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Tue Aug 18 14:24:43 2009
Reference Cavity Inspection
Rana, Koji, Yoichi
To see why the reflected beam from the RC is distorted, we took out
the periscope and the iris in front of the RC. The periscope mirrors
had some gunk and dusts on them. We blew nitrogen air onto them to
remove the dust. Since the gunk did not come off with the air, we
wrapped a Q-tip with lens cleaning paper soaked in Methanol, and wiped
the surface of the mirrors. We did this because it was hard to remove
the mirrors from the periscope (they were in a spring loaded mirror
holders. The springs were too strong to safely remove them without
damaging the mirrors).
Looking into the RC from the front mirror revealed nothing obstructing
in the path.
After the cleaning, we put the periscope back and observed the direct
reflection from the cavity (not locked). It was still distorted
exactly like before.
So we did some tests.
First we injected He-Ne to the RC. It turned out that multiple
reflections from the optical window (not AR coated for He-Ne) made it
almost impossible to investigate anything with He-Ne. But this
observation made us to suspect maybe one side of the window is not AR
We placed the periscope about 50cm away from the RC and injected the
beam from an angle, so that we can observe the direct reflection.
First, we checked the shape of the beam leaving the periscope. It was good.
We then observed the reflected beam from the RC. It was also good, no distortion.
We made sure that it was really the reflection from the mirror, not from the window
We measured the separation between the in coming beam to the cavity and the reflected beam
at two locations. From this, we can guess where the two beams intersect (the reflection point).
The estimated reflection point was far inside the RC enclosure, indicating that it was really
reflection from the front mirror of the RC.
Since we did not see any other reflection beam, we concluded that the AR coating of the window
We checked the direct reflection beam shape with several different incident angles, but the
beam shape was always good.
We put back the periscope to the original position. This time, we put a high reflectivity mirror
after the output mirror of the periscope. The beam coming out of the circulator (PBS) had a good
circular shape. But still if we let the beam reflected by the cavity, the beam shape is distorted.
Something must be happening in the RC. Unfortunately, we could not figure out what it is.
We put everything back to the original configuration, except for the iris, and the RC alignment
was already good (surprise). After Koji's final tweak, the FSS is now doing fine, but still
the reflected beam is ugly.