to drift. The sum signal (the bottom plot) decreases during this time as well. We explore this today.

Experiment 1

Our path was laser > AOM > M1 > M2 > QPD (with lenses)

Initial investigation of ramping. We want to see if we can just minimize the drift with a lens system on the QPD. This had no effect.

Experiment 2

laser > L1 > AOM > M1 > M2 > QPD

We then saw something in the manual that showed that the rise time of the AOM was related to the beam waist in the crystal. The thinking with this experiment is that if we put the focal point of the laser in the crystal (thus making the beam waist as small as possible) we can change the rise time. If the rise time is affecting this drift and time dependency that we have been observing, we can then observe this change by affecting the rise time. In this experiment we saw no effect on the ramping.

Experiment 3

laser > L1 > AOM > M1 > L2 > M2 > QPD

In this experiment we decided to put a lens on the other side of the AOM (with the AOM at the focal point of the lens) to see if this helped at all. It turns out, that it had a good effect. We saw almost no drift in the laser outside of an initial shift in position. No ramping. I think what happened is that we imaged the AOM with the second lens.

Experiment 4

laser > AOM > L1 > L2 > M1 > M2 > QPD

Now we are going to setup both lenses after the AOM so that we are imaging the AOM with L1.

Exp 4.1

  • The 1:1 telescope (L1 and L2) is situated anywhere in between.
  • We still have the ramping effect.
    • This is consistent with our previous observations because it was when we had our tweezers setup that we first noticed this. (IE the telescopes were not imaging the AOM.)

Exp 4.2

  • The AOM is at the focal point of the first lens of the 1:1 telescope
  • Still not a fix

Exp 4.3

  • AOM at focus of L1 and QPD at (f1+f2)
  • Still not a fix

Another Proof

I got the phenomenon on video and it's tough to see unless you go frame by frame. However It is quite easy to see the intensity change when the AOM voltage is increased.

  • The beam on the left is the low voltage 1st order beam and the beam on the right is the 0th order beam.

    The beam on the left is the low voltage 1st order beam and the beam on the right is the 0th order beam.

  • The voltage is bumped up to 5V from 1.3V and the 1st order beam increases in intensity.

    The voltage is bumped up to 5V from 1.3V and the 1st order beam increases in intensity.

  • A few seconds after the voltage increase, the 1st order beam is slightly less intense than previous even though nothing has changed. Throughout the process the beam has shifted positions slightly.

    A few seconds after the voltage increase, the 1st order beam is slightly less intense than previous even though nothing has changed. Throughout the process the beam has shifted positions slightly.

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