User:Steven J. Koch/Notebook/Kochlab/2009/07/15/power modulation

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Steve Koch 01:06, 16 July 2009 (EDT): Previously, the AOD power control went from 0.9V to 5.67 V max. Increasing voltage meant increasing power. The power modulator currently is the opposite of that, and not to mention it has this bipolar kind of control relative to the setpoint. I'm thinking how I want to deal with this. I'm hoping I can make 0V equal the minimum.


Steve Koch 02:53, 16 July 2009 (EDT): After doing the stuff below, I am making these decisions:

  • Going to cycle modulator while laser is at 12 amps; then not going to turn it off for a long time (weeks)
  • Going to add a global variable to feedback software that allows hard-wired inversion of the DAQ voltage. Thus, the software can set 5V, which will actually produce -5V on the output (if "invert?" set to true)
  • Power modulator probably is not working properly (see jumps in Sum signal). Could be due to sum signal, but I'm doubting it. (although I did see some jumps when jiggling those connections...h)


  • Laser 12.59 Amps setpoint; it thinks power is 1.43 W. Without modulation input, power controller maxxes out at about 893 W, which it thinks is 89%. Clearly one of the instruments is off. Holding the power meter by hand, the max power I can get is 1.166 W, which would indicate that the power modulater is incorrect. I don't know if it's misaligned, broken, or just that's the way it works. I think it's consistent, though.
  • Mod tweaked to maximum (roughly) at no input:
    • 0V = 876.7 mW
    • -1V = 888.5 (off scale)
    • -0.1V = 886.7 (still off scale)
    • -0.05V = 884.4 (still off scale (by that I mean "error" lit)
    • -0.02V = 881 (error flickering)
    • 0V = 876,7
    • 1V = 677.8
    • 2V = 477.8
    • 3V = 278.5
    • 4V = 78.96
    • 5V = 12.91 (error)
    • This seems clear that it is trying to do about 200 W per volt
  • Mod tweaked to minimum (flips between 13.65 and 14.02)
    • 5V is 11.43 error
    • 0V is 12.54 error
    • -0.02V is 16.6 error
    • -0.03V is 18.81 mW
    • -1V is 212.5 mW
    • -2V is 411.8 mW
    • -3V is 611.4 mW
    • -4V is 810.6 mW
    • -5V is 880.4 mW (error)
    • Once again, it seems that deltaP = deltaV*-200mW/V
  • Laser to 10.57 amps (says 0.54 W), modulator tweaked to minimum, about 5.166 mW
  • 0V is 4.797
    • -0.02V is 8.118 mW (and strangely, error light is on?)
    • -0.03V is 9.963 mW, no error
    • -1V is 203.6 mW
    • -2V is 364.2 mW
    • -1.5 V is 303.6 mW
    • -0.5 V is 103.6 mW
    • So, once again, it's -200 mW / V. So, I would say clearly this is what the modulator does, independent of input power. The manual is confusing and tells you that you can measure this (SFmod) but doesn't tell you what it should be.
  • Laser to 15.28 A, 2.72 mW
    • -10V modulation is an error and 1.31 W, supposedly 89.93 %. Power meter says 2.18W (Note for reference: Modulator says 1.315 W for about 2.18W (probably higher). this factor is 60%.
  • Laser 14.08 A, 2.01W
    • 1.266 W read when power meter says 1.63W or higher (78%)
  • Laser 12.92 A, 1.48 W
    • 946 mW read when power meter says 1.234 (76%)
  • Laser 10.97, 0.67 W
    • 432 read when power meter says 549 mW (79%)
  • Laser 14.08, 2.07W
    • 601.1 mW read when power meter says 770 mW (78%)
    • In this case, the power modulator is well within it's range, and there's still a discrepancy factor of 0.78 between what it's reading and what the power meter is actually saying. Best guess now is to say that actual power after modulator is 1/0.78 or 28% higher than the reading. It doesn't matter too much, I just want to know what's going on and later if we check it that it's still the same. I think we should operate at 14 Amps on the laser when we want to try to unzip first. This should be a maximum input power to the objective of 1.63 W.

Let's say optimistically that 1.6 Watt = 50 pN maximum. Then 160 mW would be 5 pN. Actually, this is annoying, I'm just going to use readings off the modulator. Let's say that 1.266 W (actually 1.63 W) = 50 pN maximum. Then, 127 mW = 5 pN and 12.7 mW = 0.5 pN. Can we achieve that with the control voltage?

  • laser to 14 amps 1.93 W
    • minimum is 18.45 mW, sum signal is 0.117 V
    • 0V, 18 mW, 117 mV (modulation input, front panel reading, sum signal)
    • -1V, 209.9 mW, 1.94 V
    • -2V, 409.9 mW, 3.8 V
    • -3V, 609.2 mW, 5.52 V, VERY unstable jumps in the range of 50 mV (is this due to the input modulation? Potentially very bad; need to come back and try this without control voltage input At the end of this note, had stabilized to 5.38 V, less jumpy
    • -4V, 808.8 mW, 7.33 V, unsteady, rising. Now is 7.37 rising...up to 7.42 now
    • -5V, 1.008 W, 9.29 V, unsteady
    • -6V, 1.2W, pinned at 10.6 V NOTE: This means if we want to go to this power, we'll need to attenuate the beam further. Not a big deal with the KG glass that Andy ordered.
    • No modulation input, manual put setpoint to 609.2. Sum signal is unstable at about 5.8, jumps as big as 60 mV (that's that expected?
    • Turned the power modulator off, and sum signal rops to about 730 mV. Jumps are of size maybe 10 mW (that's more than 1%....hmm, maybe the power modulator just can't keep up with laser? I guess we don't really care too much as long as sum signal is accurate and the jumps aren't too exceptional...a bit worried, though.)
    • for reference, turned back on power modulator, same laser current, recalibrated and set 1064 nm wavelenght. -7V gives 1.229 reading, and error.
    • -6V, 1.23W, error
    • -5V, error
    • -4V, 1.151W, pinned sum
    • -3V, 863.4 mW, sum 8.48 (ratio is 101.8 mW / Volt)
    • -2V, 575.1 mW, sum 5.61
    • -1V, 286.7 mW, sum 2.74
    • 0V, 17.58 mW error, sum 0.108 V

OK, what I just discovered is the fucking factor changes when you cycle the power on the device (or perhaps when you recalibrate it?) It's now at about -287 mW / V. WHy? What we need is a method for turning on the thing to get the desired factor. This previous time, I definitely turned on the machine when the laser power was at 14A. (but did I hit cal button after adjusting wavelength?)

  • Set laser to 11 amps (about 600 mW); cycled power; set wavelenght; power setpoint already to minimum
    • 0V, 5.74 error
    • -1V, 110.2
    • -5V, 430.7 error
    • -3V, 332.1, sum signal 4.55
    • -3.7V, 409.9, sum signal 5.66V (Note above: 409.9 supposed power was sum signal of 3.8V I think this means that the device is just fucked up as far as what power it reads?)
  • Set laser to 14 amps (about 2.07 W); cycled power on modulator (note; modulation input was connected); set wavelength; power setponit to minimum
    • -1V is 375 mW; sum is 3.4V (produces different power than above)
    • -0.765V is 286.4, sum is 2.62 (sum is roughly same as above, this is good at least)
    • disconnected modulation input, and hit cal (wavelength has now been set too)
    • same -0.765V gives 286.4, sum is 2.62. So, evidently, nothing changed upon doing "cal." I guess one lesson here is don't turn off the fucking power modulator for the next few weeks! We can also insert a front panel setting to put in the mW / volt factor. The reason this is important is because the PID parameters of the feedback loops will depend on this factor, and we don't want to have to retune those parameters all the time.
    • -1V is 374.9 mW, 3.48 V sum
    • -2V is 752.0 mW, 7.1 Sum. (ratio is 105.9)
      • Note that the power to sum ratio is similar to above when I powered on the modulator at 14 amps.
  • Set to 12 amps; waited a minute or so; cycled power; adjusted wavelength to 1064; hit cal; held down the "down" button for a while. 9.225 minimum power. Connected modulation control.
    • 0V = 9.225 mW.
    • -1V is 198.5 mW.
    • -2V is 398.1 mW, sum signal is 5.64. WHOAH: I saw a huge vertical jump, though (didn't catch the magnitude)....waited happened again, jumped to 5.77 (ouch!!! this sucks!) This is repeatable behavior upon going to 0V and back to -2. This sucks. Anyway, the conversion factor is about -200 mW / V and expect to see huge sum signal jumps until we figure this out. Bleh
  • Recycled power like above. 9.225 minimum power (but laser has increased to 1.12W)
    • 0V is 8.856 mW, 0.079 V (double-checked, and now no connection produces 8.856 mW too)
    • -1V is 198.5 mW, sum signal is 2.79 V and I did see a jump
    • -2V is 398.1 mW, 5.69 V, not seeing any jumps. ok, enough of this shit.