EDIT by KI:
The definition of the recycling gain is wrong here.
See the latest entry (#5875)
Here is a summary about the Power Recycled Michelson (PRMI).
It seems the mode matching is also one of the greatest contributor on the low recycling gain.
+ Loss = 5.3% (or effective reflectivity of 93.28% in Michleson) => Under coupling !!
+ Mode matching efficiency = 47.4 % => Really bad !!
With these values we end up with a recycling gain of 7 and a normalized REFLDC of 0.5 as observed (#5773).
Also according the incident beam scan measurement (#5773) the loss is NOT a local effect like a clipping, it is more like uniformly distributed thing.
As for the mode matching, the number indicates that approximately the half of the incident light is coming back to the REFL port without interacting with PRMI.
This is bad because the non-mode-matched light, which is just a junk light, is entering into the photo detectors unnecessarily.
In the worst scenario, those junk light may create a funny signal, for example a signal sensitive to the alignment of PRM.
The method to estimate the loss and the MM (Mode-Matching efficiency) is essentially the same as before (#5541).
One difference from the previous estimation is that the I used more realistic parameters on the transmissivity of ITMs and PRM :
PRM : T = 5.637 % (see the 40m wiki)
ITM : T = 1.384 % (see the 40m wiki)
In addition to the basic calculations I also made plots which are handy for figuring out where we are.
Quantities we can measure are the reflected light from PRMI and the recycling gain using the REFL PD and the POY PD respectively.
So I wanted to see how the loss and MM can be estimated from the measured REFL DC and recycling gain.
The plots below are the ones.
The first figure shows a contour map of the loss as a function of the measured REFL DC and recycling gain.
The white area is a place where no proper solutions can be found (for example MM can get to more than 100 or loss becomes negative).
The star mark in the plot corresponds to the place where we are now. Obviously the loss is about 5%.
If we somehow decrease the amount of the loss the star mark will mostly go up in the plot.
The second figure shows a contour map of the MM as a function of the measured REFL DC and recycling gain.
The X and Y axis are exactly the same as that of the first plot. Again the star mark represents the place where we are.
We are currently at MM=47%
Here are some solutions to bring the recycling gain higher.
We don't work on these things immediately since it requires opening of the chambers again and it will take some times.
But we should think about those options and prepare some stuff for a coming vent.
+ Refinement of the position of the mode matching telescopes. => The Recycling gain can go up to 15.
=> Assuming the loss in the cavity doesn't change, the star mark in the first plot will go to the left hand side along the "0.05" black solid line.
=> However PRMI will be still under coupled.
=> Needs an estimation about which way we move the telescopes.
+ Locate the place of the dominant loss source and reduce it somehow.
=> The recycling gain will be more than 18 if the loss reduces by a factor of more than 5.
=> Needs a clever way to find it otherwise we have to do it in the classical way (i.e. white light and trying to find dirty surfaces)