|
[Larisa and Jenne]
A few weeks ago (on the 28th of January) I had tried to measure the quantum efficiency of one quadrant of the WFS as a function of angle. However, Rana pointed out that I was a spaz, and had forgotten to put a lens in front of the laser. Why I forgot when doing the measurement as a function of angle, but I had remembered while doing it at normal incidence for all of the quadrants, who knows?
Anyhow, Larisa measured the quantum efficiency today. She used WFS2, quadrant 1 (totally oil-free), since that was easier than WFS1. She also used the Jenne Laser (with a lens), since it's more stable and less crappy than the CrystaLasers. We put a 50 Ohm terminator on the RF input of the Jenne Laser, since we weren't doing a swept sine measurement. Again, the Ophir power meter was used to measure the power incident on the diode, and the reflected power, and the difference between them was used as the power absorbed by the diode for the quantum efficiency measurement. A voltmeter was used to measure the output of the diode, and then converted to current as in the quote below.
Still on the to-do list: Replace the WFS2 diode. See if we have one around, otherwise order one. Align beams onto WFS so we can turn on the servo.
QE = (h*c)/(lambda*e) * (I/P)
Where I = (Volts from Pin1 to GND)/2 /500ohms
P = Power from laser - power reflected from diode.
h, c, e are the natural constants, and lambda is 1064nm.
Also, I/P = Responsivity
Larissa is going to put her data and plots into the elog shortly....
| Quote: |
|
Quantum Efficiency Measurement:
I refer to Jamie's LHO elog for the equation governing quantum efficiency of photodiodes: LHO 2 Sept 2009
The information I gathered for each quadrant of each WFS was: [1] Power of light incident on PD (measured with the Ophir power meter), [2] Power of light reflected off the PD (since this light doesn't get absorbed, it's not part of the QE), and [3] the photo current output by the PD (To get this, I measured the voltage out of the DC path that is meant to go to EPICS, and backed out what the current is, based on the schematic, attached).
I found a nifty 25 pin Dsub breakout board, that you can put in like a cable extension, and you can use clip doodles to look at any of the pins on the cable. Since this was a PD activity, and I didn't want to die from the 100V bias, I covered all of the pins I wasn't going to use with electrical tape. After turning down the 100V Kepco that supplies the WFS bias, I stuck the breakout board in the WFS. Since I was able to measure the voltage at the output of the DC path, if you look at the schematic, I needed to divide this by 2 (to undo the 2nd op amp's gain of 2), and then convert to current using the 499 Ohm resistor, R66 in the 1st DC path.
I did all 4 quadrants of WFS1 using a 532nm laser pointer, just to make sure that I had my measurement procedure under control, since silicon PDs are nice and sensitive to green. I got an average QE of ~65% for green, which is not too far off the spec of 70% that Suresh found.
I then did all 8 WFS quadrants using the 1064nm CrystaLaser #2, and got an average QE of ~62% for 1064 (58% if I exclude 2 of the quadrants....see below). Statistics, and whatever else is needed can wait for tomorrow.
Problem with 2 quadrants of WFS2?
While doing all of this, I noticed that quadrants 3 and 4 of WFS2 seem to be different than all the rest. You can see this on the MEDM screens in that all 6 other quadrants, when there is no light, read about -0.2, whereas the 2 funny quadrants read positive values. This might be okay, because they both respond to light, in some kind of proportion to the amount of light on them. I ended up getting QE of ~72% for both of these quadrants, which doesn't make a whole lot of sense since the spec for green is 70%, and silicon is supposed to be less good for infrared than green. Anyhow, we'll have to meditate on this. We should also see if we have a trend, to check how long they have been funny.
|
|