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 Wed Feb 4 17:45:39 2009, Yoichi, Update, General, Mode matching of the upgraded IFO
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 Wed Feb 4 19:22:57 2009, Yoichi, Update, General, Do we need off-axis parabolic mirrors ?
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Thu Feb 5 10:42:33 2009, Yoichi, Update, General, Do we need off-axis parabolic mirrors ? No way !
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Message ID: 1272
Entry time: Wed Feb 4 19:22:57 2009
In reply to: 1271
Reply to this: 1274
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| Author: |
Yoichi |
| Type: |
Update |
| Category: |
General |
| Subject: |
Do we need off-axis parabolic mirrors ? |
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I also estimated the mode matching degradation caused by the astigmatism.
Since the incident angles to the mode matching mirrors are not 0, the effective focal lengths in the incident plane and the perpendicular plane are different.
This effect leads to astigmatism of the beam.
When there is astigmatism, the maximum achievable mode matching rate becomes less than 100%.
According to my calculation, the mode matching cannot be better than 94% for the input beam.
For the output mode matching, we can theoretically achieve more than 99% even with the astigmatism.
The difference comes from the fact that the OMMT is longer, thus the incident angle is smaller.
If we don't like this 94%, we have to use off-axis parabolic mirrors, or modify the IMMT to a longer one.
I prefer to make it longer. Just 5" elongation will increase the mode matching rate to 99.4%.
We have a room for this 5" elongation.
Again, the details of the calculation are added to the Mathematica notebook below.
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