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CAL (XPcal)
takahiro.yamamoto - 4:19 Friday 01 March 2024 (28722) Print this report
laser power measurement of X-Pcal
[Fujii, YamaT]

Abstract

This is the continued work of klog#28673
For making the power budget around the AOM input of X-Pcal, we measured laser power at (1)~(4) in Fig.1.
Sum of laser power at (2)~(4) is ~11% smaller than one at (1) and this seems to be much larger than measurement accuracy.
Though we have achieved our goal of rough estimation of AOM input power for each paths, we need to continue the investigation that where the missing power goes to...

Details

We measured the laser power at (1)-(4) with the 1.6, 2.2, and 2.8W as the output settings of the laser control and measured power is as follows.
output(1)(2)(3)(4)(1)-[(2)+(3)+(4)]
1.6W1.512W0.689W0.632W0.010W0.181W
2.2W2.028W0.931W0.858W0.015W0.224W
2.8W2.546W1.119W1.068W0.019W0.340W


Sum of laser power at (2)~(4) is ~11% smaller than one at (1). This means that ~100mW mismatch in klog#28673 comes from missing transmitted light of any mirrors, missing ghost, loss at somewhere etc., doesn't come from a measurement precision. In fact, uncertainty of readout power is less than 10mW and time dependent uncertainty (probably comes from the instability of laser power) seems to be 20-30mW. Anyway, a discrepancy of 100 mW seems to be too large to be considered a measurement precision.

After these measurements, we noticed that transmitted light of the mirror marked cyan circle was very bright though one of the mirror marked magenta circle couldn't be seen at all. So we tried to measure the transmitted light at behind of the mirror marked cyan circle. But power meter didn't detect this light though the light surely enters the power meter (power density issue?). Because we didn't have a enough time to investigate this issue, we gave up today.
Images attached to this report
Comments to this report:
Shingo Fujii - 9:17 Friday 01 March 2024 (28725) Print this report

Fujii 

I had given a slightly wrong value to YamaT-san.
The corrected values are marked as red.

output (1) (2) (3) (4) (1) - [(2)+(3)+(4)]
1.6 W 1.512 W 0.689 W 0.632 W 0.010 W 0.181 W
2.2 W 2.028 W 0.931 W 0.858 W 0.015 W 0.224 W
2.8 W 2.546 W 1.188 W 1.068 W 0.019 W 0.271 W
Shingo Fujii - 17:30 Tuesday 12 March 2024 (28817) Print this report

Fujii, Dan 
 
Additional measurements (4) and (5) for making the power budget.
Since there were some points that could not be measured in the previous measurement,
we measured the points with a power meter that can measure low power.
Updates are in red.
 
(6) is the calculated difference between the laser power output and the sum of each measurement and the anticipated losses.
Ideally, this should be zero, but there is a discrepancy of ~100 mW.
The discrepancy may have resulted from a measurement precision, since it does not show a linearly increasing system with respect to output and the uncertainty of the power meter in the previous measurement is at least 3 %.

(1) (2) (3) (4) (5) (6)
1.512 W 0.689 W 0.632 W 0.035 W 0.004 W 0.099 W
2.028 W 0.931 W 0.858 W 0.048 W 0.006 W 0.114 W
2.546 W 1.188 W 1.068 W 0.062 W 0.009 W 0.130 W

 
(6) = (1) - [ PBS_loss + Lens_loss + (2) + (3) + (4) + (5)]
PBS_loss = [ (1) - (4) ] * 0.03
Lens_loss = [ (1) - PBS_loss - (5) ] * 0.006

The losses were calculated with reference to thesis (JGWdoc).

Images attached to this comment
dan.chen - 18:52 Wednesday 03 April 2024 (29065) Print this report

With Shingo

In order to measure the laser power with a one power meter (S121C from ThorLabs), we put ND filters between the HWP and the PBS for LPD.
Then we got about 350mW before the PBS = point 1.

Result

With this condition, we measured point 1, 2, and 3.
The result mean that there is about 4% loss from point 1 to 2+3, which is consistent with the power loss of the PBS for LPD.
So we think unknown loss around this area should be about 1% at an accuracy of ~1%.

Details

Measured raw data is attached.
If we apply 1% correction based on the LPD value for the measured value of C=point 2:
A-(B+C*1.01) - power to LPD = 349.8 - (172.7+160.9*0.99) - 3 ~ 15mW
Because A = 349.8, this 15mW is about 4%.
According to this thesis, the PBS has 3% loss, which is almost consistent with our measurement this time.

We could not measure the point just after the PBS because of the limited space.

Images attached to this comment
dan.chen - 16:43 Thursday 04 April 2024 (29081) Print this report

With Shingo

We removed the ND filters today.
After the work, we found the alignment changed a little, about 2 mm on ETM.
We adjusted the alignment by the picomotor in EXA chamber.

Then, we qualitatively looked at the scattered light from the PBS with an IR camera (ASI224MC) and a power meter(S121C).
The laser source power was set to be 10W.
I attached an IR camera picture which shows the PBS shines very brightly.
We also held our power meter up around the PBS and got readings of about 1 mW, depending on the position.
(The distance between the power meter and the PBS was roughly 4-5 cm.)
These results indicate the scattered light is not so small, and can have several % with a very rough calculation.

Images attached to this comment
dan.chen - 15:51 Friday 05 April 2024 (29109) Print this report

The PBS information:

  • model number: TFPC12-1047
  • link
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