T. Yamamoto, Akutsu

So let's calculate something to confirm the green beam power found at the EXA-PD.

• According to the Ushiba-kun's klog in the last night, the PD at EXA showed peaks of around 3000 counts for the transmission green.
• Thanks to the Chen Dan-kun's report, the Thorlabs PDA100A (or A2???) gain is set at 50dB.
• In the PD manual (see here), 
  • At 532 nm, 0.34 W/A, maybe.
  • With the 50dB gain, and assuming the load resistance of high-z (I believe our AA is like this), 4.75e5 V/A
• The usual conversion factor: 40V end-to-end diff input / 16bit counts = 40/2^16 = 610 uV/cnt

Considering all the above, 3000 * 40/2^16 /4.75e5 /0.34 = 11 uW.

At the same time, what power should be transferred?

• According to Tanioka-kun, the GreenX power at the Faraday output is about 10mW.
• Acoording to the wiki,
  • ITM, ETM transmission: 0.06, 0.06 each
  • PR2 transmission: 0.9?
  • PR3 reflection: 0.9?
• GV windows transmission: 0.9 each??  Please have a look here. We have three closed GV windows as of today.
• The active area of the PD is 10mm sq. (10x10 = 100mm^2)
• Gaussian beam has a spot radius of w=35/sqrt(2) mm at the PD
• According to my calculation, a Gaussian beam of power P0 and spot radius w would have a power of P within the radius r as follows:
  • P/P0 = 1-exp(-2 (r/w)^2); for simplicity, let the PD area as a 10mm radius circular; substituting r=10mm, we got P/P0 = 0.55.

Then, 10mW * 0.55 * 0.06^2 * 0.9^5 = 11.7 uW

Hmmm... too much matching. Over consistent. Maybe I can suspect myself that I misunderstood something.