User:Steven J. Koch/Notebook/Kochlab/2009/08/12/Power modulator

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Steve Koch 13:36, 12 August 2009 (EDT): It's possible the power modulator is not aligned well enough. It would be great if this is the source of the noise and if it can be fixed easily. I don't know yet, though, except that the power modulator is not working as well as it should, and that the beam seems to be entering quite a bit to the right of the entry window (maybe low too?).

Steve Koch 13:54, 12 August 2009 (EDT) Create offset array is working, but it's a little fucked up. It's weird now, because it's using the x, y, sum math in both the array and real data, so there's all kinds of shakiness to the array. Some possibilities:

  • Programatically subtract offset based on prior knowledge, and using smooth function
  • Do the PHRESH math after wiggle subtraction.
  • Laser down to 9.3 A (170 mW), and signal is still jumpy. PW set to constant transmission at 89.94% and I took out the ND. Still seeing jumps in Y and sum. Very sensitive to touching any of the first four elements (PM, M1, M2, beam expander). Using the laser card, beam definitely seems to the right of the entrance aperture, to me.
  • Laser still at 9.3 A (180mW now), and PM set to 6.18%. Signal is fucking rock-solid and not sensitive at all to touching the early optics.
    • Sum is 16.5 mV, pm 0.5 mV
    • X and Y (no math) are about the sam level of noise
    • Put the ND back in and doesn't work very well. Sum seems noisier, but it's still pretty solid)
  • Laser up to 11.44A (1.00W), PM set to 6.18%. ND "in". X&Y went way off range (-0.01 V for X, +0.0075 V for Y), need to rezero.
    • Sum is 120 mV pm 6 mV (much much noiser than w/o ND; X&Y are not noiser, which seems strange to me, since I'm not doing PHRESH math.
      • The noise on the Sum signal is very periodic, with a period of about 17 ms...this is probably not coincidentally 60 Hz.
        • 'This noise completely goes away when ND is "out"'. What the heck?

Steve Koch 17:15, 12 August 2009 (EDT): OK, So I tried centering the beam more in the power controller. I don't think it affected anything I can see. Beam is still quite unstable when attenuation is low.


I took before and after photos of the beam on the coverglass, and it doesn't look all that different. Of course, it must be aligned slightly crappier now that I have adjusted the power modulator, so we can expect the stiffness to be slightly lower.

Steve Koch 17:31, 12 August 2009 (EDT): I discovered that we can control the laser power directly via an analog input (I had thought it was only digital). Thus, I should probably try removing the power controller from the beam path and using the laser directly. If the jumpiness goes away, this would be great. Of course, it could be that the jumpiness will still be there and that the power modulator was acting as an isolator.

Steve Koch 20:40, 12 August 2009 (EDT): I can turn the power modulator off, and insert a 1/2 lambda in front of it to get the power up. The noise is crazy high at high power. I can't take out the power modulator completely, because the beam shifts position too much.

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